CA1266520A - Restoring framing in a communications system - Google Patents
Restoring framing in a communications systemInfo
- Publication number
- CA1266520A CA1266520A CA000509558A CA509558A CA1266520A CA 1266520 A CA1266520 A CA 1266520A CA 000509558 A CA000509558 A CA 000509558A CA 509558 A CA509558 A CA 509558A CA 1266520 A CA1266520 A CA 1266520A
- Authority
- CA
- Canada
- Prior art keywords
- signal
- framing
- intelligence
- key
- proper
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Lifetime
Links
Classifications
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L9/00—Cryptographic mechanisms or cryptographic arrangements for secret or secure communications; Network security protocols
- H04L9/08—Key distribution or management, e.g. generation, sharing or updating, of cryptographic keys or passwords
- H04L9/0816—Key establishment, i.e. cryptographic processes or cryptographic protocols whereby a shared secret becomes available to two or more parties, for subsequent use
- H04L9/0819—Key transport or distribution, i.e. key establishment techniques where one party creates or otherwise obtains a secret value, and securely transfers it to the other(s)
- H04L9/0822—Key transport or distribution, i.e. key establishment techniques where one party creates or otherwise obtains a secret value, and securely transfers it to the other(s) using key encryption key
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L9/00—Cryptographic mechanisms or cryptographic arrangements for secret or secure communications; Network security protocols
- H04L9/08—Key distribution or management, e.g. generation, sharing or updating, of cryptographic keys or passwords
- H04L9/0891—Revocation or update of secret information, e.g. encryption key update or rekeying
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N7/00—Television systems
- H04N7/16—Analogue secrecy systems; Analogue subscription systems
- H04N7/167—Systems rendering the television signal unintelligible and subsequently intelligible
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N7/00—Television systems
- H04N7/16—Analogue secrecy systems; Analogue subscription systems
- H04N7/167—Systems rendering the television signal unintelligible and subsequently intelligible
- H04N7/1675—Providing digital key or authorisation information for generation or regeneration of the scrambling sequence
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N7/00—Television systems
- H04N7/24—Systems for the transmission of television signals using pulse code modulation
- H04N7/52—Systems for transmission of a pulse code modulated video signal with one or more other pulse code modulated signals, e.g. an audio signal or a synchronizing signal
- H04N7/54—Systems for transmission of a pulse code modulated video signal with one or more other pulse code modulated signals, e.g. an audio signal or a synchronizing signal the signals being synchronous
- H04N7/56—Synchronising systems therefor
Landscapes
- Engineering & Computer Science (AREA)
- Signal Processing (AREA)
- Computer Security & Cryptography (AREA)
- Multimedia (AREA)
- Computer Networks & Wireless Communication (AREA)
- Synchronisation In Digital Transmission Systems (AREA)
- Television Systems (AREA)
- Communication Control (AREA)
- Facsimile Transmission Control (AREA)
- Mobile Radio Communication Systems (AREA)
- Synchronizing For Television (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
A secure communications system requires special arrangements for the establishment and maintenance of framing and synchronization.
A framing word is transmitted at A fixed frequency, and a key neces-sary to unscramble the transmission is transmitted in a fixed position in relation to the framing word. Synchronization information (reference clock bursts) is scrambled with the intelligence and may only be recovered once the key is available. When framing is being maintained, the receiver's phase-locked loop opens only during the short reference clock bursts in the incoming signal, and the framing signal generator is able to predict the occurrence of successive framing signals without reference to the incoming signal. When framing is lost, the receiver enters a "soft lock" condition in which its phase-locked loop is opened to receive the entire incoming signal and in which the framing signal generator refers solely to the incoming signal to generate framing signals.
A secure communications system requires special arrangements for the establishment and maintenance of framing and synchronization.
A framing word is transmitted at A fixed frequency, and a key neces-sary to unscramble the transmission is transmitted in a fixed position in relation to the framing word. Synchronization information (reference clock bursts) is scrambled with the intelligence and may only be recovered once the key is available. When framing is being maintained, the receiver's phase-locked loop opens only during the short reference clock bursts in the incoming signal, and the framing signal generator is able to predict the occurrence of successive framing signals without reference to the incoming signal. When framing is lost, the receiver enters a "soft lock" condition in which its phase-locked loop is opened to receive the entire incoming signal and in which the framing signal generator refers solely to the incoming signal to generate framing signals.
Description
s~
RESTORlN ~ FRA MING
IN A COMMVNICATIONS SYSTEM
TECHNICAL FIELD
This invention is concerned with achieving ~nd maint~ining proper framing and synchronization in ~ communications system, especially in secure ~ommunications system. Proper frflming exists when slots in the received time sequence are correctly sssociated with the ~ppropri-ate positions in the transmitted time sequence; proper synchronizfltion implies th~t the trflnsmitter and receiver clocks are locked to each other (so th~t bit inteKrity is maintsined).
E~ACKGROUND OF THE INVENTION
, Various types of communications systems are of such a character that proper fr~ming is necess~ry to their operation~ For example~ in time-diYision multiplex transmission Or independent digital bit streflms, correct interpret~tion of the v~lue of the incoming signal ~s one member of the transmitted symbol set, and direction of the interpleted symbol to the correct receiver, require proper framing. Likewise, in television trsnsmissionl picture elements displ~yed at the receiver must be in the same relati~e positions ~s those appeflring st the trensmitter, ~nd, for & line-and-field scanned television signal (one in which the picture being tr~nsmitted is scsnned in fields of adjacent parallel lines), proper framJng is necessary to ~chieve this. Unless the dats frames ~qre especially short (for example, individusl ASCII characters~, correct synchronization is needed to msintAin reliable communication during the periods betweell successive fr~ming sJgnals.
In ordinary (non-secure) communicstions systems, fr~ming informa-tion often takes the form of a unique word, identific~tion of which by -~ 2 --the receiver est~blishes ~ p~rticul~r inst~nt as the dfltum from which time me~surements mQy be m~de. The s~me word occurs periodic~lly, st the ssme time in relation to esch frame of dat~ (for ex~mple, ~
television fr~me or ~ield)~ Since the fr~ming ~ord occurs at ~ con-stant frequency and is identical ror e~eh fr~me, fr~ming o~ the recei~ed intelligence is e~sily aeeomplished.
This technique 13 unsultable, however, for a secure communlcations system, sin~e ~n import~nt security teAture msy be the deni~l of framing information. ~r~ming denial can be accomplished by v~rying the time at which the intelligence occurs In relstion to the frRming word (scramblJng); but some mesns must then be found of del~vering this frsming inform~tion to the intended reclpient. If the Rddressee possesses the key to unscramMe the d~t~, no problem should occur. However, a further difficulty srises when sychroniz~tivn infor-mstion (such ~s ~ rererence clock burst) is scr~mbled ~long with the intelligence. Such inform~tlon is necess~ry to the frsming process because It keeps the clock ~rom drirtlng too ~ar during the relatiYely long perlods betw~n rrsmlng pulses.
-2a-SUMMARY OF THE INVENTION
It is an object of an aspect of the present invention to achieve and maintain proper framing in a communications system.
It is an object of an aspect of the invention to achieve and maintain proper framing in a secure communica-tions system.
An object of an aspect of the invention is to recover framing and synchronization information from a scrambled intelligence si gna I .
An object of an aspect of the invention is to recover such information from an intelligence signal which is scrambled in accordance with a key contained within the scrambled intelligence signal.
Various aspects of the invention are as follows:
-2b-ln ~ receiver ~or receiving ~n intelligence signal which includes a timing signsl and Q transmitted key beuring a predetermined time reletionship to the timing sign~l, the intelligence sign~l being scr~mbled in acco!-dance with the transmitted key ~nd being of such ~
charRcter thst proper framing is necesssry ror reception, an Apparatus fs)r detecting the loss of, and restoring, proper rr~ming, sRid ~pp~r~tus comprising:
framing signal generating mean~ receiving the scrambled intelligence sign~l snd respons~Ye to a framing loss signal for outputting as the fr~mlng signal the received timing sign~l and respon-sive to the ~bsence of t~e framing loss signal for outputtlng 8S the fr~ming sign~l a loc~lly generated framing signal, the Is~c811y gener~ted framing SignAI being in substantifll coincidence with the timing sign~l except when fr~ming is improper;
key detection means receivlng ihe scr~mbled intelligence signal and the fr~ming signal for outputting as a detected key a signal derived from that por~ion of the scrambled intelligence signal which bears the predetermined tlme relationship to the framlng signsl; and ~ raming loss detection means recelrlng the scrambled intel-ligence signal during a predetermined period dependent upon the detected key for detecting 1mproper ~reming and for generating the frflming loss signsl until proper framing i~ restored.
- 2c -In a receiver for receiving an intelligence sign~l which includes 8 timing signal ~nd a transmitted key bearing a predetermined time relationship to the timing sign~l, the intelligence signal being scrambled in Qccordance with the transrnitted key and being of such a character that proper framing is necess~ry for reception, ~ method of d~tecting thP loss l~f, ~nd rest~r~ng, proper framing, s~id method e~m-prising the steps Or:
receiving the scrambled intelligence signal;
determining whether fr~ming is proper by ex~mining a predetermined section o~ the scrambled intelligen~e signal;
identifying the predetermined section by reference to detected key;
deri~ring the detected key from the portion of the scram-bled intelligence signal which be~rs the predetermined time relationship to a selected framing sign~ nd selecting the framing sign~l based on the determination of whether framing - is proper, such thet 5~
-2d-i. if framing is proper, the selected rraming signsl is 8 locally gener~ted ~r~ming signal in substantial coincidence with the received timing sigri81; and ii. otherwise, the received timing signal is the selected frsming signal.
In a receiver for receiving an intelligence Sigl)Al which includes 8 timing signsl ~nd ~ transmltted key be~ring ~ predetermined t~me relationship to the timing sign~l, the intelligence sign~l being scr~mbled in accordance with the tr~nsmitted key and being of such a charscter th~t proper framing i3 necessary for reception, A method of detecting the loss of, snd restoring, proper framing, ssld method com-prising the steps of:
receiving the scrambled intelligence signal;
genersting 8 clock signal at ~ frequen~y determined by a selected reference signal;
determining whether framing is proper by ex~mining a predetermined sectlon of the scr~mbled intelligence signsl;
identifying the predetermined section by reference to detected key;
deri~lng the detected key from the portion Gf the scram-bled intelligence slgn~l which bears the predetermined time relationship to a selected framing signal;
selecting the framing Slgrl8l bssed on the deterrnin~tion of whether fr~ming is proper, such that i. if ~raming Is proper, the sele~ted fr~ming sign~l is locally gener~ted frmaing signal in substantial colncidencz with the recei ~red timing signal; And li. otherwise, the recei~ed timing sign~l is the selected framing signal; and selecting the reference signal based on the determinstion of whether framing is proper, such that 1~i6~
-2e-i. if fr~ming is proper, the selected reference signAI is the scrflmbled intelligence sign~l only during the predetermined period;
&nd ii. otherwise, the selected reference sigrlfll i5 the entire scr~mbled intelligence sign~l.
The communicat~ons system embodying the present inrention is one transmittlng broadcast teleYislon. In order to deny programming to unauthorized re~eivers (e ~ those who have not p~id ~or the service), the broadcaster scr~mbles the lines of the tele~rision picture, ~ ~
~LZ~ S~
varies their time of occurrence relstive $o each other or to a regulflrly occurring datum, such as a timing signal produced at the transmitter. Scrambling is done in ~ccordance with ~ key, ana the key is trflnsmitted along with the scrambled picture. The synchro-nization pulses which would ordinarily be transmitted in the blanking interv~ls 3re omitted (for security), ~nd these inter~r~ls ~re used to trsnsmit the key, the audio for the program, ~nd the timing signal (encoded QS a unigue word~ For further security, the key is encrypted tusing a master key previously placed in the possession of the recipi-ent). Although the sign~l is scrambled, the tr&nsmitted key is trans-mitted with a ~onstant time relationship to the timing signal; this enables the transmitted (encrypted) key to be recovered once the ti ming signsl is recovered.
The receiver for such 8 signal includes a number o~ components.
One, a framing signal generator, gener~tes ~ framing sign~l based on the received timing signal or on previously received timing signals (extrapolsted locslly using the receiver's clock). When the generflted framing signfll is IOCQ1lY gençr~ted, b~sed on extrapolation from previously received timing signals, it is in substantial coincidence with the transmitted timing siensl (except, of course, when framing is lost).
When the received timing signsl is output ns the framing signal, no extrapolstion is necessary.
Another component is the key detector, which looks for the transmitted key at ~ predetermined time before or after the framing sign~l. The key detector 81so ~ncludes decryption circuitry for decrypting the detected key in accordAnce with the prearr~nged master key.
A third component is a gating signal generator and time base descrflmbler. This receives the ~rsming signal and the decrypted key snd c~lcul~tes from the key the ~ppropri~te ~blay to place each line of the received picture in the proper time relationship to the others ~to descrsmble the picture). This compor,ent also generstes a gating signal for use by, aMcng other components, the receiver's phase-locked loop (clock). The geting signal opens the loop at the appropriate time to admit a small portion of the received sign~l, ordinarily a reference clock burst occurring during each horizont~l blanking interval.
An import~nt component is the frarning loss detector, which determines when proper fr~ming is l~lst ~nd which then c~uses cert~in changes in the operation of the other receiver components. The frflm-ing loss detector receives the same gating signal received by the phase-locked loop and exsmines the same portion of the received signal, the reference clock burst (although ~ny other sppropri~te portion of the received signal could inste~d be examined). The ~aming loss detector then determines, from the examined portion of the received signal, whether framing is proper~ If it is, no changes are made in the receiver's operation. If it is no$, two things happen. First, the phase-locked loop is opened to the entire received signal~ so the receiver's clock m~y be corrected, if necessary. (It is possible th~t framing W8S lost due to drift of the clock). Second, the framing sign~l generator is directed to generate framing signals based only on the received timing signals and not on historical information (~ ~ prior timing signals extr~polated uslng the receiver's clock), which may no longer be valid.
With the phase-locked loop open, the receiver's clock will be corrected (because clock information is transmitted with the television signsl). And with the framing signal generator looking only st the received timing signals and not Qt its own local extrapolation of the m, receiver framing will by brought into agreement with the transmitted signsl.
E~RIEF DESCRIPTION OF THE DRAWINGS
Figure 1 illustrates three lines of a television transmission which may be received In accordsnce with this invention.
Figure 2 is a diagrsm of the ~raming and synchronization compo-nents of a television receiver employing the present invention.
!Figure 3 is a diagram of the fr~ming word detector shown in Figure 2.
Figure 4 is a diagram illustrating the operation of the burst gate gener~tor And time base descrsmbler of Figllre 2.
Figure 5 is ~ diagrsm of the soft lock detector shown in Figure 2.
DETAILED DESCR~PTION OF THE PREFERRED EM80DIMENT
Figure 1 illustrates three lines of a television transmission which may be received in ffccordance with this invention. The lines illustrated ~re lines 1-3 of the vertical blanking interv~l (VBI).
Each line shown contflins certain information -- here re~erred to 8S "horizontal dats" -- in its horizontal blanking interval (HE~I), the first 10.9 us for an NTSC signsl. Preferably the horizontal dsta includes program audio in digital form and a short burst -- 10 cycles -- at the reference clock frequency~ The clock burst occurs At a fixed point during the HBI of lines within the VBI. The synchro-nizfltion and blanking levels which would normally fill the HBI are removed.
During the video field, that is, between VBrs, the HBrs contflin the same horizontal data but differ in Qn Importsnt respect from those illustrsted. I)uring the video field the length of each HBI, and the position of the reference burst within it, are varied by a pseudor~ndom amount from the standard. This is ~ccomplished by adding or removing digital audio samples at appropriate places in the line. To avoid long-term shortages or surpluses of transmitted information, which could mount quickly, the pseudorandom variation is prearr~nged to average out to zero over each complete field.
Also during the video field, the active portion of each line (the portion not occupied by horizontal d~ta) eontA3nS the separate an~logue components (luminanS~e and chroms) of the transmitted television line.
In the VBI, of course, there is no picture to be transmitted; flnd framing in~ormation may advar,tageously be transmitted during the 12 ~
active portion of each of the VBI lines. These lines (63.S6 us long in NTSC) are occupied by a number of digit~l symbols (455, in NTSC) representing various items of information necess~ry to flchieve ~nd maintain proper frflming. Simple division will show th~t, for a televi-sion tr~nsmission with the ~bove p~rameters (one intended for an NTSC
receiver), symbols occur at ~ r~te of 7.18 MHz, twice the csl~r subcsrrIer frequency.
Line 1 of the V81, in Its active portion, consists of ~ltern~ting zeroes and ones, or maxima and minima of the transmitted symbol set (if multilevel symbols are used). Bec~use of the r~te of symbol tr~ns-mission, line 1 thereîore contains a 52.6~ us continous wave at 3.58 MHz3 the color subcarrier frequency ~nd, prefer~bly, one frequency available from the receiver's clock. (When the term "receiver" is used, it includes not only the NTSC, PAL, SECAM or other st~nd~rd televi-sion receiver but also, more bro~dly, the decoder snd other terminal equipment necess~ry to convert fln incoming television sign~l to one of the standard form~ts, such as NTSC. The clock, therefore, is prefer-ably part of the decoder portion of the receiver.) Line 1 mAy be g~ted into the recei~er's phase-locked loop to drive it into synchronism with the transmitter's clock.
Line 2 Or the VBI, in its active portion, consists of the timing si~al encoded ~s a word, Along with other information (the FD section of the line) needed to identi~y the psrticul~r field in which the line is occurring. Beginning with symbol 79, a series of first code patterns P
is transmitted. The first code pattern P in the prefelred embodiment is the following set Or eight binsry digits: 111tOOOO. This first code pattern P is tr~nsmitted 41-1/2 times, for ~ totfll of 332 symbols.
After the first code pattern P h~s been trQnsmitted 41-1/2 times, the second code p~ttern ~ is transmitted twice, in e~ lNith P. Second code j~ttern Q, in the preferred embodiment, is the complement of first code patter P, th~t is, 00001111. Since an extra half cycle of P
was transmitted, the "in phflse" requirement me~ns thst transmissions of Q begin in the middle of the second code pattern Q (i.e., 1111).
(If an integral number of pAtterns P had been trensmitted, Q transmis-sions would begin at the beginning of Q -- i.e., 0000.~ The timing pu]se is therefore encoded ss the phase revessal occurring between the last transmission of first code pattern P and the first transmission of the second code pettern Q. After the second tr~nsmission of second code pattern Ç~, a field identification waveform is trsnsmitted. This waveforrn serves to identify which of 16 fields is being transmitted.
(It msy be used to synchronize encryption elements of the system.) Line 3, in its uctive portion, contains the (hesvily error-corrected and prefersbly encrypted) key necessary to unscramble the pseudorandom variations in the position of the ten-cycle reference clock burst occurring during the HBI of esch line of the video field.
The key, which is updated once A frame, is used as the starting vector for a pseudorandom number generator circuit.
Figure 2 is a diagr&m of the frsming and sychronization compo nents of the receiver (or, as mentioned ~bove9 the decoder). The scr~mbled television signal, scrsmbled in accordance with the key transmieted in line 3 of the VB}, arrives on line 101 and is delivered to key dectyptor 103, framing word detector 105, and ph~se-locked loop 107. Detector 105 always receives the entire composite television signal so that it may always detect the transmitted timing signRI, although it will usually output ~ locally-generated ~raming signal inste~d of using the received timing signal as the framing signal.
Phss~locked loop lC7 recei~res the composite telerision signal during periods determined by burst g~te generator 109 ~s modified by soft lock detector 111). When fr~ming and synchronization are correct (hsrd lock condition), these periods are the periods of the ten-cycle reference burst in the HBI of every line. Key decryptor 103 receives the entire composite television signal so that it m~y detect the unscrsmbling key on line 3 of the VBI by computing the Amount of time which elapses sfter framing word detector 105 outputs the frarning signsl. The clock signal output from phas~locked loop 107 is delivered as necesssry to the receiver components.
Burst gate generator 109 requires both the framing signal (from detector 105) and the decrypted key (from decryptor 103) in order to determine the time of occurrence of the refer~nce clock bursts in the HBl's of the composite teleYision s~gnal. Phase-locked loop 107 is ordinarily vpened only during these periods for clvck correction. Also during these burst periods, soft lock detector 111 is permitted to receive the cornposite television signal in order to determine whether the hard lock condition is being maintAined. This is accomplished by integrating the received signal. Since the clock burst is 8 series of alternating 0 ~nd 1 symbols, after removQl of the DC component its average value should be zero~ If, upon integration by soft lock detector 111, the portion of the composite television signal received during the period of the gsting Si~lQI produced by generator 109 has a value of zero, framing is proper. If not, framing is improper; and so~t lock detector csuses the receiver to enter the "soft lock'~
condition by outputting a soft lock signal to both framing word detector 105 and phase-locked loop 107. The soft lock signal opens loop 107 to receive the entire composite television signAI, including the clock signal 3n line 1 of the Vl~l~ It also modifies the hysterisis algorithm of detector 105 so that lo~lly~enerated framing signals are ignored, and tlte framing signal output from detector 105 is the received timing signal.
- Figure 3 is a disgrAm of framing word detector 105. The composite television signal (including the encoded timing pulse), fllthough originating as a digitsl signal, is transmitted by radio in analog form. It is first filtered and amplified by, respectively, high pass filter 304 and amplifier 306. The resulting signal is next apptied to input terminal 30Q.
Digital filter 30~, an 8th~ order digital bandp~ss filter tuned to 895 kHz, is a recursive filter. (The filter frequency, 895 kHz, is the frequency at which the code patterns P and Q occur (symbol frequency 7.16 mHz divided by eight symbols per code pffttern.) The poles of filter 308 lie ex~ctly on the unit circle in the z-plane, resulting in an extremely narrow bandwidth and, hence, excellent noise reJection. Sta-bility of the filter is m~intQined by periodic~lly cleflring its stor~ge elements by means of input control logic circuit 309.
When the 895 kHz code patterns enter digital filter 3û8~ its output tends to increase. After the first code pattern P has been applied to digit&l filter 308 for a period of time between 28 us and 4~ us (depending on the le~el of noise) the output of the filter will ha~e risen to a high enough leYel to trigger threshold detector 310 and activate pQttern recognition circuit 312. As shown in Figure 3, the threshold for detecting ~ "1" at the output of digital filter 308 is a value greater than or equal to 25 "1"s. The threshold value for detecting a "0" at the output of digital filter 308 is a value less than or equal to 7. In 28 us digitel filter 308 will h~ve received 25 first code patterns P; therefore, in the absence of noise, the accumulated value for each of the four '11" positions of first code pattern P will have reached 25. In 46 us, all 41-1/2 repetitions of the first code pattern P will hsve been received.
Pattern recognition circuit 312 performs two functions. First, it examines the outputs of threshold detector 310 to determine if they were produced 8s the result of the application of an 895 kHz signal at the ir~put of the digital filter. This is simply a matter of determining whether eight sam21es of the "greater thann/"less than" signal (out of threshold detector 310) satisfy the rollowing two criteria:
a. There must be exactly four "greater than" and four "less than"; and b. There must be e3ther four "greater than" in 8 row or four "less th~n" in a row.
Once it has determined that the 895 kHz signal was present, pattern recognition circuit 312 begins locally generating its own version of the 5.~
895 kHz sigrt~ 9 that is, a series of first code patterns P. These patterns are input to exclusive-OR gate 314 along with the incoming signal from input terminsl 300 in order to lo~te the phase revers~l (see Figure 1) in the encoded synchronizetion signal.
When the ph~se reversal occurs, the output of e~clusive-OR
gate 314 wi~ ch~nge from "0" to "1". The output of exclusive-OR
g~te 314 driYes serial 12-of-16 ~roting circuit 316, whose output is active high whenever 12 of the last 16 input samples were "l"s. In the absence of noise, the point ~t which the voting circuit's output becomes active high is fixed with respect to the composite television signal (if the original synchronization signal itself is fixed, ~s it is in the preferred embodiment). However, due to noise which introduces errors in the phsse reversed section of the encoded timing word, the point at which the voting circuit's output becomes acti~re high will not necessarily be fixed.
This situation is remedied by re-synchronizing the output of voting circuit 316 with the locelly generated first code p~ttern P from psttern recognition circuit 312. This re-synchronizstion takes place in reset circuit 318. ~Without reset circuit 318, the output cf voting circuit 316 wouid be taken as the decoded timing- signal. In the pre-ferred embodiment, however, the decoded timlng signal is taken from the output o~ reset circuit 318.) Reset circuit 318 combines two item~ of information necessary accuretely to recoYer the timing SignRI.
From pattern recognition circuit 312, it receives the loc~lly-generated stream of first code pstterns P. Bec~use these patterns flre error-free, they include error-free informatlon on the exsct point during e~ch code pattern when the timing signal could occur (mid-p~ttern).
The only information missing is some identific~tion of which code psttern will ha~re the timing signal ~t its center, and this is provided by voting circuit 316. Pattern recognition circuit 312 therefore pr~
vides a one-bit-wide window, during esch cycle of first code p~ttern P, during which the timing signal m~y occur, given the correct state of 3~
voting circuit 316. 'rhe timing signal is output by reset circuit 318 in the one window which occurs during a code pattern when the voting circuit's output goes high.
The system described will regenerate accurately-timed timing sign~ls under poor sign~l ~onditions. Under even worse sign~2 condi-tions, the renge of operation of the system may be extended by introducing a regenerative circuit for replacing timing signals which have been missed or incorrectly decoded due to excessi~e noise. (This extension may be achieved only if the trsnsmitted timing signals are periodic.) In the preferred embodiment, the decoded timing signal from reset circuit 318 is used indirectly to reset system counters flS shown in Figure 3. System counters 322 operste continuously ~nd automatically reset to 0 periodically, the period being nominally the same as the period of the transmitted timing signals. When the system counters are reset to 0, they develop a locally-generated fram-ing signal on line 324 which is compared with the decoded timing signal on line 326. Comparison tskes place in hysteresis circuit 320, which counts the number of occasions when the local ~reming signal and the decoded timing signal do not coincide. ~hen the count reaches a predetermined value (5 in the preferred embodiment), the next decoded timing signal is used to reset the system counters. If the count is below the predetermined ~ralue, the decoded timing signal is not used to reset the system counters; they sre allowed to reset automatically. This arrangement causes undetected timing signals to be regenerated by the system counters and causes timing sign~ls which are occssionally incorrectly detected ~i.e., detected when no timing word W8S transmitted) to be ignored.
The key decryptor may use any aglorithm similar ~n structure to that disclosed in the Data Encryption Stflndard (FIPS Publicstion 46) of the National Bureau of Standsrds, which publicstion is hereby incorpor-ated into this applicstion by reference. The algorithm disclosed in the DES, however, is prefer~ble from a security st~ndpoint. Once the framing signal is received from detector 105, decryptor 103 ealcul~tes the time of occurrence of the active portion of line 3 of the VBI. It then looks ~t the information sppearing in the incoming Sign81 during the c~lculated time period and treats this informstion as the enerypted key, decryptlng it in ~ccord~nce with the DES (~nd the master key previously introduced into the receiYer). Ir framing is proper7 this information really will be the key, and the television signal will be able to be unscr&mbled. If framing has been lost, the portion of the trHnsmitted signal treated as the key will not provide the information needed to properly unscrumble the picture.
Figure ~ is a diagram showing the operation of burst gate generator and time base descrambler 109. The unscrambling key from decryptor 103 is used as the starting vector ~or pseudor~ndom number generator circuit 401. Circuit 401 produces (for the NTSC case) a sequence of 525 random numbers bssed on the key. These rsndom numbers are then combined, with information derived from line counter 403 which is incremented once e~ch line, in line-type selection circuit 405. This cir~uit selects which type of line ~e E~? shortened by a standnrd amount, shortened by twice the st~ndard amount, lengthened by either amount, or unchanged) is to ~ppear next9 and the information is fed to line length controller 407 which monitors the aggregate deviation ~n line lengths referenced to the st~rt of the current field and ensures that the ~ollowing two conditions are met:
1) The aggregate devi~tion does not exceed + 9us at ~ny time; and
RESTORlN ~ FRA MING
IN A COMMVNICATIONS SYSTEM
TECHNICAL FIELD
This invention is concerned with achieving ~nd maint~ining proper framing and synchronization in ~ communications system, especially in secure ~ommunications system. Proper frflming exists when slots in the received time sequence are correctly sssociated with the ~ppropri-ate positions in the transmitted time sequence; proper synchronizfltion implies th~t the trflnsmitter and receiver clocks are locked to each other (so th~t bit inteKrity is maintsined).
E~ACKGROUND OF THE INVENTION
, Various types of communications systems are of such a character that proper fr~ming is necess~ry to their operation~ For example~ in time-diYision multiplex transmission Or independent digital bit streflms, correct interpret~tion of the v~lue of the incoming signal ~s one member of the transmitted symbol set, and direction of the interpleted symbol to the correct receiver, require proper framing. Likewise, in television trsnsmissionl picture elements displ~yed at the receiver must be in the same relati~e positions ~s those appeflring st the trensmitter, ~nd, for & line-and-field scanned television signal (one in which the picture being tr~nsmitted is scsnned in fields of adjacent parallel lines), proper framJng is necessary to ~chieve this. Unless the dats frames ~qre especially short (for example, individusl ASCII characters~, correct synchronization is needed to msintAin reliable communication during the periods betweell successive fr~ming sJgnals.
In ordinary (non-secure) communicstions systems, fr~ming informa-tion often takes the form of a unique word, identific~tion of which by -~ 2 --the receiver est~blishes ~ p~rticul~r inst~nt as the dfltum from which time me~surements mQy be m~de. The s~me word occurs periodic~lly, st the ssme time in relation to esch frame of dat~ (for ex~mple, ~
television fr~me or ~ield)~ Since the fr~ming ~ord occurs at ~ con-stant frequency and is identical ror e~eh fr~me, fr~ming o~ the recei~ed intelligence is e~sily aeeomplished.
This technique 13 unsultable, however, for a secure communlcations system, sin~e ~n import~nt security teAture msy be the deni~l of framing information. ~r~ming denial can be accomplished by v~rying the time at which the intelligence occurs In relstion to the frRming word (scramblJng); but some mesns must then be found of del~vering this frsming inform~tion to the intended reclpient. If the Rddressee possesses the key to unscramMe the d~t~, no problem should occur. However, a further difficulty srises when sychroniz~tivn infor-mstion (such ~s ~ rererence clock burst) is scr~mbled ~long with the intelligence. Such inform~tlon is necess~ry to the frsming process because It keeps the clock ~rom drirtlng too ~ar during the relatiYely long perlods betw~n rrsmlng pulses.
-2a-SUMMARY OF THE INVENTION
It is an object of an aspect of the present invention to achieve and maintain proper framing in a communications system.
It is an object of an aspect of the invention to achieve and maintain proper framing in a secure communica-tions system.
An object of an aspect of the invention is to recover framing and synchronization information from a scrambled intelligence si gna I .
An object of an aspect of the invention is to recover such information from an intelligence signal which is scrambled in accordance with a key contained within the scrambled intelligence signal.
Various aspects of the invention are as follows:
-2b-ln ~ receiver ~or receiving ~n intelligence signal which includes a timing signsl and Q transmitted key beuring a predetermined time reletionship to the timing sign~l, the intelligence sign~l being scr~mbled in acco!-dance with the transmitted key ~nd being of such ~
charRcter thst proper framing is necesssry ror reception, an Apparatus fs)r detecting the loss of, and restoring, proper rr~ming, sRid ~pp~r~tus comprising:
framing signal generating mean~ receiving the scrambled intelligence sign~l snd respons~Ye to a framing loss signal for outputting as the fr~mlng signal the received timing sign~l and respon-sive to the ~bsence of t~e framing loss signal for outputtlng 8S the fr~ming sign~l a loc~lly generated framing signal, the Is~c811y gener~ted framing SignAI being in substantifll coincidence with the timing sign~l except when fr~ming is improper;
key detection means receivlng ihe scr~mbled intelligence signal and the fr~ming signal for outputting as a detected key a signal derived from that por~ion of the scrambled intelligence signal which bears the predetermined tlme relationship to the framlng signsl; and ~ raming loss detection means recelrlng the scrambled intel-ligence signal during a predetermined period dependent upon the detected key for detecting 1mproper ~reming and for generating the frflming loss signsl until proper framing i~ restored.
- 2c -In a receiver for receiving an intelligence sign~l which includes 8 timing signal ~nd a transmitted key bearing a predetermined time relationship to the timing sign~l, the intelligence signal being scrambled in Qccordance with the transrnitted key and being of such a character that proper framing is necess~ry for reception, ~ method of d~tecting thP loss l~f, ~nd rest~r~ng, proper framing, s~id method e~m-prising the steps Or:
receiving the scrambled intelligence signal;
determining whether fr~ming is proper by ex~mining a predetermined section o~ the scrambled intelligen~e signal;
identifying the predetermined section by reference to detected key;
deri~ring the detected key from the portion of the scram-bled intelligence signal which be~rs the predetermined time relationship to a selected framing sign~ nd selecting the framing sign~l based on the determination of whether framing - is proper, such thet 5~
-2d-i. if framing is proper, the selected rraming signsl is 8 locally gener~ted ~r~ming signal in substantial coincidence with the received timing sigri81; and ii. otherwise, the received timing signal is the selected frsming signal.
In a receiver for receiving an intelligence Sigl)Al which includes 8 timing signsl ~nd ~ transmltted key be~ring ~ predetermined t~me relationship to the timing sign~l, the intelligence sign~l being scr~mbled in accordance with the tr~nsmitted key and being of such a charscter th~t proper framing i3 necessary for reception, A method of detecting the loss of, snd restoring, proper framing, ssld method com-prising the steps of:
receiving the scrambled intelligence signal;
genersting 8 clock signal at ~ frequen~y determined by a selected reference signal;
determining whether framing is proper by ex~mining a predetermined sectlon of the scr~mbled intelligence signsl;
identifying the predetermined section by reference to detected key;
deri~lng the detected key from the portion Gf the scram-bled intelligence slgn~l which bears the predetermined time relationship to a selected framing signal;
selecting the framing Slgrl8l bssed on the deterrnin~tion of whether fr~ming is proper, such that i. if ~raming Is proper, the sele~ted fr~ming sign~l is locally gener~ted frmaing signal in substantial colncidencz with the recei ~red timing signal; And li. otherwise, the recei~ed timing sign~l is the selected framing signal; and selecting the reference signal based on the determinstion of whether framing is proper, such that 1~i6~
-2e-i. if fr~ming is proper, the selected reference signAI is the scrflmbled intelligence sign~l only during the predetermined period;
&nd ii. otherwise, the selected reference sigrlfll i5 the entire scr~mbled intelligence sign~l.
The communicat~ons system embodying the present inrention is one transmittlng broadcast teleYislon. In order to deny programming to unauthorized re~eivers (e ~ those who have not p~id ~or the service), the broadcaster scr~mbles the lines of the tele~rision picture, ~ ~
~LZ~ S~
varies their time of occurrence relstive $o each other or to a regulflrly occurring datum, such as a timing signal produced at the transmitter. Scrambling is done in ~ccordance with ~ key, ana the key is trflnsmitted along with the scrambled picture. The synchro-nization pulses which would ordinarily be transmitted in the blanking interv~ls 3re omitted (for security), ~nd these inter~r~ls ~re used to trsnsmit the key, the audio for the program, ~nd the timing signal (encoded QS a unigue word~ For further security, the key is encrypted tusing a master key previously placed in the possession of the recipi-ent). Although the sign~l is scrambled, the tr&nsmitted key is trans-mitted with a ~onstant time relationship to the timing signal; this enables the transmitted (encrypted) key to be recovered once the ti ming signsl is recovered.
The receiver for such 8 signal includes a number o~ components.
One, a framing signal generator, gener~tes ~ framing sign~l based on the received timing signal or on previously received timing signals (extrapolsted locslly using the receiver's clock). When the generflted framing signfll is IOCQ1lY gençr~ted, b~sed on extrapolation from previously received timing signals, it is in substantial coincidence with the transmitted timing siensl (except, of course, when framing is lost).
When the received timing signsl is output ns the framing signal, no extrapolstion is necessary.
Another component is the key detector, which looks for the transmitted key at ~ predetermined time before or after the framing sign~l. The key detector 81so ~ncludes decryption circuitry for decrypting the detected key in accordAnce with the prearr~nged master key.
A third component is a gating signal generator and time base descrflmbler. This receives the ~rsming signal and the decrypted key snd c~lcul~tes from the key the ~ppropri~te ~blay to place each line of the received picture in the proper time relationship to the others ~to descrsmble the picture). This compor,ent also generstes a gating signal for use by, aMcng other components, the receiver's phase-locked loop (clock). The geting signal opens the loop at the appropriate time to admit a small portion of the received sign~l, ordinarily a reference clock burst occurring during each horizont~l blanking interval.
An import~nt component is the frarning loss detector, which determines when proper fr~ming is l~lst ~nd which then c~uses cert~in changes in the operation of the other receiver components. The frflm-ing loss detector receives the same gating signal received by the phase-locked loop and exsmines the same portion of the received signal, the reference clock burst (although ~ny other sppropri~te portion of the received signal could inste~d be examined). The ~aming loss detector then determines, from the examined portion of the received signal, whether framing is proper~ If it is, no changes are made in the receiver's operation. If it is no$, two things happen. First, the phase-locked loop is opened to the entire received signal~ so the receiver's clock m~y be corrected, if necessary. (It is possible th~t framing W8S lost due to drift of the clock). Second, the framing sign~l generator is directed to generate framing signals based only on the received timing signals and not on historical information (~ ~ prior timing signals extr~polated uslng the receiver's clock), which may no longer be valid.
With the phase-locked loop open, the receiver's clock will be corrected (because clock information is transmitted with the television signsl). And with the framing signal generator looking only st the received timing signals and not Qt its own local extrapolation of the m, receiver framing will by brought into agreement with the transmitted signsl.
E~RIEF DESCRIPTION OF THE DRAWINGS
Figure 1 illustrates three lines of a television transmission which may be received In accordsnce with this invention.
Figure 2 is a diagrsm of the ~raming and synchronization compo-nents of a television receiver employing the present invention.
!Figure 3 is a diagram of the fr~ming word detector shown in Figure 2.
Figure 4 is a diagram illustrating the operation of the burst gate gener~tor And time base descrsmbler of Figllre 2.
Figure 5 is ~ diagrsm of the soft lock detector shown in Figure 2.
DETAILED DESCR~PTION OF THE PREFERRED EM80DIMENT
Figure 1 illustrates three lines of a television transmission which may be received in ffccordance with this invention. The lines illustrated ~re lines 1-3 of the vertical blanking interv~l (VBI).
Each line shown contflins certain information -- here re~erred to 8S "horizontal dats" -- in its horizontal blanking interval (HE~I), the first 10.9 us for an NTSC signsl. Preferably the horizontal dsta includes program audio in digital form and a short burst -- 10 cycles -- at the reference clock frequency~ The clock burst occurs At a fixed point during the HBI of lines within the VBI. The synchro-nizfltion and blanking levels which would normally fill the HBI are removed.
During the video field, that is, between VBrs, the HBrs contflin the same horizontal data but differ in Qn Importsnt respect from those illustrsted. I)uring the video field the length of each HBI, and the position of the reference burst within it, are varied by a pseudor~ndom amount from the standard. This is ~ccomplished by adding or removing digital audio samples at appropriate places in the line. To avoid long-term shortages or surpluses of transmitted information, which could mount quickly, the pseudorandom variation is prearr~nged to average out to zero over each complete field.
Also during the video field, the active portion of each line (the portion not occupied by horizontal d~ta) eontA3nS the separate an~logue components (luminanS~e and chroms) of the transmitted television line.
In the VBI, of course, there is no picture to be transmitted; flnd framing in~ormation may advar,tageously be transmitted during the 12 ~
active portion of each of the VBI lines. These lines (63.S6 us long in NTSC) are occupied by a number of digit~l symbols (455, in NTSC) representing various items of information necess~ry to flchieve ~nd maintain proper frflming. Simple division will show th~t, for a televi-sion tr~nsmission with the ~bove p~rameters (one intended for an NTSC
receiver), symbols occur at ~ r~te of 7.18 MHz, twice the csl~r subcsrrIer frequency.
Line 1 of the V81, in Its active portion, consists of ~ltern~ting zeroes and ones, or maxima and minima of the transmitted symbol set (if multilevel symbols are used). Bec~use of the r~te of symbol tr~ns-mission, line 1 thereîore contains a 52.6~ us continous wave at 3.58 MHz3 the color subcarrier frequency ~nd, prefer~bly, one frequency available from the receiver's clock. (When the term "receiver" is used, it includes not only the NTSC, PAL, SECAM or other st~nd~rd televi-sion receiver but also, more bro~dly, the decoder snd other terminal equipment necess~ry to convert fln incoming television sign~l to one of the standard form~ts, such as NTSC. The clock, therefore, is prefer-ably part of the decoder portion of the receiver.) Line 1 mAy be g~ted into the recei~er's phase-locked loop to drive it into synchronism with the transmitter's clock.
Line 2 Or the VBI, in its active portion, consists of the timing si~al encoded ~s a word, Along with other information (the FD section of the line) needed to identi~y the psrticul~r field in which the line is occurring. Beginning with symbol 79, a series of first code patterns P
is transmitted. The first code pattern P in the prefelred embodiment is the following set Or eight binsry digits: 111tOOOO. This first code pattern P is tr~nsmitted 41-1/2 times, for ~ totfll of 332 symbols.
After the first code pattern P h~s been trQnsmitted 41-1/2 times, the second code p~ttern ~ is transmitted twice, in e~ lNith P. Second code j~ttern Q, in the preferred embodiment, is the complement of first code patter P, th~t is, 00001111. Since an extra half cycle of P
was transmitted, the "in phflse" requirement me~ns thst transmissions of Q begin in the middle of the second code pattern Q (i.e., 1111).
(If an integral number of pAtterns P had been trensmitted, Q transmis-sions would begin at the beginning of Q -- i.e., 0000.~ The timing pu]se is therefore encoded ss the phase revessal occurring between the last transmission of first code pattern P and the first transmission of the second code pettern Q. After the second tr~nsmission of second code pattern Ç~, a field identification waveform is trsnsmitted. This waveforrn serves to identify which of 16 fields is being transmitted.
(It msy be used to synchronize encryption elements of the system.) Line 3, in its uctive portion, contains the (hesvily error-corrected and prefersbly encrypted) key necessary to unscramble the pseudorandom variations in the position of the ten-cycle reference clock burst occurring during the HBI of esch line of the video field.
The key, which is updated once A frame, is used as the starting vector for a pseudorandom number generator circuit.
Figure 2 is a diagr&m of the frsming and sychronization compo nents of the receiver (or, as mentioned ~bove9 the decoder). The scr~mbled television signal, scrsmbled in accordance with the key transmieted in line 3 of the VB}, arrives on line 101 and is delivered to key dectyptor 103, framing word detector 105, and ph~se-locked loop 107. Detector 105 always receives the entire composite television signal so that it may always detect the transmitted timing signRI, although it will usually output ~ locally-generated ~raming signal inste~d of using the received timing signal as the framing signal.
Phss~locked loop lC7 recei~res the composite telerision signal during periods determined by burst g~te generator 109 ~s modified by soft lock detector 111). When fr~ming and synchronization are correct (hsrd lock condition), these periods are the periods of the ten-cycle reference burst in the HBI of every line. Key decryptor 103 receives the entire composite television signal so that it m~y detect the unscrsmbling key on line 3 of the VBI by computing the Amount of time which elapses sfter framing word detector 105 outputs the frarning signsl. The clock signal output from phas~locked loop 107 is delivered as necesssry to the receiver components.
Burst gate generator 109 requires both the framing signal (from detector 105) and the decrypted key (from decryptor 103) in order to determine the time of occurrence of the refer~nce clock bursts in the HBl's of the composite teleYision s~gnal. Phase-locked loop 107 is ordinarily vpened only during these periods for clvck correction. Also during these burst periods, soft lock detector 111 is permitted to receive the cornposite television signal in order to determine whether the hard lock condition is being maintAined. This is accomplished by integrating the received signal. Since the clock burst is 8 series of alternating 0 ~nd 1 symbols, after removQl of the DC component its average value should be zero~ If, upon integration by soft lock detector 111, the portion of the composite television signal received during the period of the gsting Si~lQI produced by generator 109 has a value of zero, framing is proper. If not, framing is improper; and so~t lock detector csuses the receiver to enter the "soft lock'~
condition by outputting a soft lock signal to both framing word detector 105 and phase-locked loop 107. The soft lock signal opens loop 107 to receive the entire composite television signAI, including the clock signal 3n line 1 of the Vl~l~ It also modifies the hysterisis algorithm of detector 105 so that lo~lly~enerated framing signals are ignored, and tlte framing signal output from detector 105 is the received timing signal.
- Figure 3 is a disgrAm of framing word detector 105. The composite television signal (including the encoded timing pulse), fllthough originating as a digitsl signal, is transmitted by radio in analog form. It is first filtered and amplified by, respectively, high pass filter 304 and amplifier 306. The resulting signal is next apptied to input terminal 30Q.
Digital filter 30~, an 8th~ order digital bandp~ss filter tuned to 895 kHz, is a recursive filter. (The filter frequency, 895 kHz, is the frequency at which the code patterns P and Q occur (symbol frequency 7.16 mHz divided by eight symbols per code pffttern.) The poles of filter 308 lie ex~ctly on the unit circle in the z-plane, resulting in an extremely narrow bandwidth and, hence, excellent noise reJection. Sta-bility of the filter is m~intQined by periodic~lly cleflring its stor~ge elements by means of input control logic circuit 309.
When the 895 kHz code patterns enter digital filter 3û8~ its output tends to increase. After the first code pattern P has been applied to digit&l filter 308 for a period of time between 28 us and 4~ us (depending on the le~el of noise) the output of the filter will ha~e risen to a high enough leYel to trigger threshold detector 310 and activate pQttern recognition circuit 312. As shown in Figure 3, the threshold for detecting ~ "1" at the output of digital filter 308 is a value greater than or equal to 25 "1"s. The threshold value for detecting a "0" at the output of digital filter 308 is a value less than or equal to 7. In 28 us digitel filter 308 will h~ve received 25 first code patterns P; therefore, in the absence of noise, the accumulated value for each of the four '11" positions of first code pattern P will have reached 25. In 46 us, all 41-1/2 repetitions of the first code pattern P will hsve been received.
Pattern recognition circuit 312 performs two functions. First, it examines the outputs of threshold detector 310 to determine if they were produced 8s the result of the application of an 895 kHz signal at the ir~put of the digital filter. This is simply a matter of determining whether eight sam21es of the "greater thann/"less than" signal (out of threshold detector 310) satisfy the rollowing two criteria:
a. There must be exactly four "greater than" and four "less than"; and b. There must be e3ther four "greater than" in 8 row or four "less th~n" in a row.
Once it has determined that the 895 kHz signal was present, pattern recognition circuit 312 begins locally generating its own version of the 5.~
895 kHz sigrt~ 9 that is, a series of first code patterns P. These patterns are input to exclusive-OR gate 314 along with the incoming signal from input terminsl 300 in order to lo~te the phase revers~l (see Figure 1) in the encoded synchronizetion signal.
When the ph~se reversal occurs, the output of e~clusive-OR
gate 314 wi~ ch~nge from "0" to "1". The output of exclusive-OR
g~te 314 driYes serial 12-of-16 ~roting circuit 316, whose output is active high whenever 12 of the last 16 input samples were "l"s. In the absence of noise, the point ~t which the voting circuit's output becomes active high is fixed with respect to the composite television signal (if the original synchronization signal itself is fixed, ~s it is in the preferred embodiment). However, due to noise which introduces errors in the phsse reversed section of the encoded timing word, the point at which the voting circuit's output becomes acti~re high will not necessarily be fixed.
This situation is remedied by re-synchronizing the output of voting circuit 316 with the locelly generated first code p~ttern P from psttern recognition circuit 312. This re-synchronizstion takes place in reset circuit 318. ~Without reset circuit 318, the output cf voting circuit 316 wouid be taken as the decoded timing- signal. In the pre-ferred embodiment, however, the decoded timlng signal is taken from the output o~ reset circuit 318.) Reset circuit 318 combines two item~ of information necessary accuretely to recoYer the timing SignRI.
From pattern recognition circuit 312, it receives the loc~lly-generated stream of first code pstterns P. Bec~use these patterns flre error-free, they include error-free informatlon on the exsct point during e~ch code pattern when the timing signal could occur (mid-p~ttern).
The only information missing is some identific~tion of which code psttern will ha~re the timing signal ~t its center, and this is provided by voting circuit 316. Pattern recognition circuit 312 therefore pr~
vides a one-bit-wide window, during esch cycle of first code p~ttern P, during which the timing signal m~y occur, given the correct state of 3~
voting circuit 316. 'rhe timing signal is output by reset circuit 318 in the one window which occurs during a code pattern when the voting circuit's output goes high.
The system described will regenerate accurately-timed timing sign~ls under poor sign~l ~onditions. Under even worse sign~2 condi-tions, the renge of operation of the system may be extended by introducing a regenerative circuit for replacing timing signals which have been missed or incorrectly decoded due to excessi~e noise. (This extension may be achieved only if the trsnsmitted timing signals are periodic.) In the preferred embodiment, the decoded timing signal from reset circuit 318 is used indirectly to reset system counters flS shown in Figure 3. System counters 322 operste continuously ~nd automatically reset to 0 periodically, the period being nominally the same as the period of the transmitted timing signals. When the system counters are reset to 0, they develop a locally-generated fram-ing signal on line 324 which is compared with the decoded timing signal on line 326. Comparison tskes place in hysteresis circuit 320, which counts the number of occasions when the local ~reming signal and the decoded timing signal do not coincide. ~hen the count reaches a predetermined value (5 in the preferred embodiment), the next decoded timing signal is used to reset the system counters. If the count is below the predetermined ~ralue, the decoded timing signal is not used to reset the system counters; they sre allowed to reset automatically. This arrangement causes undetected timing signals to be regenerated by the system counters and causes timing sign~ls which are occssionally incorrectly detected ~i.e., detected when no timing word W8S transmitted) to be ignored.
The key decryptor may use any aglorithm similar ~n structure to that disclosed in the Data Encryption Stflndard (FIPS Publicstion 46) of the National Bureau of Standsrds, which publicstion is hereby incorpor-ated into this applicstion by reference. The algorithm disclosed in the DES, however, is prefer~ble from a security st~ndpoint. Once the framing signal is received from detector 105, decryptor 103 ealcul~tes the time of occurrence of the active portion of line 3 of the VBI. It then looks ~t the information sppearing in the incoming Sign81 during the c~lculated time period and treats this informstion as the enerypted key, decryptlng it in ~ccord~nce with the DES (~nd the master key previously introduced into the receiYer). Ir framing is proper7 this information really will be the key, and the television signal will be able to be unscr&mbled. If framing has been lost, the portion of the trHnsmitted signal treated as the key will not provide the information needed to properly unscrumble the picture.
Figure ~ is a diagram showing the operation of burst gate generator and time base descrambler 109. The unscrambling key from decryptor 103 is used as the starting vector ~or pseudor~ndom number generator circuit 401. Circuit 401 produces (for the NTSC case) a sequence of 525 random numbers bssed on the key. These rsndom numbers are then combined, with information derived from line counter 403 which is incremented once e~ch line, in line-type selection circuit 405. This cir~uit selects which type of line ~e E~? shortened by a standnrd amount, shortened by twice the st~ndard amount, lengthened by either amount, or unchanged) is to ~ppear next9 and the information is fed to line length controller 407 which monitors the aggregate deviation ~n line lengths referenced to the st~rt of the current field and ensures that the ~ollowing two conditions are met:
1) The aggregate devi~tion does not exceed + 9us at ~ny time; and
2) The aggregate deviation at the end of the field must be zero.
The line length controller then provides information to horizontal counter and decoder 409, enflbling counter/decoder ~0~ to produce the correet line store control signals for the current line snd to c~lcul~te the location in each HBI of the reference clock burst and output the burst gates 8t the appropriate times.
5~) Figure 5 is a block diagram showing details of soft lock detector 111. The framing pulse frsm fr~ming word detector 105 arrives on line 501 and is used to reset both local subc~rrier regenerator 503 Rnd th~ counters in counter/decoder circuit 505.
Circuit 505 produces ~n output which is active during lines 22 through 42 of every tield. This output is gated with the burst gate signal ~rriving on line 507, in AND gAte 509, to produce a pulse which is ~ctive only during the burst on field lines 22 through 42. This signal is then used to en~ble miss counter 511 whenever the locslly gener~ted subcsrrier (from generator 503) does not mRtch the composite television signal (arriving on line 513). When the output from miss counter 511 exceeds ~ preset threshold, the output of threshold detector 515 becomes active~ sending the soft lock signal to fræming word detector 105 and phase-locked loop 1û7.
Although illustrative embodiments of the present inYention l~ve been described in det~il with reference to the accompanying draYvings, it is to be understood that the invention is not limited to those precise embodiments, and th~t various changes ~nd modific~tions m~y be effected therein by one skil~ed in the ~rt without dep~rting from the scope or spirit of the invention.
The line length controller then provides information to horizontal counter and decoder 409, enflbling counter/decoder ~0~ to produce the correet line store control signals for the current line snd to c~lcul~te the location in each HBI of the reference clock burst and output the burst gates 8t the appropriate times.
5~) Figure 5 is a block diagram showing details of soft lock detector 111. The framing pulse frsm fr~ming word detector 105 arrives on line 501 and is used to reset both local subc~rrier regenerator 503 Rnd th~ counters in counter/decoder circuit 505.
Circuit 505 produces ~n output which is active during lines 22 through 42 of every tield. This output is gated with the burst gate signal ~rriving on line 507, in AND gAte 509, to produce a pulse which is ~ctive only during the burst on field lines 22 through 42. This signal is then used to en~ble miss counter 511 whenever the locslly gener~ted subcsrrier (from generator 503) does not mRtch the composite television signal (arriving on line 513). When the output from miss counter 511 exceeds ~ preset threshold, the output of threshold detector 515 becomes active~ sending the soft lock signal to fræming word detector 105 and phase-locked loop 1û7.
Although illustrative embodiments of the present inYention l~ve been described in det~il with reference to the accompanying draYvings, it is to be understood that the invention is not limited to those precise embodiments, and th~t various changes ~nd modific~tions m~y be effected therein by one skil~ed in the ~rt without dep~rting from the scope or spirit of the invention.
Claims (8)
1. In a receiver for receiving an intelligence signal which includes a timing signal and a transmitted key bearing a predetermined time relationship to the timing signal, the intelligence signal being scrambled in accordance with the transmitted key and being of such a character that proper framing is necessary for reception, an apparatus for detecting the loss of, and restoring, proper framing, said apparatus comprising:
framing signal generating means receiving the scrambled intelligence signal and responsive to a framing loss signal for outputting as the framing signal the received timing signal and respon-sive to the absence of the framing loss signal for outputting as the framing signal a locally generated framing signal, the locally generated framing signal being in substantial coincidence with the timing signal except when framing is improper;
key detection means receiving the scrambled intelligence signal and the framing signal for outputting as a detected key a signal derived from that portion of the scrambled intelligence signal which bears the predetermined time relationship to the framing signal; and framing loss detection means receiving the scrambled intel-ligence signal during a predetermined period dependent upon the detected key for detecting improper framing and for generating the framing loss signal until proper framing is restored.
framing signal generating means receiving the scrambled intelligence signal and responsive to a framing loss signal for outputting as the framing signal the received timing signal and respon-sive to the absence of the framing loss signal for outputting as the framing signal a locally generated framing signal, the locally generated framing signal being in substantial coincidence with the timing signal except when framing is improper;
key detection means receiving the scrambled intelligence signal and the framing signal for outputting as a detected key a signal derived from that portion of the scrambled intelligence signal which bears the predetermined time relationship to the framing signal; and framing loss detection means receiving the scrambled intel-ligence signal during a predetermined period dependent upon the detected key for detecting improper framing and for generating the framing loss signal until proper framing is restored.
2. The apparatus of claim 1 further comprising gating signal generating means receiving the framing signal and the detected key for generating a gating signal during the predetermined period, said framing loss detection means receiving the gating signal
3. The apparatus of claim 1 further comprising clock generator means for generating a clock signal in phase with an input signal, said clock generator means being responsive to the framing loss signal to receive as the input signal the entire scrambled intelligence signal and being responsive to the absence of the framing loss signal to receive as the input signal the scrambled intelligence signal only during the predetermined period, said clock signal being received by said framing signal generating means, said key detection means and said framing loss detection means.
4. The apparatus of claim 1 wherein the transmitted key is encrypted, said key detection means including decryption means for decrypting that portion of the scrambled intelligence signal which bears the predetermined time relationship to the framing signal.
5. The apparatus of claim 1 wherein said framing signal generating means comprises a hysteresis circuit to output the received timing signal as the framing signal in response to the framing loss signal only when a predetermined number of consecutive locally generated framing signals are not in substantial coincidence with timing signals.
6. In a receiver for receiving an intelligence signal which includes a timing signal and a transmitted key bearing a predetermined time relationship to the timing signal, the intelligence signal being scrambled in accordance with the transmitted key and being of such a character that proper framing is necessary for reception, a method of detecting the loss of, and restoring, proper framing, said method com-prising the steps of:
receiving the scrambled intelligence signal;
determining whether framing is proper by examining a predetermined section of the scrambled intelligence signal;
identifying the predetermined section by reference to a detected key;
deriving the detected key from the portion of the scram-bled intelligence signal which bears the predetermined time relationship to a selected framing signal; and selecting the framing signal based on the determination of whether framing is proper, such that i. if framing is proper, the selected framing signal is a locally generated framing signal in substantial coincidence with the received timing signal; and ii. otherwise, the received timing signal is the selected framing signal.
receiving the scrambled intelligence signal;
determining whether framing is proper by examining a predetermined section of the scrambled intelligence signal;
identifying the predetermined section by reference to a detected key;
deriving the detected key from the portion of the scram-bled intelligence signal which bears the predetermined time relationship to a selected framing signal; and selecting the framing signal based on the determination of whether framing is proper, such that i. if framing is proper, the selected framing signal is a locally generated framing signal in substantial coincidence with the received timing signal; and ii. otherwise, the received timing signal is the selected framing signal.
7. In a receiver for receiving an intelligence signal which includes a timing signal and a transmitted key bearing a predetermined time relationship to the timing signal, the intelligence signal being scrambled in accordance with the transmitted key and being of such a character that proper framing is necessary for reception, a method of detecting the loss of, and restoring, proper framing, said method com-prising the steps of:
receiving the scrambled intelligence signal;
generating a clock signal at a frequency determined by a selected reference signal;
determining whether framing is proper by examining a predetermined section of the scrambled intelligence signal;
identifying the predetermined section by reference to a detected key;
deriving the detected key from the portion of the scram-bled intelligence signal which bears the predetermined time relationship to a selected framing signal:
selecting the framing signal based on the determination of whether framing is proper, such that i. if framing is proper, the selected framing signal is locally generated framing signal in substantial coincidence with the received timing signal; and ii. otherwise, the received timing signal is the selected framing signal; and selecting the reference signal based on the determination of whether framing is proper, such that i. if framing is proper, the selected reference signal is the scrambled intelligence signal only during the predetermined period;
and ii. otherwise, the selected reference signal is the entire scrambled intelligence signal.
receiving the scrambled intelligence signal;
generating a clock signal at a frequency determined by a selected reference signal;
determining whether framing is proper by examining a predetermined section of the scrambled intelligence signal;
identifying the predetermined section by reference to a detected key;
deriving the detected key from the portion of the scram-bled intelligence signal which bears the predetermined time relationship to a selected framing signal:
selecting the framing signal based on the determination of whether framing is proper, such that i. if framing is proper, the selected framing signal is locally generated framing signal in substantial coincidence with the received timing signal; and ii. otherwise, the received timing signal is the selected framing signal; and selecting the reference signal based on the determination of whether framing is proper, such that i. if framing is proper, the selected reference signal is the scrambled intelligence signal only during the predetermined period;
and ii. otherwise, the selected reference signal is the entire scrambled intelligence signal.
8. The method of claim 6 wherein the transmitted key is encrypted and said step of deriving the detected key comprises decrypting that portion of the scrambled intelligence signal which bears the predetermined time relationship to the selected framing signal.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US06/736,305 US4817142A (en) | 1985-05-21 | 1985-05-21 | Restoring framing in a communications system |
| US736,305 | 1985-05-21 |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA1266520C CA1266520C (en) | 1990-03-06 |
| CA1266520A true CA1266520A (en) | 1990-03-06 |
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ID=24959365
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| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000509558A Expired - Lifetime CA1266520A (en) | 1985-05-21 | 1986-05-20 | Restoring framing in a communications system |
Country Status (12)
| Country | Link |
|---|---|
| US (1) | US4817142A (en) |
| EP (1) | EP0221930B1 (en) |
| JP (1) | JPS63501113A (en) |
| CN (1) | CN1011655B (en) |
| AU (1) | AU600899B2 (en) |
| CA (1) | CA1266520A (en) |
| DE (1) | DE3689627T2 (en) |
| DK (1) | DK169192B1 (en) |
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| MX (1) | MX166007B (en) |
| WO (1) | WO1986007225A1 (en) |
| ZA (1) | ZA863231B (en) |
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| JPH04115790A (en) * | 1990-09-05 | 1992-04-16 | Sharp Corp | Synchronizing circuit |
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| MA23381A1 (en) * | 1993-12-03 | 1995-07-01 | Scientific Atlanta | MULTI-SERVICE DATA RECEIVER ARCHITECTURE |
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| US20060023874A1 (en) * | 2004-07-29 | 2006-02-02 | International Business Machines Corporation | Method and multiline scrambled clock architecture with random state selection for implementing lower electromagnetic emissions |
| US8971305B2 (en) * | 2007-06-05 | 2015-03-03 | Qualcomm Incorporated | Pseudo-random sequence mapping in wireless communications |
| US8358782B2 (en) * | 2009-06-26 | 2013-01-22 | Eastman Kodak Company | Method for displaying a video of a scene |
| US10514719B2 (en) | 2017-06-27 | 2019-12-24 | Biosense Webster (Israel) Ltd. | System and method for synchronization among clocks in a wireless system |
| CN112690007B (en) * | 2019-01-16 | 2022-04-05 | 华为技术有限公司 | Method and equipment for converting burst signal into continuous signal |
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-
1985
- 1985-05-21 US US06/736,305 patent/US4817142A/en not_active Expired - Lifetime
-
1986
- 1986-04-21 WO PCT/US1986/000824 patent/WO1986007225A1/en active IP Right Grant
- 1986-04-21 AU AU57715/86A patent/AU600899B2/en not_active Ceased
- 1986-04-21 JP JP61502391A patent/JPS63501113A/en active Pending
- 1986-04-21 DE DE3689627T patent/DE3689627T2/en not_active Expired - Fee Related
- 1986-04-21 EP EP86902747A patent/EP0221930B1/en not_active Expired - Lifetime
- 1986-04-30 ZA ZA863231A patent/ZA863231B/en unknown
- 1986-05-13 MX MX002468A patent/MX166007B/en unknown
- 1986-05-20 CA CA000509558A patent/CA1266520A/en not_active Expired - Lifetime
- 1986-05-21 CN CN86103445A patent/CN1011655B/en not_active Expired
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1987
- 1987-01-19 DK DK027087A patent/DK169192B1/en not_active IP Right Cessation
- 1987-01-21 FI FI870236A patent/FI85206C/en not_active IP Right Cessation
Also Published As
| Publication number | Publication date |
|---|---|
| FI85206C (en) | 1992-03-10 |
| AU5771586A (en) | 1986-12-24 |
| DK169192B1 (en) | 1994-09-05 |
| CA1266520C (en) | 1990-03-06 |
| CN86103445A (en) | 1986-12-03 |
| DK27087A (en) | 1987-01-19 |
| WO1986007225A1 (en) | 1986-12-04 |
| EP0221930A1 (en) | 1987-05-20 |
| FI870236A0 (en) | 1987-01-21 |
| MX166007B (en) | 1992-12-16 |
| EP0221930B1 (en) | 1994-02-09 |
| DE3689627D1 (en) | 1994-03-24 |
| EP0221930A4 (en) | 1989-09-11 |
| CN1011655B (en) | 1991-02-13 |
| FI870236A (en) | 1987-01-21 |
| DK27087D0 (en) | 1987-01-19 |
| DE3689627T2 (en) | 1994-09-15 |
| ZA863231B (en) | 1986-12-30 |
| FI85206B (en) | 1991-11-29 |
| US4817142A (en) | 1989-03-28 |
| AU600899B2 (en) | 1990-08-30 |
| JPS63501113A (en) | 1988-04-21 |
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| MKLA | Lapsed | ||
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Effective date: 19990308 |