CA1243359A - Adaptive expanders for fm stereophonic broadcasting system utilizing companding of difference signal - Google Patents

Adaptive expanders for fm stereophonic broadcasting system utilizing companding of difference signal

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Publication number
CA1243359A
CA1243359A CA000497176A CA497176A CA1243359A CA 1243359 A CA1243359 A CA 1243359A CA 000497176 A CA000497176 A CA 000497176A CA 497176 A CA497176 A CA 497176A CA 1243359 A CA1243359 A CA 1243359A
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CA
Canada
Prior art keywords
signal
difference signal
difference
variable gain
output
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
Application number
CA000497176A
Other languages
French (fr)
Inventor
Aldo G. Cugnini
Daniel W. Gravereaux
David W. Stebbings
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BROADCAST Tech PARTNERS
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BROADCAST Tech PARTNERS
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Publication of CA1243359A publication Critical patent/CA1243359A/en
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Classifications

    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04HBROADCAST COMMUNICATION
    • H04H20/00Arrangements for broadcast or for distribution combined with broadcast
    • H04H20/44Arrangements characterised by circuits or components specially adapted for broadcast
    • H04H20/46Arrangements characterised by circuits or components specially adapted for broadcast specially adapted for broadcast systems covered by groups H04H20/53-H04H20/95
    • H04H20/47Arrangements characterised by circuits or components specially adapted for broadcast specially adapted for broadcast systems covered by groups H04H20/53-H04H20/95 specially adapted for stereophonic broadcast systems
    • H04H20/48Arrangements characterised by circuits or components specially adapted for broadcast specially adapted for broadcast systems covered by groups H04H20/53-H04H20/95 specially adapted for stereophonic broadcast systems for FM stereophonic broadcast systems
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04BTRANSMISSION
    • H04B1/00Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
    • H04B1/06Receivers
    • H04B1/16Circuits
    • H04B1/1646Circuits adapted for the reception of stereophonic signals
    • H04B1/1692Circuits adapted for the reception of stereophonic signals using companding of the stereo difference signal, e.g. FMX

Abstract

ABSTRACT

A stereophonic broadcasting system incorporating companding of the difference signal in which both the usu-al difference signal and a compressed version of the dif-ference signal are transmitted to one or more remote re-ceivers. At the receiver the usual, unchanged, difference signal is used as a reference signal for controlling the expansion of the compressed difference signal so as to cause the amplitude of the companded difference to equal the level of the usual difference signal. Thus, the ex-pander is adaptive to any compression characteristic that might be employed at the transmitter. Further, the avail-ability of the usual uncompressed difference signal at the receiver enables the adaptive decoding of dynamic parame-ters of the received signal, such as frequency response and attack and recovery times, so that all of the parame-ters of the original signal can be restored automatical-ly.

Description

~33~

ADAPTIVE EXPANDERS FOR FM STEREOPHONIC BROADCASTING
S~STEM UTILIZING COMPANDING OF DIFFERENCE S_GNAL

BACKGROUND OF THE INVENTION

This invention relates to FM stereophonic ~road-casting systems and, more particularly, to adaptive ex-panders for FM stereophonic broadcasting systems utilizing companding of the difference signal.
It i5 known from U.S. Pat. No. 4,485,483 of Emil L~ Torick and Thomas B. Keller, to compand the difference signal in a modulated channel that is in quad-rature with the c~annel normally used for stereo in such a way as not to increase thebandwidth requirements Eor transmission. In the system disclosed in tniS patent lQ (hereinafter sometimes referred to as the "ToricX/Keller system"2 the usual left and rignt signals are convention-ally matrixed to obtain conventional sum ~MI and differ-ence (Sl signals. The dtfference signal is used to ampli-tude-modulate a first su~-carrier signal and at the same time is applIed to a compressor ~hich compresses its dy namic range in accordance with a given law to produce a co~pressed difference signal S~. The compressed differ-ence signal S' is used to amplitude-modulate a second sub-carrier signal, preferably of the same frequency but in quadrature phase relationship with the first. Suppressed-carrier, dou~le-sideband modulation of each sub-carrier is employed, with the frequency of the sub-carrier signal be-ing suffictently hign to assure a frequency gap between the lo~er sidebands of the modulated sub-carrier signals and the M signal. A conventional low-level pnase refer-~3~

ence pilot signal of a frequency lyin~ with~n the frequen-cy gap is employed for detection purposes at the receiv-er. The M signal, the two modulated sub-carrier signals, and the pilot are modulated onto a high-frequency carrier Eor transmission purposes. The recei~er includes a demod-ulator for deriving the M signal, the normal difference signal S and the compressed difEerence signal S', and an expander for complementarily expanding the derived com-pressed difference signal. The expanded noise-reduced version of the d~ference signal is combined with the de-rived sum signal M to obtain the original left ~1) and right (RI signals. The receiver also includes switch means for applying the normal dif~erence signal S, instead of the expanded version of the derived difference signal, to the com~ining means for enabling the receiver for re-production of conventional stereophonic signals. Compand-ing of the difference signal S gives 22dB to 26dB signal-to-noise improvement in the transmission and enables the stereo listener to en~oy the same signal-to-noise ratio as does t~e conventional monophonic listener. This amount of reduction of received noise gxeatly increases the efec-tive stereo service area by improving the quality of the signal recei~ed ~y listeners located with~n the service area.
~5 Commonly assigned patent application Ser.
No~ 4~7,163 filed concurrently herewith ~y applicant David W. Ste~ings, discloses an improved FM
stereo system that is similar to the Torick~Keller system in that the usual diference signal S and a compressed 3Q difference signal S~ are ~oth transmitted. However, un like the Torick~Keller system, in which only the expanded 3~

version of -the received compressed difference signal is matrixed with the sum signal M to obtain the original L
and R signals, the receiver according to the Stebbings disclosure matrixes a noise-reduced difference signal, de-rived by combining and expanding the received differencesignals S and S', with the derived sum signal M to obtain the original L and R signals. Since the signal content of the signals S and S' is the same, by combining them the effective level of the received difference signal is in-creased by 6dB, whereas the noise is increased by only3dB, resulting in a net 3db improvement in signal-to-noise ratio.
The transmission of the uncompressed difference signal S (necessary for compatibility with existing sys-tems), coupled with the utilization at the receiver ofboth the uncompressed and the compressed difference sig-nals, allows any desired companding law to be used in the compressor for the difference signal. For example, in-stead of being limited to the 2-to-1 slope typical of the "CX" companding system described in commonly assigned Pat. No. 4,376,916, or to the characteristics of other known companding systems, it is possible to use a ccmpres-sor having an infinity-to-one or similar companding char-ac-teristic which provides a subjectively assessed 1OdB to 12dB improvement against program modulated noise for a given amount of noise reduction over that realizable with such prior art companders.
A primary object of the present invention is to pro-vide an improved FM stereophonic broadcasting system which better utilizes the greater channel capacity of the Torick/Keller system while still realizing its improved signal-to-noise advantage.

Another object of the invention is to provide an adaptive expander for use in khe receiver having the capa-bility o-E adapting to any compressor characteristic that may be employed at the transmitter.
Yet another object is to provide an expander for use in an FM stereophonic system that is capable of adap-tive frequency response decoding and adaptive attack ancl recovery time decoding so that all parameters of the orig-inal signal can be restored automatically regardless of the compression law used in transmission.

:

~3~
~5- C-153 1 -A

SUM~RY OF THE IN~IENTION

The FM stereophonic broadcasting system of the present invention utilizes the Torick/Keller concept of transmitting both the usual difference signal S and a com-pressed difference signal S' and the Stebbings concept of 5 combining the usual difference signal and the expanded version of the compressed difference signal at the receiv-er -to achieve greater noise reduction and improved signal-to-noise ratio. The present invention is based on the realization that because the normal difference signal is transmitted completely unchanged (for compatibility with existing receivers) i-t can be used at the receiver as a reference signal for controlling the expander so as to cause the amplitude of the companded difference signal to equal the level of the normal difference signal and thus 15 insure proper dematri~iny with the received sum signal M.
This feature offers the important advantage of the expand-er being adaptive to any compressor characteristic tha-t might be employed at the transmitter, from which it fol-lows that should companding characteristics other than those initially selected for use at the transmitter be de-veloped in the future, it would not be necessary to re-place existing receivers to explolt them; rather, it would be necessary only to change the transmitter to incorporate a new compressor and the expander would adapt to it.
Similarly, the uncompressed dif:Eerence signal available at the receiver allows adaptive decoding o:E dy-namic parameters of the received signal, such as fre~uency response and attack and recovery time constants, so that all of the parameters of the original s.ignal can be re-stored automatically whatever transmission system is used.

5~

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features, and advantages of the inv ention, and a better understanding of its construction and operation, will be had from the following detailed des-cription when considered in conjunction with the accomp-anying drawings, in which:
FIG. 1 is a frequency diagram o:E the composite baseband signal developed in accordance with the princi-ples of the present invention;
FIG. 2 is a si.mplified block diagram oE a trans-mitting terminal for generating and transmitting the com-posite signal of FIG. 1;
FIG. 3 shows the steady state compression and ex-pansion characteristics of three different companding laws to which the expander of the invention is capable of adap-ting;
FIG. 4 shows the steady state compression and ex-pansion characteristics of another companding law useful in the system;
FIG~ 5 is a simplified block diagram of a receiving terminal including the adaptive expander of the inven-tion;
FIG. 6 is a simplified block diagram of an alterna-tive form of adaptive expander embodying the invention;
FIG. 7 is a simplified block diagram of a modified version of the adaptive expander shown in FIG. 6;
FIG. 8 is a block cliagram of a feecl-forward type of adaptive expander embodying the principles of the inven-tion;
FIG. 9 is a simplified block diagram of a parallel-type frequency correcting adaptive expander;

3~

FIG. 10 is a simplified block diagram of a series-type frequency correcting adaptive compander; and FIG. 11 is a simpllfied block diagram of an adap~
tive expander having automatic adaptive time constantsO

3~

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

In common with the Torick/Keller system and the system disclosed in the aforementioned Stebbings applica-tion, in the transmitter of the present system a com-pressed difference signal (hereinafter designated S') is added to the conventional composite FM signal utilized in the existing two-channel stereo system approved by the FCC. The compressed difference signal is conveniently transmitted as a double-sideband suppressed 38kHz quadra-ture sub-carrier signal S'cos~t. Thus, the composite0 baseband signal may be represented by the equation:
em = M ~ psin~t + Ssin~t ~ S'cos~t Eq.(1) where p represents the amplitude of the pilot signal and ~ = 2~o38k~z~ As is evident from the frequency spectrum of the composite signal illustrated in FIG. 1, the quadrature sub-carrier requires no additional spectrum space and, as will be seen, imposes no penalty in modulation potential because of the adaptive quality of the expander.
A transmitter for generating this composite signal is illustrated in FIG. 2 which, in the interest of simpli-city, omits some of the more conventional transmitter cir-cuits. Two audio frequency signals L and R, derived from a stereo source (not shown), are applied via usual 75 ~sec pre-emphasis networks 6 and 8 respectively, to the inputs of a conventional matrix network 10 consisting, for exam-ple, of a network of summing amplifiers arranged to pro-duce at the output terminals of the matrix the two audio signals M = (1, ~ R) and S = (L - R). The monophonic sum signal M is applied via a first delay device 11 to one in-~L2~3~o~

put o:E an adder 12 r and the stereophonic difference sig-nal S is applied via a second delay device 13 to the input of a first modulator 14, and also to the input of a com-pressor 16 of a noise-reducing companding system; the com-pressor may be any of several types which will be des-cribed later. The compressed difference signal~ designat-ed S', is appl.ied to the input o:E a second modulator 1 8 r the output of which is delivered to adder 12 where it is linearly combined with the monophonic sum signal M and the siynal from modulator 14. The delays introduced by delay devices 11 and 13 are such as to insure that the M signal and the two modulated signals arrive at the adder simul-taneously~
The sub-carrier and pilot signals are derived from a carrier generator 20 which provides a sine wave signal having a frequency of 38kHz which is applied to modulator 14 and also to a phase shift network 22 of known construc-tion for providing a 90 phase displacement between the sub-carrier signal applied to modulator 18 and the sub-carrier applied to modulator 1~. Modulators 14 and 18 are suppressed-carrier amplitude-modulators of known construc-tion and serve to amplitude-modulate the two sub-carriers with respective audio frequency signals so as to produce the two double-sideband, suppressed-carrier, amplitude-modulated sub-carrier signals Ssin~t and S'cos~t. These two signals are then combined in adder 12 with the sum signal M and a 19kHz sine wave pilot signal, also derived from carrier generator 20. The co:mposite signal appearing at the output of adder 12, ha~ing the amplitude co-effici-ents shown in FIG. 2, is then applied to the FM exciter of a transmitter 22 and freguency modulated onto a high fre-quency carrier for transmission to one or mor~ remote re~
ceivers.

~l2~3~

The compressor 16 may have any of several known companding laws, four of which are disclosed in the afore-mentioned co-pending application of David W. Stebbings.
Actually, the construction of the compressor is not criti-cal to the operation of the system in that the expander used at the receiver is capable of adapting to any com-pressor characteristic. Suffice it to say, then, that the compressor 16 is designed to have a compression character-istic that maximizes the signal-to-noise improvement. By way of example, the compressor may have the ininity-to-one compression characteristic illustrated in the signal level diagram o~ FI~. 3, wherein the input signals to be processed for compression are represented along the ab-scissa between -60dB and a standard operating level of OdB. The absolute value of the input voltage rises along the abscissa from left to right up to the rated level.
The output levels for compression or expansion are repre-sented along the ordinate between -60dB and OdB. The ab-solute value of the output voltage rises along the ordin-ate, from the bottom to ~he top, up to the rated level.
Curve 30 represents the relation between the output and input levels of the normal difference signal S which, since it is transmitted unchanged (i.e., not subjected to compression) has a gain slope of one. Curve 32 represents the relationship between the input and output signal lev-els of the compressor 16 and shows that for input signal levels in the range between -60dB and about -32dB the characteristic has a slope of one; thus, input signal lev-els lower than about -32dB relative to the rated level are not compressed but receive a fixed gain of about 26 dB.
For input levels higher than about -32dB, the characteris-tic has a compression slope of infinity; that is, begin-ning at the knee 34 of the characteristic, in this example 33~

set at -32ds, the output level is the same reyardless of the level oE the input signal, which in this example is at a level of -6.7dB relative to the rated level. Thus it is seen that the dynamic range of lnput signals between -60dB
and -32ds has been reduced to a range between -AOdB and -6.7dB for the output signals, and that Eor input signal levels greater than about -32dB the output level remains constant at -6.7dB. The vertically oriented arrow 36 extending between the characteristic 30 for the S signal and the compressor characteristic 32 represents the approximately 26dB gain increase in the difference signal S' as compared to the difference signal S re~uired to com-pensate for the noise penalty for stereophonic programming relative to monophonic.
The infinity-to-one compre~sion law for the differ-ence signal is preferable to the 2:1 or the 3:1 compres-sion characteristics, also shown in FIG. 3, typically em-ployed in the "CX" companding system described in Pat.
No. 4,376,916, especially in the respect that it gives a significant relative improvement against program modulated nolse .
As another example, the compressor 16 may be de-signed to have the compression characteristic 40 shown in the signal level diagram of FIG. 4, which extends with un-ity slope up to within a dB or two of 100~ modulation andthen gradually drops back from that level as the level of the difference signal (represented by curve 38 having a slope of one) rises such that addition oE the S and S' signals yives an infinity-to-one characteristic (depicted by dotted line characteristic 42) that is always main~
tained within a small fraction of a dB of 100~ modula-tion. The re-entrant characteristic of the curve 42 can ~133~

be quite easily derived by subtracting the difference sig-nal S from the output of a regular infinity-to-one com-pressor having the characteristic illustrated in FIG. 3.
'rhis re-entrant characteristic not only permits maximum possible modulation of the sub-carrier signal 7 it also in-sures that the sub-carrier is maintained at substantially constant amplitude over a wide dynamic range of program level, making it possible to transmit at 100~ modulation all of the time, which assures the best possible S/N con-dition at the receiver.
FIGS. 3 and 4 illustrate but four examples o~ com-pressor characteristics that the expander of the invention is capable of adapting to to produce a noise-reduced ex-panded difference signal at the receiverO Another exam-ple is the compression characteristic of the dbx Inc. com-pander that has been adopted as the standard in the Rlec-tronic Industries Association system for stereo televis-ion. Whatever compression characteristic is utilized, the difference signal (L - R) is used/ unchanged, to ~mplitude-modulate one sub-carrier signal and the same difference signal, but compressed according to the desired characteristic, is used to amplitude-modulate a quadrature sub-carrier, and then both are frequency modulated on-to a high frequency carrier Eor transmission.
FIG. 5 is a block diagram of a receiver in associa-tion with a preferred form of adaptive expander according to the invention. Again, for purposes of simplicity, some of the more conventional FM receiver circuits (e.g., RF
and IF stages and discriminator networks) have not been shown and will be only briefly mentioned as necessary. A
received FM signal is ampliEied in the RF and IF stages (not shown) of a receiver/FM demodulator 50, and demodu-lated in any of the known FM detection circuits (not ~3~

shown) to derive the audio signals contained in the re-ceived signal, namely, the signals M9 S, S' and the pilot signal, each with the indicated amplitude co-efficient.
The monophonic sum signal ~ is separated from the higher frequency components of the composite signal by a low-pass filter 52 and applied as one input to a de-matrixer cir-cuit 54 of conventional design. The remaining components of the composite signal are selected by a bandpass filter 55 designed to pass Ere~uencies in the band Erom 19kHz to 53kHz and to reject frequencies outside this band, and then applied to an "S" demodulator 58 and an "S"' demodu-lator 60. The pilot signal is derived by known means (not shown) and applied to a carrier generator 62 which regen-erates quadrature versions thereof, namely, sin~t and cos~t, which are applied to demodulators 58 and 60, re-spectively, so as to obtain the signals S and S' ~which has the function of ~L - R) established by the compressor at the transmitter), respectlvely.
The availablity at the receiver of both the un-changed difference signal and the compressed versionthereof enables the use of an expander that adapts to the compression characteristic, whatever its form, this being accomplished by using the received unchanged difference signal S as a reference level for developing a control signal for a variable gain element connected to receive the compressed difference signal S' and for producing a noise-reduced difference signal for application to de~
matrixer 5~.
In the embodiment of the expander shown in FIG. 5 the received unchanged difference signal S is full-wave rectified by a rectifier 6~ to produce a direct current signal having a level proportional to the amplitude of the ~3~P~3 difference signal S. This direc~ current signal is ap-plied to an integrator 66 which preferably ls of the com-plex form described in commonly assigned Pat. No.
4,376,916, which includes a network of at least three sig-nal paths having differing time constants, the networkhaving a common input terminal for receiving the rectified signal and a common output kerminal at which -the control signal is developed. All except one of the signal paths each includes a diode for causing the respective path to be conductive only in response to the rectified signal ex-hibiting a change in amplitude sufficient to exceed a pre-determined voltage, and the said one signal path conducts in response to any diEferential in voltage between the in-put and output terminals. The output signal from integra-tor 66 which describes the envelope information of the un-changed difference signal S, is applied as one input to a comparator 68 which, for e~ample, may be an operational amplifier 707 with the signal from integrator 66 applied to its positive input.
The compressed difference signal S' produced at the output of demodulator 60 is applied as one input to an ad-der 72 in which it may, if desired, but not necessarily, be added to the unchanged difference signal S; the dotted line connection 74 signifies the optionality of adding the two signals together at this point. Assuming that the un-changed difference signal is not added to the compressed difference signal, the compressed difference signal is simply transmitted to the input of a variable gain element 76, the output of which is applied to a second Eull-wave rectifier 78 which produces a direct current output signal having a level proportional to the amplitude of the signal appearing at the output of variable gain element 76. This ~f13~

direct curren~ signal is applied to a second integrator ~0, which may have the same construction as integrator 66 but, on the other hand, it need not have the same time constants as those used in integrator 66, nor -those used in compressor 16 at the transmitter, the reasons for which will shortly become evident. A settable reEerence voltage level for this loop is established by a device ~2 labeled VreE , which element may be embodied in the rec-tifier circuit 78. The signal developed in the chain including rectifier 78 and integrator 80 is applied as a second in-put to comparator 68, more specifically, to the negative input of operational amplifier 70. Any output signal from comparator 68 representing a difference in the amplitude o~ the two applied signals is amplified by a suitable am-plifier 8~ to produce a control signal which is applied tothe control element of variable gain element 76 to control the gain thereof.
In operation, if the levels of the output signals from integrators 66 and 80 are the same, there will be no change in the magnitude of the control signal applied to variable gain element 76 which, in turn, signiEies that the level of the output signal from the variable gain ele-ment 76 corresponds to that of the unchanged diference signal S. Viewed in another way, the function of compara-tor 68 is analogous to that of the threshold level of con-ventional expanders in that no change is made to the vol-tage level determined by the unchanged difference signal S
until the signal level is above the so-called ~cnee of the transfer characteristic. In the present ctrcuitf the threshold, instead of beiny a preset Eixed signal as is the case in conventional expanders, is a signal derived from the unchanged difference signal S, which, of course, 3~
-16- C-1531-~

varies in amplitude with certain attack and recovery times, but is at the level that the compressed signal Sl should be. Thus, instead of there being a fixed reference voltage, the comparator 68 provides a changing reference level which dynamically varies with the changes in level of the unchanged difference signal S itself. The purpose of the ~reE. device 82 is to establish a fixed gain dif-ference equal to the fixed gain difference of the compres-sor employed at the transmitter so as to preclude variable gain effects on signals below a certain level, that is to say, to provide the equivalent oE a "knee" to offset er-rors that could occur if the signal-to-noise rztio of the difference signal S is not sufficiently high. At input signals levels above such "knee", the level of the signal appearing at the output of variable gain element 76 is de-termined by the amplitude of the control signal developed in comparator 68 which, in turn, is dependent on the am-plitude of the unchanged difference signal S. By virtue of controlling the gain of variable gain ~lement 76 with a control signal proportional to the difference between the level of the compressed difference signal and the un-changed difference signal, the output of variable gain element 76 is the signal S, except that it has been sub-jected -to noise reduction by reason of the expansion of the compressed difference signal that takes place in the control loop. The signal S produced at the output of var-iable gain element 76 is applied to the input of an op-tional de-emphasis circuit 86 (which is used only if pre-emphasis is used in the transmitter) and the output there-of is applied via a switch 88 to the second input of de-matrixer 5~.

33~

The level of the signal S appearing at the output of variable gain elemen-t 76 is set by the Vref. device 82 and arnplifier 84 to be equal to the level of the re-ceived unchanged difference signal S and using that signal as a reference level gives the expander -the important ad-vantage of being capable oE adapting to any of a wide va-riety of companding laws. Thus, in the event a new type of compression transfer characteristic were to be devel-oped in the future, it would not be necessary to scrap the described expander and replace it with a new receiver ex-pander in order to take advantage of the new transEer characteristic. Because its operation depends only on the level of the unchanged difference signal S, any type of compression law currently available, or as may be devel-oped in the future, would be satisfactorily decoded by theadaptive system of FIG. 5.
Unlike most existing companding systems, which re-quire a very specific law for compression and a specific complementary law for expansion to achieve satisfactory performance, this adaptive system is not critically sensi-tive to the compression law, and even time constants are not as important, because the receiver always has a reEer-ence level derived from the unchanged difference signal S.
Reverting now to FIG. 5, in addition to operating as previously described, the receiver is fully compatible with conventional monophonic and two-channel (biphonic) stereophonic broadcasts. When a monoaural broaclcast is be-ing received, the output of receiver/FM demodulator 50 comprises only the monaural signal M consisting of (L
R). This signal is selected by low pass Eilter 52 and applied to de-matrixer 5~, and since no signal is applied ~3~

to the second input of the de-matrixer, only the signal M
appears at each output oE the de-matrixer for application to the left and right loudspeakers, respectively.
For enabling the receiver to reproduce a received conventional two-channel stereo signal, the switch 88 preEerably is automatically actuated Erom the position shown -to the dotted line position so as to connect the output of ~emodulator 58 to the second input of de-matrix-er 5~. Such automatic switching can be achieved, for ex-ample, by any oE several known techniques, such as modu-lating the pilot tone, or adding a separate identification signal, when a compressed difference signal is also being transmitted; a detector in the receiver ~not shown) re-sponsive to the identification signal produces a signal for actuating the switch 88 from the solid line position to the dotted line position. Thus, when a conventional -two-channel stereo signal is received, the M signal, as before, is applied to one input oE de-matrixer 5~ and the S signal derived from demodulator 78 are combined in the de-matrixer to produce output signals 2L and 2R, the am-plitude of each of which is then reduced by one-half prior to application to the left and right loudspeakers.
FIG. 6 shows an alternative configuration of the adaptive expander of FIG. 5, the difEerence being that in-stead of having two integrators, each following a respec-tlve full-wave rectifier and the outputs of the integra-tors applied to the comparatorl the outputs of the recti-fiers are applied to the comparator and its output applied to a single integrator. More particularly, the uncom-pressed difEerence signal S Erom demodulator 58 is appliedto a first Eull-wave rectiEier 90 and is also applied as one input to an adder 92 in which it is preEerably added ~3~
-19- C-1531~A

to the compressed diEference signal S' from demodu].ator 60. The direct current signal from rectiEier 90, which has a level proportional to the amplitude of -the diEfer-ence signal S, is applied to one input of a comparator, such as a difference amplifier 94. A settable reference voltage level Eor this loop is established by a device 96 labeled Vref 1~ which elemen-t may be embodied in the circuit of rectifier 90, and the purpose of which will be described presently. The signal Erorn adder 92, represent-ing the sum of signals S and Sl, is appliecl to the inputoE a variable gain element 98, the output of which is ap-plied to a second full-wave rectifier 100 which produces a direct current output signal having a level proportional to the amplitude of the ou-tput signal from variable gain element 98. This direct current signal is applied to the other input of difference amplifier 94. A second voltage reference device 102, designated Vref 2/ establ.ishes a reference voltage level for this loop. Any output signal from the comparator representing a difference in the am-plitude of the two applied signals (after taking into ac-count the reference voltage levels established by elements 96 and 102) is applied to an integrator 104, preferably of the complex type described above. The output signal from integrator 104 is amplified by a suitable amplifier 106 to produce a control signal which is applied to the control element of variable gain e].ement 98 to control the gain thereof. ~s in the expander of FIG. 5, the .Eunction of the di:Eference amplifier is analogous to that oE the threshold level of conventional expanders in that no change is made to the voltage level determined by the un-changed difference signal S until the signal level is above the knee of the transfer characteristic. Vol-tage ~33~

reference Vref.1 is provided to limit the lower ~nee of the expander as .in conventional expanders, and Vref 2 is provided to cut off its associated loop, because otherwise the loop gain would try to make the output of variable gain element 98 to be constant at the level of the uncom-pressed dif:Eerence signal S. Otherwise, the operation is similar to that o:E the adaptive expander of FIG. 5 and produces a noise-reduced difference signal S at the output of variable gain element 98 for application to de-matrixer 54.
FIG. 7 shows yet another implementation of the same basic adaptive expander which effects a saving in compon-ents as compared to either of the previously described im-plementations in that it re~uires only one full-wave rec-tifier and one complex integra-tor to achieve substantially the same results. More particularly, in this case the un-compressed difference signal S is directly applied to one input of a difference amplifier 110 and is added to the compressed difference signal S' in an adder 112 to produce a sum signal which is applied to the input of a variable gain element 114. The output signal from variable gain element 114 is applied to a gain element such as an ampli-fier 116, the output of which is applied to the second in-put of difference amplifier 110. Any resulting difference signal is full-wave rectif:ied by a rectifier 118 and the resulting direct current signal~ after level adjustment by a voltage level setting device 120, is applied to an inte-grator 122~ again of complex type, the output of which is amplified by a suitable amplifier 124 to develop a control signal for variable gain element l14. As before, the out-put of the variable gain element is a noise-reduced signal S for application to the de-matrixer.
2~1~3~

The gain elemen-t 116 is included to avoid signal ambiguity at the output of full wave rectifier 118 which would occur unless steps are taken to make sure that the signal ~rom variable gain element 114 is always either larger or smaller than the difference signal S. Other-wise, if the two signals appl.ied to di.Eference amplifier 110 should be equal, the output signal from the full-wave rectifier cannot know which signal was the larger. To eliminate this possibility for ambiguity the gain element 10 I 16 is connected in the loop following the variable gain element 11~ to make sure that the signal in this loop is always dominant over the difference signal S. Because gain element 116 is inside the control loop for variable gain element 114 it will automatically wash out because of the feedback nature of the system, and will not affect the amplitude of the final noise-reduced signal S appearing at the output of variable gain element 114. Alternatively, a suitable gain element can be inserted in the S signal path to the difference amplifier to insure that this signal is always dominant, and inserting in the output line to the dematrixer an attenuator to offset the gain of the element in the S signal path.
FIG. 8 is a block diagram of yet another implemen-tation oE an adaptive expander embodying the invention, this one being of the .Eeed-forward type as opposed to the three feedback types just described. Although the feed-forward type is subject to the restrictions, not appli-cable to the feedbac]~ type, that the variable gain e:lement must have a calibrated and prescribed transfer character-istic, and the control signal must be developed with ap-propriate gain and offsets to match that law, i-t can oper-ate satis:Eactorily if such restrictions are taken into ac-count. The feed-forward embodiment has many elements in ~ 2 ~

common with the previously described adaptive expanders but are arranged differently as will now be described.
Specifically, the uncompressed difference signal S is full-wave rectified by a rec-tifier 130, the output of which is applied as one input to a difference amplifier 132, the level of the rectified signal being settable by a voltage level setting device 134 designated Vref 1- The uncompressed difference signal S is also added to the com-pressed difference signal S' in an adder 136 and the re-sulting sum signal applied to a second full-wave rectifier 138 and also to the input of a variable gain element 140.
The direct current signal from rectifier 138, after being subjected to level setting by a voltage level setting de-vice 142 designated Vref 2 and which may be embodied in the rectifier circuitry, is applied to the other input of difference amplifier 132. ~ny difference signal appearing at the output of amplifier 132 is applied to an integrator 144 of the complex type described earlier, the output of which is applied as one input t.o a "law" generator 146, to which is also applied a reference voltage Vref 3 derived from a suitable voltage source 148. For a given change in level of the signal from integrator 144, the "law" genera-tor 146 produces the correct output voltage or current for application to the control element of variable gain ele-25 ment 140 -to insure that gain element 140 gives the correct amount o:E gain or attenuation as may be necessary at any instant o:E time. For example, since the variable gain element typically is logarithmic, the law generator 146 would contain a logarithmic ci.rcuit element to match the changes in the signal from integrator 144 to decibels o:E
attenuation in the variable gain element 1 4n . Thus, the output oE variable gain element 140 is a noise-reduced dif~erence signal S for application to the dematrixer.

~3~

The availability at the receiver of the uncom-pressed difference signal also enables the design of a frequency correcting adaptive expander, one form of which is shown in the block diagram of FIG. 9. The principle of the illustrated Erequency correcting expander is to util~
ize the uncompressed difference signal to adaptively ex-pand a compressed signal containing unknown frequency re~
sponsive elements. For example, the Dolby A companding system has four companded channels, each with a distinct and separate frequency band which is compressed indepen-dently according to its own signal levels. The function of the system of FIG. 9 is to separate such a multi-chan-nel signal into corresponding separate channels and using the uncompressed difference signal, which contains all of the signal information of the compressed diEference sig-nal, to separately adaptively expand the multiple channel signals, and thereaEter combining the resulting noise-reduced channel signals ~o obtain a noise-reduced and fre-quency corrected difference signal for app]ication to a dematrixer. ~o this end, the uncompressed difference sig-nal S and the compressed difference signal S' are summed in an adder 150 and the resulting signal applied to an ar-ray of filters including a low pass fil-ter 152 designated LPF1 and a multiplicity of band pass filters 154, 156 and 15~ respectively designated BPF2, BPF3 and BPFN, de-signed to pass successively higher frequency bands in the audio frequency spectrum. The uncompressed difference signal S is applied to an equivalent set of filters 160, 162, 16~ and 166 respectively designated LPF1, BPF2, ~PF3 and BRFN and having the same pass bands as the corre-spondingly designated filters in the other set. Each of ~2~
~2~- C-1531-A

the filters receiving the sum signal from adder 150 is connected to its own variable gain elemen~ 172, 174, 176 and 178, respectively, the outputs oE which are applied to respective full-wave rectifiers 182, 184, 186 and 188.
The direct current signals from these rectiEiers are applied to one input of a respective difference amplifier 188, 190, 192 and 19~. Similarly, the frequency bands of the uncompressed diEference signal S passed by filters 160, 162, 16~ and 166 are full-wave rectified by respec~
tive rectiEiers 100, 102, 104 and 106 and the respective direct curren-t output signals are applied to the other in-pu-t terminal of difEerence amplifiers 1907 192, 194 and 196, respectively. The difference signals delivered by the difference amplifiers are applied to respective com-plex integrators 198, 200, 202 and 20~ and the output sig-nals therefrom after suitable amplification (not shown) are applied as control signals for variable gain elements 172, 17~, 176 and 178, respectively. It will have been observed that the system of FIG. 9 is a group of four adaptive expanders each constructed according to the block diagram of FIG. 6, except that each is surrounded with a low pass filter or a band pass Eilter. The outputs of the multiple variable gain elements are combined in an adder 206 which combines all of the separately adaptively ex-panded signals together to produce at the output the totalnoise-reduced and frequency corrected difference signal S
for application to the dematrixer. Thus, the system of FIG. 9 consists of a multiplicity of adaptive expanders, each of which operates on its own part o e the frequency spectrum as defined by the associated Eilters. In prac-tice, the filters need not have s-teep rolloff characteris-tics and may be, for example, oE the order of 6dB or 12dB

~.?~ 3~

per octave. It will be understood that FIG~ 9 illustrates an ultimate general case of a frequency correcting adap~
tive expander and that it may be modified to have more or less expanders to meet a specific application.
FIG. 10 is a block diagram of a frequency-correct-ing adaptive expanding system for providing essentially the same function as that achieved with the sys-tem of FIG. 9, except in this case the filtering process is arranged in series rather than in parallel. Also, rather than showing a general case, FIG. 10 sho~s how the system can be applied to the agreed ~IA/dbx Inc. stereo tele-vision system in which the audio difference signal is com-pressed by a compressor which has two variable gain ele-ments arranged in series and includes de-emphasis and bandpass networks for effectively dividing the audio fre-quency spectrum into two bands. Thus, for enabling the adaptive expansion of a dbxjcompressed signal, the system of FIG. 10 has two variable gain elements connected in series and includes two sets of filters for dividing the audio frequency spectrum into substantially the same bands as are used in the dbx compressor. More specifically, the uncompressed difference signal S, which again is used as a reference signal, is applied to a first filter 210, and may optionally be added to the compressed difference sig~
nal S' in an adder 212; the resulting signal is applied to a second filter 214 havin~ the same pass characteristics as filter 210. In practice, bo-th of filters 210 and 214 may be de-emphasis filters having characteristics compar-able to those which establish the upper band in the dbx Inc~ compressor. ~s in the arrangement of FIG. 6, the se-lected band of the siynal S is full wave rectified by a rectiEier 216 and the selected band of the signal from ad-der 212 is applied to a variable gain element 218. Thls ~L2~33~$~

variable gain elemen-t operates on a selected high fre~uen-cy band and functions as a variable de-emphasis element.
The overall output of the expander is rectified by a rec-tifier 220, and the direct current output signals Erom rectifiers 216 and 220 are applied to respective inputs of a difference amplifier 222, the output of which is applied to a complex integrator 22~ which develops a control sig-nal for variable de-emphasis element 218. The remaining portion of the spectrum of difference signal S is applied to a second filter 226 and the frequency-corrected and noise-reduced signal delivered by variable de-emphasis element 218 is applied to a wide band variable gain ele ment 23~. The output of element 234, which is the output of the expander, is also applied to a bandpass filter 228 which, again, has the same pass characteristics as filter 226. ~s before, the selected band of signal S is full-wave rectified by a rectifier 230 and the resulting direct current signal is applied as one input to a difference am-plifier 232, and the signal passed by filter 228 is ap-plied to a full-wave rectifier 236, the output of which is applied to the other input of the difference amplifier 232. The output signal from the difference amplifier is applied to a complex integrator 238, the output of which after suitable ampliEication (not shown) constitutes the control signal for variable gain element 23~. Thus, it is seen that the system of FIG. 10 is similar in function to the system of FIG. 9 except that the individual adaptive expanders are connected in series with filter or variable de-emphasis elements connected between them, and the full noise-reduced and frequency corrected signal S appears at the output of -the last variable gain element (i.e.~ ele-ment 23~) in the chain. It is to be understood that in ., ~2'~3~

the interest of clarity only two series-connected expand-ers have been shown, and that additional stages can be provided should the application require.
Since the uncompressed difference signal S contains all of the dynamics of -the original signal which may, for example 7 be music, it can be used in an adaptive expander to determine what attack and recovery times were used in the transmitterls compressor, whatever its type. ~y way of background, in the design of companding systems there is always a compromise between the attack and recovery times. Considering first the attack time, it can not be so short that the rate of change of gain of the signal would be so large during signal correction as to produce an audible click due to the rapid amplitude modulation of the signal. Another disadvantage oE an attack time that is too short is that sharp peaks in ~Lhe signal will tend to over modulate the transmitter. Accordingly, attack times of 10 to 20 milliseconds are typically used in com-panding systems in order to maintain a higher average mod-ulation level by, in effect, missing the peaks on the pro-gram signal while at ~he same time not continuously over-loading the transmitter, as would be the case if the attack time is too long.
Now briefly considering the attack and recovery times oE companding systems, in order to keep the modula-tion of the transmitter at a maximum, the attack time of the compressor should be as short as possible so that upon occurrence of a sudden high level transient the compressor is gain adjusted downward in order not to over modulate.
If the recovery time were long and the transient followed immediately by a passage of quiet music, the expander would take too long to reduce gain, and the transmission ~3~

channel noise would be heard fading away. On the other hand, a recovery time khat is too short is undesirable be-cause -there will be gain modulation during each cycle, or cycle-~ollowing of low frequencies and consequent increase in distortion. These conflicting requirements of attack and recovery times are usually met by making compromised choices or by usin~ complex integrators, such as the one used in the "CX" companding system described in commonly assigned Pat. No. 4,376,916, which has multiple and com-plex attack and recovery times Eor different signal condi-tions.
The above-outlined characteristics of companding systems are equally present when considering an adaptive expander because it is not possible to make the attack time of the adaptive expander infinitely short, nor its recovery time very long. If this could be done the output dynamics of the signal would be good because if, or in-stance, the reference signal S had a transient that went up and then recovered in a certain way, the compressor at the transmitter would alter those dynamic ratîos and an adaptive expander would, if not limited by recovery or at-tack time in-tegrators, seek to establish the correct sig-nal le~el and put the output right. However, in practice, for a low frequency transmitted signal such an adaptive expander itself would cycle-ollow and cause distortion.
Consequently, even an adaptive expander must have attack and recovery time elements, preferably of the complex type as shown and described in the embodiments discussed thus far. However, it will be seen that it is possible to adapt the integrator of the adaptive expanc1er to the attack and recovery time constants of the compressor at the transmitter.

~3~

Referring to FIG. 11, which illustrates in block diagram form a feedback type of a~aptive expander corre-sponding generally to the arrangement shown in FIG. 6, the uncompressed difference signal S is full-wave rectified by a full-wave rectifier 240 and the direct current output signal, after being subjected to level setting by a vol-tage reference element 242 (which may be incorporated in the rectifier circuitry), is applied as one input to a difference amplifier 244. The uncompressed and compressed difference signals are summed by an adder 246 and the re-sulting signal is applied to a variable gain element 248.
As before, the output signal from the variable gain ele-ment is full-wave rectified by a rectifier 250 and its dc output signal adjusted in level by a voltage reference de-vice 252 and thereafter applied as the second input todifference amplifier 244. The output signal from the dif-ference amplifier is applied to a complex integrator 254 which includes a variable attack time element 256 and a variable recovery time element 258. The output of the in-tegrator, after suitable amplification by an amplifier260~ becomes the control signal for variable gain element 248. Again, as before, the output of the variable gain element is the noise-reduced signal S and, as will be seen, is also adapted to the dynamics of the original sig-nal.
When the signal at the output of difference ampli-fier 244 becomes large on a transient basis, it is known that there is a large error in either the attack or the recovery time constants of integrator 254, and for the reasons discussed above it is desirable that the integra-tor be ad~usted to always go for the longest possible attack time and the longest possible recovery time. This P~3 is accomplished by also applying the output signal from difference amplifier 2~4 to two sensi.ng amplifiers:
an attack time error ampliEier 262 and a recovery time error amplifier 26~. Each of the sensing amplifiers 262 and 264 has an associated voltage reference element 266 and 268, respectively, which establishes a voltage level against which the ou-tput signal from the amplifier is com-pared to determine the magnitude of the error signal :Erom difference amplifier 2~. It is possible to distinguish whether the error is an attack time error or a recovery time error because the polarity of the error signal for attack time is the opposite of the polarity of the error signal Eor recovery time. Thus, the error signal can, in either case, be integrated to develop a control signal for altering the attack and recovery time constants of in-te-grator 25~. To this end, the error signal from sensing amplifier 262 is applied to an attack error integrator 270, which develops a control signal which is applied to the control element of variable attack time element 256 of integrator 254, and the error signal output of sensing am-plifier 26~ is applied to recovery error integrator 272, which develops a recovery time control signal for adjust-ing the recovery time of variable recovery time element 258. Variable attack time element 256 may take the form of an RC circuit including a series resistor and a capaci-tor connected to a ground reference in which the resistive element is controllably variable; Eor example, the resis-tor may be either a variable current source or a field effect transistor (FET) arranged to be con-trolled by the control signal from the attack error integrator. Thus, for example, if the attack time of integrator 25~ had ini-tially been set to have a long time constant and a signal ~3~

is received having a short attack time, a large attack time error would be sensed, and if it exceeded the level es-tablishe~ by Vref 266, the control signal would change the resistance of the variable resistance element in a direction to shorten the attack time constant. Mean-while, the attack error integrator 270 would very slowly relax to ayain exhibit a long attack time constant.
~ he time constant of the variable recovery time element 258 may be controlled by the parallel combination oE a resistor and a capacitor for discharging the main in-tegrating capacitor; again, the resistor may -take the form of either a variable curren-t source or a variable resis-tor, or similar circuit element~ adapted to be controlled by the control signal developed by the recovery error in-tegrator 272. As in the case o~ the variable attack timeelement, the controllable resistor in element 258 would normally be set to exhibit its longest possible recovery time constant (within reason) and to be shortened in re-sponse to the recovery time control signal. It will now be apparent that the system of FIG. 11, which is basically the same as that of FIG~ 6 except that the complex inte-grator is replaced by a controllable complex integrator, will automatically adapt to the attack and recovery dynam-ics oE the compressed signal, all made possible by using the uncompressed difference signal S as a reference.
It will, of course, be understood that the three general types o~ adaptive expander described above are not mutually exclusive and, in fact, they can be combined in a variety of permutations and combinations. F~r example, one or the other of the frequency-correctiny arrangements of FIGS. 9 and 10 could be combined wlth the just des-cribed adaptive attack and recovery time expander fea-ture. Also, it will be recognized by ones skilled in the companding art that the described adaptive techniques for expanding compressed signals are equally applicable to compressors should a situation arise requiring them.
It should now be evident that the transmission of the uncompressed difference signal S along with a COM~
pressed version S' oE the diEference signal, enables u-til-ization of the uncompressed difference signal at the re-ceiver as a reference signal to make an expancler adaptive to any companding law and which enables decoding of dynam-ic parameters of the received signal, such as frequencyresponse, and attack and recovery time constants, so that all of the parameters of the original signal can be re-stored automatically, regardless of the transmission sys-tem.
Although several specific embodiments of -the inven-tion have been illustrated and described, they are exemp-lary only, and such variations and modifications as will now be suggested by those skilled in the art will be un-derstood as forming a part of the present invention inso-far as they fall within the spirit and scope of the appended cl~ims.
For example, the invention may be used in other than the described frequency modulation systemr including amplitude modulation, phase modulation, delta modulation, pulse and pulse code modulation. The carrier may be any of a variety of forms, including visible or infrared light, and multiple~ed satellite distribution systems.
The various functions shown in the block diagrams may be implemented in many ways so as to achieve ideal and/or ro-bust characteristics; for example, the full-wave rectifier and integrator typically form an envelope detector of which various implementations, inc]uding r.m.s. detectors, Q~

are availableO Also, although the described use of a quadrature channel permits the transmission of the com-pressed diEference signal without requiring an increase in bandwidth, if additional bandwidth is available, it is then possible to use an additional carrier, sufficiently removed in frequency from the S subcarrier, to contain the compressed difference signal.

Claims (12)

1. An adaptive expander for use in the receiver of a biphonic FM stereophonic broadcasting system which is adapted to receive a stereo difference signal S and a com-pressed version S' of said stereo difference signal, said expander comprising:
an electronically controllable variable gain device having input, output and control terminals, means for applying said compressed difference sig-nal S' to the input terminal of said variable gain device, and control signal generating means for generating a control signal responsively to both said compressed dif-ference signal S' and said difference signal S and apply-ing the control signal to the control terminal of said variable gain device for causing the gain between the in-put and output of the variable gain device to have a value dependent on the value of said control signal.
2. An adaptive expander as defined in claim 1, wherein said control signal generating means includes means in-cluding comparator means for dynamically comparing a sig-nal proportional to the signal produced at the output ter-minal of said variable gain device against a signal pro-portional to said difference signal S for producing an er-ror signal proportional to differences between said sig-nals S and S', and means for processing said error signal to produce said control signal.
3. An adaptive expander as defined in claim 2, wherein said means for applying said difference signal S' to said variable gain device includes means for summing said difference signal S and said compressed difference signal S' and applying the resulting sum signal to the input ter-minal of said variable gain device, and wherein said control signal generating means in-cludes rectifier means for causing said control signal to be a direct current signal which substantially follows dy-namic variations of the error signal produced by said com-parator means.
4. An adaptive expander as defined in claim 3, wherein said rectifier means comprises first rectifier means responsive to the difference signal S for producing a first rectified signal having a level proportional to the amplitude of and which substan-tially follows dynamic variations of the difference signal S, and means for applying said first rectified signal to said comparator means, and second rectifier means responsive to the signal produced at the output terminal of said variable gain de-vice for producing a second rectified signal having a lev-el proportional to the amplitude of and which substantial-ly follows dynamic variations of the output signal from said variable gain device, and means for applying said second rectified signal to said comparator means.
5. An adaptive expander as defined in claim 4, wherein said control signal generating means includes complex in-tegrator means consisting of a network of signal paths having differing time constants for modifying said control signal to minimize program modulated noise in spite of rapid changes in level of said signals S and S'.
6. An adaptive expander as defined in claim 5, wherein said complex integrator means comprises a single integra-tor circuit having an input connected to receive the error signal from said comparator means and having an output coupled to the control terminal of said variable gain de-vice.
7. An adaptive expander as defined in claim 5, wherein said complex integrator means comprises:
a first integrator circuit having an input connected to receive said first rectified signal and having an out-put coupled to said comparator means, and a second integrator circuit having an input con-nected to receive said second rectified signal and having an output coupled to said comparator means.
8. An adaptive expander as defined in claim 3, wherein said difference signal S is directly applied to said com-parator means, wherein said rectifier means comprises a single rectifier responsive to said error signal for producing a rectified error signal having a level proportional to the amplitude of and which substantially follows dynamic vari-ations of said error signal, and wherein said control signal generating means fur-ther includes complex integrating means consisting of a network of signal paths having differing time constants for modifying said rectified error signal to produce said control signal.
9. An adaptive expander as defined in claim 1, wherein said means for applying said compressed difference signal S' to said variable gain device comprises means for sum-ming said difference signal S and said compressed differ-ence signal S' and applying the resulting sum signal to the input terminal of said variable gain device, and wherein said control signal generating means in-cludes comparator means for dynamically comparing a signal proportional to said sum signal against a signal propor-tional to said difference signal S for producing an error signal proportional to differences between said signals S
and S', and means for processing said error signal to produce said control signal.
10. An adaptive expander as defined in claim 9, wherein said control signal generating means further includes first rectifier means responsive to the difference signal S for producing and applying to said comparator means a first rectified signal having a level proportional to the amplitude of and which substantially follows dynam-ic variations of the difference signal S, second rectifier means responsive to said sum sig-nal for producing and applying to said comparator means a second rectified signal having a level proportional to the amplitude of and which substantially follows dynamic vari-ations of said sum signal, and wherein said means for processing said error signal includes integrator means consisting of a network of sig-nal paths having differing time constants.
11. An adaptive expander as defined in claim 10, where-in said error signal processing means further includes circuit means for modifying the output signal from said integrator means to produce a control signal which is adapted to the gain characteristics of said variable gain element.
12. An adaptive expander as defined in claim 5, where-in the means for applying the difference signal S to said first rectifier means includes first filter means for ap-plying to said first rectifier means a selected frequency band of the difference signal S, and wherein the means for applying the output signal from said variable gain device to said second rectifier means includes second filter means having substantially the same pass characteristics as said first filter means for coupling said selected frequency band of said output signal to said second rectifier means.
CA000497176A 1985-01-04 1985-12-09 Adaptive expanders for fm stereophonic broadcasting system utilizing companding of difference signal Expired CA1243359A (en)

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JPS61502160A (en) 1986-09-25
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