US 2602885 A
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July 8, i952 E. H. ARMSTRONG RADIO SIGNALING 3 Sheets-Sheet l Filed March 50, 1946 INVENTOR.
v idw/'n hi Armsronq BY Zvw ATTORNEYS July 8, 1952 E. H. ARMSTRONG 2,602,885
RADIO SIGNALING Filed March 30, 1946 3 sheet sheet 2 AAAAAA VVVVVV ATTOR July 8, 1952 E. H. ARMSTRONG 2,602,885
' RADIO SIGNALING Filed March 50, 1946 4 3 Sheets-Sheet I5 Amp/07er Am//f/er ATTORN Patented July 8, 1952 UNITED STATES PATENT-.j ioF'l-lc Edwin H. Armstrong, New York, N. Application March so, 194c, seria1N. 658,403
Y 1 Claim. (Cl. Z50-6) preset the relative levels of the loud speaker output at which the musical parts of the program as distinguished from advertising talks are reproduced and maintaining meanwhile the desired difference in level automatically in accordancel with the character of material transmitted from the broadcast station.
This invention is particularly useful in F. M. broadcasting because it has been noted that it is possible to run the levels of the musical part of the programs higher than would be possible with an ordinary amplitude modulation or standard broadcast transmission without distress to the ear because of the much lower percentage of distortions in the F. M. system.
This invention is an improvement on my U. S. Patent No. 1,941,067 which describes a method of accomplishing the basic purpose Vof the idea expressed here, wherein, whenever an advertising talk is broadcast, there is transmitted simultaneously an inaudible tone from the transmitter which, when received by the listeners vset actuates a relay which cuts out the driving coil of the loud speaker, thereby silencing its output. The purpose described herein could be accomplished by connecting across the relay contacts a variable resistance which could be adjusted to values of different impedances in circuit with the coil but it is` desirable to employ electronic means to do this rather than these more or less mechanical means.
Referring now to the iigures which form a part of this specification Figure 1 illustrates one form of the invention which shows the general theory of the system;
Figure 2 illustrates a form of this invention in which a minimum number of tubes are employed to carry out the idea, a matter of considerable importance in the manufacture of broadcast receivers;
Referring now specifically to Figure 1, II represents the ordinary F. M. receiver, 8 the restorer, 9 the first audio amplifier stage, I0 the final audio stage, Il the loud speaker, I2 a high pass iilter for blocking out the low frequency program currents, but Which passes the superaudible frequency transmitted simultaneously with the advertising talks, I3 an amplifier for this frequency. I4 a sharp circuit, or series of sharp circuits for selecting it from all other frequencies, I5 a rectifier arranged with a threshold solthat small static disturbances or any residual components of the program will not be rectified, I6 a D. C. filter having a substantial time constant so that impulses of short duration cannot produce variations in the amplification of the audio system at a rate which will be audibly perceptible, and I'I a voltage divider for adjusting the amount of control voltage to be applied to the control of amplification of the amplifier 9.
The basic principle vof operation is along the lines laid down in the patent previously referred to. In accordance with the teachings of that patent an inaudible frequency is transmitted with a low degree of modulation along with the main program material whenever it is desired to give the listener a choice of selectively cutting out, if he so desires, the program material. In this present arrangement which is embodied in a wide band frequency modulation system. an auxiliary modulating signal of the order of 20,000 cycles having a swing of flveto fifteen or twenty kc. may be eiectively employed.
This frequency is received in the ordinary manner, demodulated by the discriminator 6 and the detector 1, selected out from the program material of lower frequency by the high pass filter I2, amplified by the tube I 3 and passed thru a sharply tuned filter to separate it from program harmonic contents, shock excitation currents such as those produced by automobile ignition, etc.
' The output voltage of the iilter is applied to a Figure 3 shows a modification of a portion of Fig. 2 for the purpose of securing additional control of the silencing effect, as hereinafter explained.
Figure 4 illustrates the generallarr'angement of the transmitting system, and corresponds to Figure 1 f Patent A#1,941,067 hereinabove referredto.
Similar characters of reference designate similar parts in'each ofthe several-views.' y
' audible rate.
biased back rectiiier or rectifier with a threshold or minimum operational voltage which is obtained in a manner'which will be described hereinafter. The twenty kc. current, after rising to a level suicient to overcome the threshold, is rectified, and converted into a direct current. 'I'his current is passed thru'a low pass filter having a sufficiently long time constant so that the rate of change of currentpassing thru it varies at a su-b- This insures the system from having noise produced in the audio amplifiers by disturbances which have succeeded in passing the various selective means previously mentioned that Y guard againstthem. The `output of the iilter is the reproduction when the auxiliary control is s in use and without introducing noise into the speaker system when it is applied.
Referring now specifically to Figure 2,- there is illustrated therein an arrangement for earrying out the purpose of the invention infa'simple manner. One tube only is required Aand 4the result is achieved without the introduction o'f' distortion or noise in the speaker system.
In vthisiigure represents the discrimina-tor or selective Y'system-'detector loutput and `21, 22 the usual restorer for compensating for a preemphasized transmitted signal. l23 is the usual audio amplification control and 24 the rst stage of the audio amplier. Thisaniplifier is Vof the pentode type for reasons whichwill -appear hereinafter and its bias is obtained by a combination of a cathode dropping resistor 25 and a resistance "26 Y'connected betweencathode and the source of plate potential.v The grid fleak consists of the resistors 28, 3d, the last -of which may be cut out by means of the switch '45.
'The output of the -ampliiier 24 Vis developed across the plate loadj28ian'd passed Von to the succeeding stage-inthe usual manner. With the exceptions `noted the parts Vof the system `just described represent the usual detector output and first' amplifier oi an ordinary Ystandard The additional-`features are embodied in the system El -`j4"5. Referring now to this part of the figure-'31, 32, 33, r34 represents a Yhigh pass filter connected'between the detector output and theamplier '315. y3lvis a bias resistor which, as will be hereinafter\V explained in more detail, furnished thejbias voltagefor the pentode am*- plier -and'fth'e-thresholdbias for the diode 4G. 58, 39 representsa tuned coupled circuit adj-usted to select the auxiliary frequency and exclude all undesired frequencies; 4|, 42 and 4.3 represent -a low Apass filter designed to separate out the rectified D. C. current'output'of the'diode 43 from all variablecurrent cornponents'so Ythat no variation of potential at an audible rate "can occur across the output potentiometer, which controls thefamount of voltage applied to the suppressor 46 of the amplifier 24. 45 is a switch for applying the control voltage developed across the iilter output vto the control grid `of the ampliiier 24 simultaneously with its application to the suppressor 4E`of that amplifier.
The operation of the system willbe understood from the followingdescription. `When Athe lauxiliary frequencyji'sjapplied at the transmitting end'of the system `a frequency deviation 'at some inaudible 'rate 'and ofthe order rof ve or ten per cent of lthemaximum program deviation is imparted to thewave. This'develops, under ordinary circumstancjean E. M. of the order of l volt across thedetector Voutput which "is selected outbythe highpasstlter '3l-L34 and amplified Aby A'the Ytube r35. The amplified voltage of inaudible `frequency is impressed across 'the diode '40 jand is off 'sufficientlyhigh level vto'overcome the thresholdiimpos'ed bythe'negative 'bias across the resistance 36. The direct current component is selected out by the iilter 4|, 42, 43 which has a suiiciently long time constant to permit the voltage across the potentiometer 44 to build up at an inaudible rate. This voltage or any desired fraction of it is impressed in the negative polarity on the suppressor grid 46 of the amplifier .24: so as toreduce` its amplification factor Whenever Vthe'auxiliary frequency is transmitted by an amount depending on the setting of the potentiometer 44. This can be accomplished Without the introduction of distortion-over a substantial volume range without diiiiculty; Where, however, it is desired to completely cutoff the speaker system when the auxiliaryfreq'uency is on, a larger negative voltage than is readily available for use on the suppressor may be required. In this event the negative voltage supplied to the suppressor may also be Vimpressed -on the control grid of vthe tube 24 by means ofthe switchf45. This switch may 'be arranged to be controlled synchronously with the potentiometer adjustment 44 so as to be cut in when the point of maximum negative voltage application to the suppressor is reached.
VUnder these conditions the amplification of the vtube 24 is reduced to zero.
AIt will be Vunderstood that, the, tetrode 35 and thediode4-`can,beenclosed in a single envelope so that -a single tube is all that is needed to carry out the invention in all its aspects.
It should -be noted that the negative bias for the amplifier Z4 is obtained by a combination of vcathode 4dropping resister and plate bleeder resistance -26. The effect of this arrangement is to keep a constant bias von the control grid of the amplifier regardless of Ythe change in the current through the -tube when a negative voltage is supplied tothe suppressor grid.
Referring now to Fig. 3, which shows the-control -m'ea-ns forgeiecting the alteration in level betweenthe different parts of the program, an alternativev methedof obtaining the threshold Voltage 'for 'the diode 40 is illustrated. In Fig. 2 Lthe threshold is, as Yhas'been already described, 'obtained by'utiliz'ing'the grid bias voltage of the amplifier B5. Under some `circumstances this threshold may 4not'be Asufficiently high. A vmeth- 'od or" Aobtaining a threshold 4which may be ad- 'justed -to any desirable level is shown in Fig. 3 V`by utilizationo'f vapot-entiometer 50, arranged tosupplypositive'lvoltage to the cathode of the tube 4i) lby connecting it in the manner shown, between theanode source of supply Vvoltage and ground. -A by-pass condenser -5I supplies a path of low impedance for the super-audible -frequency lcurrentjfin accordance with standard practice. The closer 'the lmidpoint vof the potentiometer is moved to the source of anode potential, the higher the threshold. This threshold potential lhas `the eiTect-of rendering the rectier less sensitive 'to noise currents Vand enabling more ueffective separation of the inaudible 4control current from the noise current, when signal intensities become low.
It is 'possible to utilize Vthe amplification control mechanism 3 IAM, as shownin Figs. 2 and 3, to effect noise silencing when the receiver is tuned from one broadcast station to another by so'designing-the relative amplification of the amplifier 35an'd the band'width'of the 'selective circuits 38-39 'that a proper proportion is obtained between the noise and control frequency voltages which `are appliedtothe diode .40 with its respective threshold; The variable potentiometer 50 provides a simple practical means of selecting the proper threshold voltage for various noise conditions.
Referring now to Figure 4 which illustratesr simply the transmitting end of the system 6 l62 represents the announcement microphone and its associated amplifying system, and (i3-64 represents the studio microphone and amplifier for picking up the musical part of the program. 65-66 is a switch for throwing over from one microphone to the other, and vl' is a simultaneously operated switch for throwing on to the modulating input H which may extend, for example, to an F. M. radio transmitter (not shown) a superaudible modulating frequency from the source 66--66-70. A frequency of the order of 20,000 cycles may be used. This frequency is chosen so that it will be inaudible and will not, therefore, affect the reception on ordinary re ceivers.
I have described what I believe to be the best embodiments of my invention. I do not wish, however, to be confined to the embodiments shown, but what I desire to cover by Letters Patent is set forth in the appended claim.
A radio receiver adapted to receive and reproduce currents of program frequencies and control signal currents of an inaudible frequency accompanying parts of the program, comprising means for receiving and reproducing such program frequencies together with such inaudible frequency when present, means for selecting said inaudible frequency from the program frequencies, rectifying means responsive to currents of the resulting selected inaudible frequency having more than a predetermined level and unresponsive to currents below said level for producing a direct current control voltage, direct current filter means for filtering out variations in the resulting direct current control voltage, an amplifier for the reprofrequency selective duced program frequencies, said amplifier having a suppressor grid, manually operable adjusting;
means for adjustably selecting any desired value of said direct current control voltage between an upper and a lower limit thereof, and circuit means for applying the selected control voltage to the suppressor grid of said amplier to control the level of amplification thereof, said amplifier having a control grid with means for ap plyng a constant bias thereto, said bias means including a cathode dropping resistor and means for applying a voltage drop to said cathode resistor regardless of the change in current through the amplifier when a negative voltage is supplied to the suppressor grid.
EDWIN H. ARMSTRONG.
REFERENCES CITED' The following references are of record in the nie of this patent:
UNITED STATES PATENTS Number Name Date 1,941,067 Armstrong Dec. 26, 1933 1,988,614 Tuczek Jan. 22, 1935 1,999,359 Hopkins Apr. 30, 1935 2,061,982 Roberts Nov. 24, 1936 2,105,789 Linsell Jan. 18, 1938 2,156,846 Getaz May 2, 1939 2,199,634 Kich May 7, 1940 2,245,652 Dickert June 17, 1941 2,250,596 Mountjoy July 29, 1941 2,284,064 Morgan May 2, 1942 2,321,651 Caraway June 15, 1943 2,370,216 Worcester Feb. 27, 1945 2,392,672 Koch Jan. 8, 1946 2,404,338 Worcester July 16, 1946 2,431,306 Chatterjea et al. Nov. 25, 1947 2,441,880 Goodale et al May 18, 1948 Brown Aug. 16, 1949