CA1145032A - Receiver for stereophonic television sound transmission - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A receiver for a compatable stereophonic television sound transmission system having left and right audio signals in conjunction with a television broadcast wherein video information is conveyed on an amplitude-modulated carrier in a frequency channel having defined frequency limits. The system includes at a transmitter location a multiplex generator for generating a composite signal having a first component representative of the sum of the audio signals, a second component comprising an amplitude-modulated suppressed carrier subcarrier signal representative of the difference between the audio signals, and a pilot component representative of the phase and frequency of the suppressed carrier. The composite signal is utilized to frequency-modulate a sound carrier to develop an RF signal component which is added to the television channel at a discrete frequency spacing from the video carrier. The receiver includes a tuner for converting the transmission channel to an intermediate frequency, a filter for separating the sound signal therefrom, and a detector for deriving the composite signal from the sound signal. The composite signal is demodulated in a stereo demodulating stage to develop the left and right audio signals.

Description

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SPECIFICATION

Background of the Invention The present application relates generally to sound transmission systems, and more particularly to a receiver for the stereophonic sound transmission system described in copending Canadian parent Application No. 271,170 filed February 7, 1977.
The transmission of stereophonic sound together with a conventional television picture transmission greatly enhances the realism and entertainment value of the program being transmitted. Various systems and apparatus have been proposed for such transmissions including various compatible subcarrier-type systems wherein left-plus-right (L~R) information is conveyed on the regular frequency-modulated sound channel of a composite television broadcast signal, and left-minus-right (L-R) information is conveyed on a sub-carrier.
One such system, which was described in "Simul-taneous Transmission of Two Television Sound Channels", NHK Laboratories Notes, Serial No. 132, February 1970, by Yasutaka Numaguchi, Yashitaka Ikeda, and Osamu Akiyama, conveyed L-R information on a single-sideband amplitude-modulated subcarrier frequency-modulated on the standard NTSC aural carrier. To simpliEy the synchronous detection required for demodulating the sub-carrier in this system, the subcarrier was generated at a frequency of 23.625 KHz, or one and one-half times the 15.75 KHz horizontal scanniny frequency of UOS~ monochrome television broadcasts, enabling the missing subcarrier to be generated - 1- `~

~5~3~ ( , in the receiver L-R demodulator by sampling the horizontal deflection signal. This system was found to he unsatisfactory, primarily because of insufficient subchannel bandwidth, poor channel separation and ambiguity in development of the left (L) -and right (R) audio signals at the receiver.
Another system proposed for stereophonic television sound transmission utilized a frequency-modulated subcarrier centered at ~1 5 KHz, or twice the horizontal scanning fre-quency. This subcarrier, when frequency-modulated on the NTSC-standard aural carrier, pro~ided an L-R bandwidth of 12 KHz However, when it was attempted to add stereophonic demodulation capability to-the 4.5 MHz soun~ channel of standard intercarrier-type television receivers to recover the L-R component, video signal component contamination resulted to an extent that satisfactory L-R audio signals could not be obtained without extensive modification of the receivers. Applying such subcarrier signals to conventional splLt-sound receivers, wherein separate intermediate frequency ~IF) channels are provided for video and sound components, is not practical since the 410 25 MHz sound IF output of conventiona~ mo~ern TV tuners is abo~e the range at which presently employed sound channel IF filters can achieve the required effectiveness.
Another system, which was proposed in U.S. Patent 3,099,707 to R. B. Dome, utilized an amplitude-modulated suppressed-carrier subcarrier component, centered at 23.525 KHz . _ .

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- to avoi~ inter~erence with harmonics of the horiæontal ... . .
scanning signal, frequency-modulated on the sound carrier.
To facilitate regenerating the subcarrier for demodulation . :. .
` purposes at the receiver a 39.375 KHz pilot signal was trans--- 5 mitted which, when combined with the 15.75 KHz horizontal .
scanning signal present in the receiver, resulted-in genera-~- tion of the suppressed 23.625 KHz carrier. This system did . . .
not provide-satisfactory performance in that the bandwidth of the L-R channel was limited to ~ KHz with symmetrical side-.: .
bands. Attempting to r,crease available bandwidth by the use . . ~ . .
- o~ assymmetrical sidebands was not practical because this ~ introduced a principa~ harmonic of the horizontal scanning - signal into the upper sideband of the L-R comp~nent.
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- - Two additional systems, which d~fered from those : .~ , . . .
proposed in the afore-described systems in that they employed a subcarrier centered at 31.5 KHz, or twice the horizontal .
.-:.
- scanning frequency, were shown in U.S. Patents 3,046,329 to ; . , . Reesor and 3,219,759 to R. B. Dome. The first system was a single-sideband system which necessitated the provision of complex filtering and demodulation circuitry in th~
` receiver i~ unacceptably narrow L-R channel bandwidt~ was -- to be avoided_ The second system, like other intercarrier systems, was susceptible to`video signal component contamin-- ation in the sound channel. Furthermore, both of these

2 systems required connection to or at least non-destructive sampling of the horizontal deflection signal within the .

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receiver, necessitating in the case of an add-on adapter a modification of the receiver and the provision of an additional cable to a converter, thereby increasing installation cost and reducing the versatility of the converter.
In contrast, the system of the aforesaid parent Application No. 271,170 utilizes an amplitude-modulated double-sideband suppressed-carrier 38 KHz subcarrier L-R component frequency-modulated on the main aural carrier together with a 15 KHz bandwidth L+R component and a 19 KHz pilot carrierO
This forms a composite signal which is similar to that employed in stereophonic FM broadcasts in the United States. The use of this system simplifies the demodulation process at the receiver, and provides a signal which is compatible with conventional non-stereophonic sound television receivers. Also, the proposed system lends itself to use with self-contained converters of a design and construction which may be readily utilized in conjunction with existing monochrome or color television receivers.
It is a general object of the present invention to provide a new and improved receiver for stereophonic sound information transmitted in conjunction with a standard television transmission, and which has improved performance and is not unduly subject to interference from an accompanying video transmission.

It is another object of the present invention to provide receiving apparatus for receiving a subcarrier-type compatible stereophonic transmission which apparatus can be conveniently installed on an existing television receiver with minimal modifications to the receiver.
It is another object of the present invention to provide apparatus for receiving a subcarrier-type stereophonic television sound transmission which can be economically constructed using standard commercially available components.
Summary~of the Invention The invention is directed to a receiver for receiving stereophonic sound transmissions included on a television broadcast channel of defined frequency limits, wherein the sound transmissions comprise a sound carrier frequency-modulated by a composi-te signal including a first component representative of the sum of the left and right source signals, a second amplitude-modulated subcarrier component representative of the difference between the left and right signals, the subcarrier component having upper and lower sidebands centered about a suppressed carrier, and a pilot component representative of the phase and frequency of the suppressed carrier. The receiver includes tuner means for converting the television broadcast channel to an intermediate frequency channel including an intermediate frequency sound signal sound bandpass filter means for separating the sound signal from the intermediate frequency channel, sour-d detector means for deriving from the intermediate frequency sound signal a composite signal including the first, second and -third components, and stereo demodulator means for deriving -the left and right source signals from the composite signal.

Brief Description of the Drawings The features of the present invention which are believed to be novel are set forth with particularlity in the appended claims. The invention, together with the further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings, in the several figures of which like reference numerals identify like elements, and in which:
Figure 1 is a functional block di.agram of the transmitting portion of a stereophonic television sound transmission system constructed in accordance with the invention of aforesaid Application No. 271,170.
Figure 2 is a graphic presentation of the frequency spectrum of a standard U.S. television channel.
Figure 3 is a graphic presentation of the composite signal generated by thestereophonic television sound trans-mission system of Application No. 271,170.
Figure 4 is a functional block diagram of a stereo-phonic multiplex generator for use in the stereophonictelevision sound transmission system of Application No. 271,170.
Figure 5 is a graphic presentation of the frequency spectrum of a television sound channel showing the effect thereon of stereophonic sound transmission in accordance with Application No. 271,170.
Figure 6 is a functional block diagram of a single-conversion converter for allowing reception of stereophonic television sound transmissions in accordance with the present invention.

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Figure 7 is a functional block diagram of a stereo demodulator for use in thestereophonic television sound transmission system of the Application NoO 271,170.
Figure 8 is a functional block diagram of a double-conversion converter for use in receiving stereophonic tele-vision sound transmissions in accordance with Application No. 271,170.
Figure 9 is a rear elevational view of a television receiver and adapter in accordance with the present invention.
Figure 10 is a functional block diagram of the stereophonic television sound adapter shown in Figure 9.
Figure 11 is a rear elevational view of a television receiver and adapter for adapting the receiver to receive stereophonic television sound transmissions in accordance with the invention.
Figure 12 is a functional block diagram of the stereophonic television sound adapter shown in Figure 11.
Figure 13 is a functional block diagram of a television receiver incorporating means for receiving stereo-phonic television sound transmissions in accordance withthe present invention.
Figure 14 is a functional block diagram of a converter for allowing reception of stereophonic television sound transmissions in accordance with the present invention on a standard stereophonic FM broadcast receiver.
Figure 15A is a schematic diagram partially in functional block form of the transmitter portion of a system for bilingual television sound transmission in accordance with Application No. 271,170.

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Figure 15s is a schematic diagram partially in functional block form of the receiver portion of a system for bilingual television sound -transmission in accordance with the invention.
Figure 16 is a graphic presentation of various transmission standards as applicable to a stereophonic television sound transmission system cons-tructed in accordance with Application No. 271,170.
Figure 17A is a schematic diagram partially in functional block form of the transmitter portion of a system for L-R component enhancement in accordance with Application No. 271,170.
Figure 17B is a schematic diagram partially in functional block form of the receiver portion of a system for L-R component enhancement in accordance with the invention.
Figure 18A is a functional block diagram of the transmitter portion of the stereophonic television sound transmission system of Application No. 271,170 incorporating means for L-R component enhancement for improved performance.
Figure 18B is a functional block diagram of the receiver portion of the stereophonic television sound trans-mission system of the invention incorporating means for compensating for L-R component enhancement.
Figure l9A is a functional block diagram of the transmitter portion of the stereophonic television sound transmission system of Application No. 271,170 showing means for Dolby type B encoding incorporated therein.
Figure l9B is a functional block diagram of the receiver portion of the stereophonic television sound trans-mission system of the invention showing means for Dolby type B decoding incorporated therein.

Figure l9C is a functional block diagram of a Dolby type B signal processing stage suitable for use in the stereophonic television sound transmission system of the invention.
Description of the Preferred Embodiment Referring to the Figures, and particularly to Figures 1-6, a stereophonic sound transmission system con-strueted in aeeordanee with Applieation No. 271,170 may be employed in eonjunetion with an aural transmitter 20 and a visual transmitter 21, whieh may be eonventional in design and eonstruetion. The radio frequeney (RF) output signals from the two transmitters are combined in a eonventional RF
signal diplexer 22 and radiated by means of a eommon trans-mitting antenna 23.

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Video source signals such as may be obtained from cameras, film chains, video tape recorders or the like, are applied to the visual transmitter 21 to produce an amplitude-modulated (AM) RF output signal 24 having the bandpass characteristic shown in Figure 2. At the same time, left (L) and right (R) stereophonic audio signals, such as may be obtained from microphones, tape decks, turntables, or the like, and which typically represent the sound level at two different locations in the scene being televised, are applied to the aural transmitter 20. Within this transmitter these signals are applied to an stereo multiplex generator 25, which may be similar in construction and operation to those utilized in conventional stereo FM broadcast transmitters.
The output of generator, in accordance with the invention, consists of a composite signal which includes a direct L+R
audio component 27, a double-sideband L-R component 28 consisting of upper and lower sidebands 28a and 28b centered at 38 KHz, and a 19 KHz pilot component 29, as shown in Figure 3. This signal is applied to conventional RF modu-later and amplifier circuits 30 within aural transmitter 20 to develop a frequency modulated RF output signal 26 centered 4.5 MHz from the video signal, as shown in Figure 2.
Referring to Figure 4, the multiplex generator 25 may include a pair of pre-emphasis networks 31a and 31b and a pair of 17 KHz low pass filters 33a and 33b for the L and R
channels, respectively. As is well known to the art, the ~4~3æ

pre-emphasis circuits impose a frequency response character-istic on the L and R signals which emphasizes the hlgher frequencies to improve the signal-to-noise ratio of the transmitted program. The low pass filters 33a and 33b serve to prevent input signals exceeding 17 KHz from affecting the 19 KHz pilot and L-R components. The left and right audio signals from filters 33a and 33b are applied to a synchronous switching stage 35 wherein they are alternately sampled to develop the L+R and L-R components in a manner well known to the art. The operation of switching stage 35 is controlled by a 38 KHz square wave signal, which is generated by means of a 76 KHz crystal-controlled oscillator stage 36 and a 2:1 frequency divider stage 37.
After further filtering in a 60 KHz low pass filter 38 to remove harmonics which may exist in the composite signal above 53 KHz, the output of the synchronous switching stage 35 is combined with a 19 KHz pilot signal in a summing stage 39.
The pilot signal is derived by means of an additional 2:1 frequency divider stage 40 and 19 KHz low pass filter and phase adjustment stage 41 to assure precise time coincidence with the 38 KHz sampling action of switching stage 35.
Referring to Figure 3, in the basic system contem-plated by Application No. 271,170 the L+R signal component 27 generated by stereo multiplex generator 25 preferably has a frequency range extending from 50 Hz to 15 KHz and an amplitude sufficient to produce a maximum sound carrier frequency deviation of 22.5 KHz. The L-R signal component 28 consists of lower and upper side band components 28a and 28b preferably centered about a 38 KHz suppressed carrier and 30 extending from 23 to 37.95 KHz and 38.05 to 53 KHz, respectively, each having an amplitude sufficient to produce a maximum frequency deviation of 11.25 KHz in the sound carrier. The 19 KHz pilot component 29, which is preferably centered between the lower sideband 28b and the L+R component 27, is trans-mitted at a frequency deviation of 2.5 KHz in the sound carrier. As shown in Figure 3, for the illustrated embodi~
ment the total bandwidth required by the composite signal is 53 KHz and, by reason of the amplitude limitations imposed on the L+R, L-R, and pilot components, the maximum deviation of the sound carrier is 25 KHz.
In accordance with the invention of Application No. 271,170, the frequency of the pilot carrier may be increased to 5/4 the horizontal scanning rate of the video transmission (Fh), or 19.6875 KHz in the case of U.S. monochrome transmission and 19.66783 KHz in the case of U.S. color transmissions.
This centers the suppressed carrier between the second and third harmonics of the horizontal scanning frequency, which harmonics have been found to be a principal cause of inter-ference in prior-art stereophonic sound systems which lacked adequate video component rejection. This also reduces interference to the L R component to a single component at approximately 7.8 KHz instead of three components at 6.5 KHz, ~æ

9~Z5 KHz an~ 2_75 KHz (~eat between the 6.5 KHz and 9.25 KHz components perceived by a listener) as with a 19 K~z pilot . .
carrier.

A stereophonic sound converter 50 for receiving . .. .
stereophonic sound transmissions in accordance with the inven-- tion is shown in Figure 6. This converter operates indepen-dently of the television receiver, having an input for direct connection to a conventional television antenna 5L and L and R
... .
audio outputs for connection to an external stereo amplifier --~ 10 and speaker system. The RF signals intercepted by antenna 51.... .
~-- are app-ied to a user-adjustable tuner 52 within the converter wherein the desired television channel is selected, amplified .
~- and converted to a suitable intermediate frequency, in this case 10.7 MHz. The intermediate frequency (IF) signal is applied to an IF amplifier stage 53, wherein additional . ~ . . . .
` amplification and limiting are provided. The amplified IF
signal is applied to a conventional FM detector stage 54 ; wherein a composite audio signal having L~, L-R and pilot signal components as depicted in Figure 3 is developed in a 2~ manner w~ll known to the art. In addition, detector 54 may :
`-- also develop an automatic frequency control ~AFC) voltage for ~ .
- application to tuner 52 to maintain the tuner properly tuned to the desired station, and the IF amplifier stage 53 may - develop an AGC signal which is applied to tuner 52 to main-` 25 tain a constant signal level.

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In order to obtain the L and R audio signals neces-sary for driving a stereo amplifier and speaker system the ; . composite signal from detector 54 is applied to a stereo d.emodulator 55, which serves as the counterpart of the stereo multiplex generator 25 in the transmitter. In its most basic form the demodulator 55 may include a commercial integrated-circuit (IC) type stereo demodulator circuit similar to those con~only employed in stereo FM broadcast receivers together with necessary de-emphasis circuits for the L and R audio outputs of the circuit. Referring to Figure 7, within such a demodulator circuit the composite signal lS typically ampiified by a buffer amplifier 60 and applied to a phase-detector 61, which comprises part of a phase-lock loop. The phase-lock loop includes a low-pass filter 62, a DC amplifier 63, a voltage-controlled 76 KHz oscillator 64, two 2:1 fre-quency divider stages 65 and 66, and a phase correction circuit 67 ~hose output is presented as a second input to the phase detector 61 for compàrison with the composite input signal. The phase-lock loop is designed to lock onto the lg KH~ pilot carrier and produce its 38 KHz second harmonic in correct phase to control synchronous switch stage 6g.
The synchronous switch 68 alternately samples the composite stereo input signaI at a 38 KHz rate, synchronized precisely in time and in the same sequence as the correspondinq samples are assembled by the synchronous switch 35 in stereo multiplex generator 25 (Yigure 1) in forming the composite - 14 ~

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signal at the transmitter. As is well known to the art, this results in the L and R audio signals being developed at the output of the swi~ch, and these derived audio signals are applied to:respective ones of two'de-emphasis networks 69a and 69b wherein a predetermined frequency response charac-teristic'is introduced-to compensate for the pre-emphasis characteristic introduced at the transmitter. ~he L and R
audio outputs of the synchronous switch may be applied to an external stereo amplifier and speaker system, or alternatively applied 'o suitable audio amplifier stages and~or spea~ers 'provlded within the converter.
-A dual-convers~on version of the stereophonic television sound converter is shown in Figure 8. In this embodiment tuner 52 converts the selected television broadcast signal to an IF signal which includes a video component centered'at 45.75 MHz an~ a sound component centered at 41.25 MHz. This signal lS applied to a 41.25 MHz bandpass fil-ter 70 wherein the sound component 1S separated and applied to a mixer stage 71. ' Within mixer stage 71 the IF sound component is combined with a 51.95-MHz, or alternatively, a 30.55 ~lHz continuous wave signal from an oscillator stage 72 to develop a second IF signal at 10.7 MHz. This signal is ampliried and amplit-ude-limited in a conventional 10.7 ~1Hz IF amplifier stage 53 prior to application to an FM detector 54, wherein '25 it is converted to a composite stereo signal having L1R, L-R and pi,lot signal components as depicted in Figure 3. In .
addition, as in the previously described single-conversion converter of Figure 6, IF amplifier stage 53 may develop an AGC voltage for application to tuner 52 and detector 54 may develop an AEC voltage for centering the IF frequency, the AFC voltage being applied to either oscillator 72 or tuner 52.
Image rejection for the dual-conversion converter is superior to that for the 10.7 MHz IF single-conversion converter, in that the 41.25 MHz first intermediate frequency provides greater separation between the frequency of the received signal and the lmage frequencies to which the rèceiver is subject. With the 41.25 MHz IF channel the receiver is subject to a primary image band 82.5 MHz removed from the- received broadcast and a potential secondary inter-ference band 20.625 MHz removed from the broadcast which can be doubled in the mixer and thereby pass through the IF
amplifier. Fortunately, frequencies that far removed are efficiently rejected by normal tuner selectivity in the dual-conversion converter. However, in the single-conversion converter these primary and secondary frequency bands are removed from the desired signal by only ~1.4 MHz and 5 ~5 MH~ respectively, the proximity of the latter being such that a portion of the video spectrum of the selected channel and an adjacent channel may pass through the IF amplifier to contaminate the sound channel. Therefore, the tuner for the single-conversion converter must exhibit far greater selectivity than that utilized in the dual-ccnversion converter.

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The problem o~ discriminating against secondary interference in the single-conversion recei~er may be ameliorated by adopting an IF slightly greater than 10.7 MHz.
- For convenience, it may be desirable to perform channel selection at the converter for both the sound and video portions of a broadcast. To this end, the converter may take the form of an adapter 90 such as than shown in Figures 9 and lO This adapter includes suitable RF circuitry for receiving both the audio and video portions of the signal, 1~ and for concurrently supplying the converted video IF signal to a convèntional television receiver 94 to permit reproduction of the ~ideo scene. The adapter 90, which, except for an additional RF output circuit extending to the television receiver, may be similar in design and construction to the lS converter shown in Fi~ure 8, includes a tuner 91 for converting signals intercepted by the television receiving antenna 51 to an intermediate frequency. The intermediate frequency signals, corresponding to those commonly employed in a television receiver, i.e. 41.25 MHz ~or the sound carrier and 45.75 L~rz for the video carrier, are amplified in an RF amplifie~ 92 and coupled through a coa~ial cable ~3 to the television receiver 94. Within the television receiver the coaxial cable 93 .~ay terminate in an isolation network 95 which serves to couple - the signals to UHF input of the VHF tuner 96a of the .ele-~25 vlsion receiver~ The VHF tuner is coupled in a conventional manner to the main chassis 99 of the receiver, which r,ay be :
. conventio~a~ i~ a~l respects. The main chassis dev~lops a .. video output signal for driving a picture tube 97.
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The balance of the stereophonic sound converter 90 is similar in construction and operation to the converter S shown in Figure 8. As in the converter of Figure 8, the - 41.25-MHz output of RF amplifier 92 is applied through a -. 41.25 MHz bandpass filter 70 to the input of mixer stage 71.
.- There, the IF s.ignal is heterodyned with a continuous wave -51.95 M~æ signal developed by an oscillator 72. As a result a 10.7 ~Hz-IP sisnal lS developed whicn is applied to the 10.7 ~Hz I~ amplifier 53. The amplified IF output signal ` . rom this stage is applied to an FM detector 54 wherein a - composite audlo signal having L~R, L-R and pilot components is derived. The composite audio signal is applied to a stereo ` lS. demodulator stage 55 wherein L and R audio signals are ~ . . .
developed for application to external stereophonic amplifying .equipment (not shown).
In operation, tuner 91, which may consist of a con-ventional turret or bandswitch type discrete channel tuner of the type cormrnonly incorporated in consumer television receivers~ i5 set to a desired channel and the intermediate frequency output from the tuner is routed through RF amplifier 92,-cable 9~ and isolation network 95 t~ the input of the . television receiver VHF tuner 96a. This interconnection can usually be readily accornplished, since the V~IF tuner 96a is ordinarily connected to the UHP tuner 96b by means of a ~ s~

coaxiaL cabl~ 93a having plugs on at least one end, so that it is only necessary to unplug this cable and plug in the isolation network to complete the connection. The isolation network serve~ to isolate or decouple chassis grounds as well as to match impedances. VHF tuner 96a, when switched to its UHF position, serves only to pass the signal from the adapter 90 to the television receiver main chassis 99.
Adapter 90 develops L and R audio signals while the television receiver 94 operates in a normal manner to produce a picture on picture tube 97. Since the oper~tion of the television receiver has no effect upon the reception of the stereophonic sound signal, instability or poor signal quality within the receiver cannot depreciate the quality of ; the reproduced sound. The automatic gain control (AGC) cir-cuits of the television receiver remain in effective control -- of video level with this arrangement, and while tuner 91 is adjusted`to optimize picture quality in television receiver 94, the quality of the reproduced sound is automatically and inde-pende~tly optimized by AFC and AGC circuits of the adapter.
~0 It will be appreciated that instead of the multi-channel tuner, it is also passible to utilize a single-channel tuner for receiving a special interest channel. Obviously, this arrangement results in simplification and reduced manu-facturing costs for the adapter, making the pac~a~ attractive for promotional and special interest uses.

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A version of the stereophonic sound adapter for . use in conjunction with a conventional television receiver, wherein channel-selection is accomplished within the receiver, is shown in Figures 11 and 12. This arrangement allows the option of concealing the adapter 100 wi~hin the television receiver cabinet, as shown in Figure 11. The television receiver, as shown in Figure 12, may be conventional ln design . and construction, comprising a receiving antenna 51, a tuner 101, television receiver circuits 103, a picture tube 104, and a loudspeaker .105. To facilitate operation of the adapter the intermediate frequency.output-signal from tuner 101 i5 non-destructively sampled by 2 pick-up 102 and conveyed through . a coaxial cable 106 to the input of a variable-gain RF amplifier 107. The pick-up 102 may consist of a high-impedance voltage . 15 pick-up coupled to the signal path, or alternatively a low-impedance current pick-up which may be inserted in series with . the signal path-by unplugging the existing cable between the tuner and the main chassis and plugging in the pick-up.
The output of RF amplifier 107 is applied through . 20 a 41.25 MHz sound~ bandpass ~ilter 108 to a first mixing or heterodynin~ stage 109, wherein this signal is heterodyned with a 51.95 MHz continuous wave signal supplied by an oscil-lator 112 to develop a 10.7 ~z IF signal. As in the previously - described converters, this signal is applied to a 10.7 MHz IF
.25 amplifier 113 wherein it is amplified and amplitude-limited, and from there to a conventional FM detector stage 114. The - 20.- .

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-- composite output signal from detector 114 is applied to the-... .
-, non-inverting input of a differential amplifier 118, whose - output is in turn applied to a stereo demodulator stage 119 , to develop L and R audio output signals for connection to an , 5 external stereophonic audio amplifier system.
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To provide'improved performance, an optional 45.75 ,, ` MHz video bandpass filter 110 may be connected to the output ' of amplif'ier lU7. The video signal passed by thls filter is ' ' mixed in a second mixing or heterodyning stage 111 with the ,' 10 51.95 MHz continuous wave signal developed by oscillator 112 . . .
. .
to form a 6.2 MHz IF signal. The 6.2 MHz siynal is amplified and amplitude-limited in a 6.2 MHz IF amplifier 115 and ' applied to a conventional FM detector stage 116 wherein an '~ output signal indicative of frequency shift in the video ', 15 channel i5 developed The output slgnal is applied through a 500 Hz Low-pass filter 117 to the inverting input of differ-ential amplifier 118, causing the output of this amplifier to correspond to the difference between the composite signal `~ from detèctor 114 and the low frequency signal from'detector ``~ 20 116.
The effect of subtracting the low frequency audio component optionally derived from the 6.2 l~lHz IF signal is to cancel out some or all of the effects of any extraneous FM
' modulation which exists in the sound channel at the output of television receiver tuner 101 as a result of microphonics or AC power supply'harmonics~ The 6.2 MHz circuits are effective .

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for this purpase because the video carrier is relatively free of fre~uency modulation components below 500 Hz, therefore any such frequency modulation finding its way into FM detector 116 is necessarily due to an undesirable effect common to both signal paths, and therefore should be cancelled out of the principal sound channel by differential amplifier 118.
The necessary circuitry for receiving stereophonic television sound broadcasts transmitted in accordance with the invention may also be provided as an integral part of a television receiver, as shown in Figure 13. In the illus-trated receiver, which except ~or its sound channel may be conventional in structure and operation, televisio~ trans-missions are intercepted by an antenna 51, and amplified and converted by a conventional tuner 120 to an intermediate frequency. The 45.75 MHz video portion o~ the IF signal is amplified by a 45.75 ~z video IP amplifier 121, and then applied to a video detector 122 wherein viaeo information in the intercepted signal is derived. The video signal from detector 122 is ampllfied in a conventional video amplifier stage lZ3- an~ applied to a picture tube 124 to control the brightness of the electron beam thereon. The horizontal and vertical scanning of the electron beam is controlled by con-ventional deflection circuits 125 which receive sync;lronizin~
pulses from video detector 122.
The sound signal appears at the output of tuner 120 as a 41,25 MHz IF siqnal. This signal is separated from the _ 22 -45 75 MHz video siynal by a 41 25 M}Iz sound bandpass filter 126 and applied to a mixing or heterodyning stage 127. In mixing stage 127 the 41.25 M~Iz sound IF signal i5 com~ined with a 30 55 MHz, or alternatively a 51.95 MHz, continuous wave signal from an oscillator 128 to develop a 10.7 ~z IF
signal. This signal is ampllfied and amplitude limited in a 10.7 MHz IF amplifier stage 129 and applied to an FM detector 130 wherein a composite signal containing L*R~ L-R and pilot components as depicted in Figure 3 is developed. The com-.
posite signal is applied to a stereo demodulator 131 wherein L and R audio signals are developed FM detector 13Q also develops an AFC voltage which is applied to appropriate fre-quency control circuitry in oscillator stage 128 to maintain the I0.7 MHz IF signal centered in the IF channel regardless - .
` 15 of the fine tuning of tuner 120.
The stereo demodulator 131, which contains both - stereo demodulation and de-emphasis circuitry, such as those described in connection with the previously described stereo-phonic sound converters and adapters, reproduces from the composite s~gna-l the R and L au~io signals developed at the program source. These audio signals are applied to respective inputs of audio amplifiers 132 and 133 wherein they are amplified to a level suitable for driving respective-loud speakers 134 and 135. Preferably, these speakers are located to the right and left of picture tube 124 as shown to provide a realistic stereo effect during viewing of the television receiver.

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The sound channel of the television receiver is dual-conversion in designt the first conversion stage being con-tained in the tuner 120. For this application, with present technology, a single-conversion sound channel would be sub-5- stantially .inferior ~y reason of the high IF output frequency (41.25 MHz) of available television tuners, and the difficulty of building filters, amplitude-limiters, and FM detectors capable of operating at that frequency while meeting the stringent requirements of the IF amplifiers for high-fidelity stereophonlc sound. Co.. ~ined-bandwidtll, pass-Dand phase-. linearity, and skirt attenuation design requirements are outside.of practical technical and/or consumer market economic ranges, using present-day RLC, ceramic, and crystal filters, . although it is contemplated that new filter technology may 15. ultimately meet these fiiter requirements. Lowering the 41.25 MHz output frequency of modern television tuners is not an attractive alternative, since superior image rejection and adequate video channel bandwidth are important advantages of the higher IF fre~uency.
A dual-conversion sound channel retains the superior image-rejection advantage o the standard 4L.25 MHz tuner outpu~ frequency, while simultaneously exploiting the advan-- tages of a low second-conversi.on IF output frequency to achieve improved limiting and FM detection. Further~ore, a dual-conversion sound channel more effectively isolates the video and sound channels while independently optimizing both . _ ~4 -3~
by means of AFC and AGC signals derived in the respective channels. It will be appreciated that frequencies other than 10.7 MHz may be utilized for the second IF channel for optimum performance, the principal advantage of the 10.7 MHZ fre-quency being for the present the ready availability of 10.7 MHz IF amplifier components.
Stereophonic television sound signals transmitted in accordance with the invention of Application No. 271,170 can be received by a conventional FM stereo broadcast receiver by means of the adapter 80 shown in Figure 14. The converter includes an RF amplifier 81 to which the RF signal intercepted by the receiving antenna 51 is applied, and a mixer stage 82, wherein the amplified signal is heterodyned with a continuous wave output signal from an oscillator 83. The RF amplifier 81, mixer 82 and oscillator 83 together function as a tuner 84, the operating frequency of RF amplifer 81 being adjusted to the desired television broadcast channel and the frequency of oscillator 83 being adjusted to operate at a frequency removed from the television channel sound carrier such that the sound difference frequency, when tripled, fall within the 88-108 MHz FM broadcast band. In the illustrated embodi-ment this intermediate frequency is 30 MHZ.
The intermediate frequency output signal from mixer 82 is applied to a 30 MHZ IF amplifier stage 85 wherein it is amplified and amplitude-limited prior to being applied to a 1~45;~32 - tripler and~gQ M~z filter stage 36. To maintain the con- verter 80 centered on the desired channel the 30 ~1Hz output - signal from IF amplifier stage 85 may be applied to an FM
- detector ~7 to develop an AFC signal for application to oscillator 33~ The output of tripler 86, ~hich constitutes a stereophonic signal having modulation characteristics ~-. similar to those of a standard stereophonic FM broadcast signal, is applied to the antenna input terminal of a.con-ventional FM stereo tuner (not shown). The output of the FM
stereo tuner, which consists of L and R audio output signals, may be applied to a conventional stereo amplifier~ and then to left and right loudspeakers which preferably are placed on either side of the television screen on ~-Ihich the video portion of the received broadcast is being viewed. A selector switch (not shown) may be included in the output circuitry ........... ...... of adapter 80 to facilitate connecting the FM tuner to an FM receiving antenna (not shown) when the adapter is not in use.
Since the frequency deviation of the third harmonic of the 30 MHz IF signal is three times the 25 KHz maximum d~iation of th~ TV sound carrier, the 75 ~Hz ma~im~m deviation `prescribed for standard F~I broadcasts is obtainec in.the resulting 90 MHz signal. For example, assuminS reception of TV channel 11, the sound carrier of the received signal is located at 203.75 MHz and the video carrier is at 199.25 MHz.
. 25 This dictates an oscillator frequency of 233.75 MHz, resulting in an intermediate frequency video carrier at 3~.5 M~z and a , ~ 26 -3~:

sound carrier at 30 M~lz The 34.5 MHz video c'arrier is eliminateh in the IF amplifier stage, leaving only .the 30 MHz sound carrier for tripling to 90 MHz in tripler 86, and reception on F~ ~roadcast channel Zll at 90 MHz. Since the , 5 pilot is, in accordance with the inventiont established at ; 19 XHz the same demodulator circuits utilized in the tuner for demodulating standard F~ broadcasts serve to demodulate the stereophonic television sound- signal, Filters for use in the 30 MHz IF amplifier 85 are within the practical design capabilities of recent surface-wave technology, and provide a particularly good application for a filter~of sin X configuration. F~ detection at the 30 MHz frequency is not a problem in this application, since ; that functlon is performed externally within the stereo tuner.
-The technique of increasing frequency'deviation - by utilizing,a harmonic of the desired signal provides the : basis for improving the performance of the limitor'and dis-criminator stages of an FM receiver. This i5 because increasing the frequency deviation of the modulated inter-. mediate carrier'effectively increases the level of the developed output signal. To illustrate application of this technique, the second converslon from 41.25 MHz to 10.7 ~z in the pre-viously described television.sound converter of Figure 8 can, be accomplished by selecting 46.60 MHz as the frequency of oscillator 72, thereby obtaini.ng a difference frequency Oc 5.35 MHz at the output of mixer 71. The 10.7 MHz IF amplifier , _ 27 -iQ3;~:

53, beins now tuned to the second harmonic of 5.35 MHz, provides twice the fre~uency deviation of the transmitted signal to the FM detector 54 The amplitude of the 10.7 ~z second harmoni.c thus extracted need not equal the amplitude of the 5.35 MHz fundamental to rece~.ve the full ~enefit of the increase'd'deviation for maximum signal-to-noise improvement.
All that is required is that it exceeds the minimum threshold -level of IE amp~ifier 53 so that good limiting action is obtained. It should be obvious to those skilled in the art that by designing for other suitably lower difference frequency outputs from mixer. 71, still higher order harmonics can be extracted by the 10.7 MHz IF amplifier, yielding proportion-ately increased frequency deviations.
In accordance with another aspect of the invention, 15 the stereopho'nic television sound system of the invention can - be utilized for bi-lingual progran~ing.. As shown in Figure l5A, assum'ing that the sound portion of a television broadcast is to be broadcast simultaneously in two different lanyuages A and B, the A sound source is connected through resistances 2a 150 and 151 to th-e inverting inputs of first and second *ifferential ampli~iers 152 and 153, respectively The. B
; sound source is connected throu~h a resistance 154 to the inverting input of amplifier 152 and through a resistance 155 to the non-inverting input of amplifier 153. The non-inverting inputs of amplifiers 152 and 153 are connected to ~round by resistances 156 and 157, respectively, and the inverting .
~ 28 -~ S~3~ l inputs are connected to the outputs of their respective amplifiers by-resistors 158 and 159, respectively. The outputs of amplifiers 152 and 153 are connected to the L
and R audio inputs o~ the system stereo generator 147, which may be identical in construction and operation to the stereo generator 25 shown in Figure 4~
As a result of this matrixing arrangement language B modulates what was formerly the 38 KHz L-R sub-carrier channel, and Ianguage A modulates what was formerly the L+R
main channel. The l9 KHz pilot component is transmittea as it was during the transmission o~ stereophonic program material.
At the receiver, as shown in Figure 15B, the L and R
audio outputs o~ the system stereo demodulator 148 are applied through respectiv~ resistances 160 and 161 to the inverting and non-inverting inputs of a differential amplifier 162.
- The output of amplifier 162 is coupled back to the inverting input terminal by a resistance 164 and the non-inverting input is connected to ground by a resistance 163. Resis-tances 160, 16L, 163 and 164 form a matrix in combination with amplifier 162 to generate- a signal corresponding to language B
at the output o~ the amplifier. Language A can be obtained ` at either of the output terminals of the stereo demodulator - 148 by conditioning the demodulator for monophonic operation.
A three-pole three-position mode selection switch 165 may be provided to select the signal to be amplified by an external .two channel audio amplifier 166 and applied to loudspeakers 167 .

3~
and 168, and to condition the demodulator for monophonic operation during reception of language A.
With this arrangement, it is contemplated that language A would normally be the majority or domestic language, since the L+R channel on which it is conveyed is compatibly received by existing monaural television receivers.
At the receiving end the matrixing circuitry can be constructed as an adapter 149 which can be readily added to or incorporated in existing receivers, such as those depicted in Figures 6 and 8, to enable selective reproduction of either language A
or language B. It should also be noted that the bilingual system can also be used in conjunction with standard FM stereo broadcasts. In this case the adapter 149 is connected between the L and R audio outputs of the stereo FM receiver and the stereo amplifying system.
From the preceding discussion it will be realized that the basic stereophonic television sound transmission system described requires only the addition of a stereo FM
multiplex generator to existing television sound transmission equipment, and the addition of a converter or adapter to existing television receiving equipment. However, by modifying certain parameters of the heretofore described system in accordance with further features to be subsequently described, improved sound transmission ispossible in conjunction with such existing equipment. Such modifications are feasible at this time since commercial P3~
.

: .
. .
- stereophonic television ~roadcasts are presently non-existent, : .
and engineering standards concerning such broadcasts have not been established. Therefore, in anticipation of, and . . .
as a basis for establishing such standards, it is appropriate to examine the characteristics of the modulated sound carrier . .
... .
generated by the tra~smission system in detail to determine what standards provi~e for optimum transr~lission of stereo-, ...... . . .
.
- phonic sound without detri~ent to picture quality.
Referring to Figure 3, the maximum ~re~uency devi-. .: . . .
~` 10 ation of either L-R component is 50% that of the L+R main .
.
channel component, this reduction being the result of the L-R
energy being spread over two sidebands which span twice the --- bandwidth of the main channel. This has the effect of reducing the modulation lndices of the L-R channel relative to the main . . - .
h 15 channel. Moreover, the L-R channel modulation indices are . . .
~ further reduced by the well known l/f decrease of the modula-: . .
tion index with increasing modulation frequency. This is graphically illustrated in logarithmic format by r igure 16, wherein Curve A is a plot o~ modulation index vs. modulation frequency (measured fro~ .he sound carrier) for the conditio~
of constant maximum frequency deviation (22 5 K~z). Curve R
- is a similar plot, except ~at, in accordance ~ith the above-~, .
mentioned maximum frecuency deviation limi.s o, FigLre 3, the modulation indices of the L-R channel are de~ressed 50 - 25 ~6 db.), while the L~R resion remains identical to Curve A.

~i 93~:

Modulation index curve B represents modulation at the 100% level for the proposed transmission system based upon a uniorm audio spectral energy distribution. In practice the distribution of energy peaXs in audio program material falls off with increasing frequency. I'his is shown by Curve C, wherein only lower frequency peaks attain the 100~ modulation level of curve B. The form of curve C is actualIy that of a de-emphasis network having a time constant of 25 microseconds, that curve having been found to best 10~ approximate the energy distribution in modern audio programs as shown by Ray M. Dolby, Optimum Use of Noise Reduction in ` ~M Broadcasting, Journal of the Audio ~ngineering Society, Vol. 21, No. S, June 1973, and D. P. Robinson, Dolby B-Type Noise Reduction for FM Broadcasts, Journal of the Audic Engineering Society, Vol. 21, No. 5, June 1973. These references demonstrate that the conventional 75 microsecond time constant presently prescribed by U.S. F~l radio standards is outmoded, being based on the frequency distribution of program material as it existed at an earlier time using ~0 equipment and methods which are now obsolete.
Referring a~ain to Figure 16, curves X and Y depict the effect on frequency response o~ pre-emphasis networks having respective time constants of 75 and 2S microseconds-- in the L~R region, the L-R region having ~een omitted for reasons of clarity. Curve D illustrates .he effect of a 75 microsecond pre-emphasis network on modern program material ~ L5~3;~ \ -- (~s represented by curve C). Curve D is obtained by sub-. tracting curve C from cur~e X, with curve ~ as the baseline.
Over-modulation is that portion of cur~e D which exceeds the !~
100% modulation line (curve B), being prominent at high audio modulation frequencies of both the L~R and L-R bands.
It can be concluded from curve D that the result of applying the 75 microsecond pre-emphasis required by U.S.
standards in present-day FM broadcasting has been overcom-pensation of the high frequencies, requiring either amplitude F
~ limiting of hLgh ~requencies r or substantial under-modulating of mid and low frequencies to a~oid over-modulation of the transmitter. The penalty in the irst instance is diminished high frequency response when the program material is de-emphasized prior to reproduction at the receiver. The penalty in the latter instance is reduced broadcast coverage.
In interpreting curve D it shollld be considered that - the ordinate in Figure 16 is the phase modulation angle of the sound carrier, which may be taken as an indication of tolerance by the transmitted signal to noise interference 0 along the transmitter-receiver radiation path. Curve D
reveals that, when 75 microsecond pre-emphasis i5 applied, the overall noise tolerance of the L-R sidebands ls substan-tially inferior to that of the main channel, the modulation frequencies in the region of 38 XHz being particularly defi-cient. This latter deficiency, together with over-modulation of high audio frequencies, are present-day proble~is of FM
stereo broadcasting.

- . ~ 33 - -3~

~ 75 microsecond pre-emphasis were to be adopted as a standard for stereophonic television sound, the problem of L-R noise susceptibility would be more serious than is presently the case for F,~l stereo broadcasts, since FM broad-- 5 cast standards specify 75 KHz as the maximum frequency deviation, whereas the corresponding ma~imum specified for television sound transmission is only 25 KHz. For comparison, curve E, which results from subtracting curve C from curve Y, with curve B as the baseline, shows the effect of applying 25 microsecond pre em,phasis to the representative program content of curve C. It will be noted that curve E coincides at all frequencies with curve B,which can be interpreted as lndicating that, with 25 microsecond pre-emphasis, the program can be transmitted with essentially 100% modulation at all frequencies. It is also apparent that adopting 25 micro-second pre-emphasis eliminates the 38 KHz-centered noise tolerance deficiency and high frequency over-modulation characteristics of 75 microsecond pre-emphasis. However, the overall L-R noise suscèptibility (relative to that of the L+R main channel) remains low, as evidenced by the 9.6 dh drop between the-low point (15 XHz) of the L+R channel and the high point (23 KHz) of the L-R channel, as shown in Figure 16.
. . . One way to increase L-R noise tolerance is shown by curve F, wherein L-R signal amplitude has been increase`d by a factor of 3` (9.6 db) relatlve to curve E, ~hile the L+R

_ 3~ -3~
channel amplitude remains unchanged. Another way to improve L-R noise susceptibility is illustrated by curve G, which results from first enchancing the L-R portion of curve E by a factor of 2 (6 db), then applying 7.5 microsecond pr~-emphasis to the overall stereo composite signal. This raises the relative modulation level of the stereo channel and adjusts the slope to increase the relative signal strength at the high frequency end of the L-R spectrum. Since pre-emphasis having a 7.5 microsecond time constant has its greatest effect above approximately 20 KHz, its effect on the L~R main channel is minimal.
Thus, the basic transmission system of Application No. 271,170 may be significantly improved with respect to sound fidelity, broadcast coverage, and signal-to-noise ratio of the L-R channel, by 1) employing an audio pre-emphasis time constant of 25 microseconds rather than the 75 microsecond time constant required by the present FM and TV broadcast standards, 2) enhancing only the L-R component of the composite stereo signal, while maintaining the L+R main channel unchanged, and 3) applying additional pre-emphasis to the composite signal in combination with selective enhancement of the L-R
region to adjust or eliminate, as desired, the negative slope of the L-R spectrum.
Figure 17A shows cixcuitry for enchancing the ampli-tude of the L-R channel without affecting the L+R main channel, ~5~;~Z

thereby achieving the improved signal-to-noise performance - characteristic of curve F in Figure 16. In this further -- aspect of the invention, additional circuitry 140 is inter-posed between the L and R program sound sources and the L
` 5 and R audio inputs of the stereo multiplex generator 25 at - the studio. Alternatively, this additional circuitry could be incorporated within the multiplex generator itself, the internal pre-emphasis circuits of which may be converted to a 25 microsecond tlme constant. At the receiver additional - 10 circuitry lkl, which compensates for the effect of the trans-mitter circuitry 140, is added as shown in Figure 17B. The receiver compensating circuitry 141 may be a simple adapter - connected between the L and R audio output terminals of the - receiver and the corresponding inputs of an external stereo amplifier/speaker system, or may be incorporated within the stereo demodulator 55 of the receiver.
The enhancement circuit shown in Figure 17A utilizes t~Q differential amplifiers 142 and 142, Amplifier 142 has its nan-inverting input connected to the L sound source and amp~Ifie~ l~ has its non-in~ertlng input connected-to the R sound source_ The output of amplifier 142, henceforth designated L', is~connected to the left audio input of the multiplex generator 25, and by an impedance Za to its invert-ing input. The output of amplifier 143, henceforth designated R', is similarly connected to the right audio input of the multiplex generator 25 and by an impedance Zb-to its inverting .. . . . . _ . . . . .............. . .. . . .
_ 36 _ \

input. The inverting inputs of amplifiers 142 and L43 are interco~nected by an impedance Zc.
If the three impedances are arranged to be resistive and of equal value (Za = Zb = Zc), the L-R audio amplitude increases by a factor of 3, while-the L+R audio amplitude remains unchanged. This follows since ` - L' = 2L-R R' = 2R-L
- and L ' + ~ = 1 and L - R' = 3 L+R L-R

1-0 The relationship is shown in the following tabulation for ~arious input combinations wherein each unit corresponds to an 11.25 KHz deviation:

L R L' R' L+R L'+R' L-R L'-R' ~ 1 -1 2 1 1 -1 -3 As shown in Figure 17B, a compensating circuit 141 consisting 2~ o three Lmpedances Za', Zb', and Zc' and a pair of audia amplifTers ~44 and 145 may be provided at the receiver to restore the h and R audio signals to the amplitude relation-ship they ha* prior to the L-R enhancement introduced at the transmitter. The L' audio output signal from demodulator 55 is coupled to the input of audio amplifier 144 by impedance Za' and the R' audio output signal from the demodulator is coupled 3~

by impedance Zb' to the input of audio amplifier 145. The inputs of amplifiers 144 and 145 are connected together by impedance Zc'. As in the previously described en~odiments, the composite signal developed within the converter or S adapter is applied to demodulator 55.
If, as illustrated above, Za, Zb, and Zc are made equal and resistive in the enhancement circuit, Za', Zb', and Zc' will also necessarily be equal and resistive, although they need not have the same absolute resistance as Za, Zb and Zc. ~ith this arrangement the L and R audio output signals at the recei~er will be restored to the same amplitude reIa-. tionship they had prior to L-R enhancement at the transmitter.
. - Although the 9.6 db tfactor of 3~ enhancement of the L-R channel has been shown and discussed, it will be lS appreciated that by selecting other .values for Za,.Zb, and Zc at the studio, a greater or lesser enhancement of the L-R
component can be achieved. For example, for the factor of 2 (6 db) enhancement illustrated by curve G of Figure 16, it is necessary that R~-ZRb=2Ra with the result that 2~ L/ = 3L-R R' - 3R-L
` 2~ . 2.
and L+R -- =1 and L R = 2 As described earlier, curve G results from the -25 combination of a 6 db enhancement, with a 25 microsecond audio pre-emphasis then applied prior to multiplexi~g, followed by 3~

... . .
.. an additional pre-emphasis applied to the composite stereo . signal after multiplexing. As shown in Figure 18A the enhance-. .
- ment and audio pre-emphasis may be accomplished by means of a .. circuit 170 situated ahead of the system stereo multiplex .... .
S generator 25 (assuming no pre-emphasis within the generator), .. . . . .
. while a pre-emphasis network 171 for the composite signal may . be located between the multiplex generator and the trans-- mitter modulator/ampli~ier circuits 30. For curve G the ... : . .
.~ composite signal pre-emphasis time constant is 7.5 micro-seconds.
. . .
. Referring to Figure 18B, to compensate at the . . .
.. receiver for the pre-emphasis of the comp~site signal intro-~- duced a~ the. transmitter, a 7.5 micros.econd de-emphasis - network 17.2 may be incorporated ahead of the demodulator 55.lS. Audio de-emphasis an~ the earlier described audio de-enhance-. ment circuits of the receiver may be incorporated in a ; . circuit 173 at the output of demodulator 55.
. It will be appreciated that t~e composite signal conditions represented by curves F and G of Fisure 16 have ; 20 been presente~ as a.means of illustrating various techniques which may ~ combined-in v~rious degrees to shape the com-posite.signal spectrum so as to.optimize stereophonic per-formance, It.is anticipated that such techniques would ultimately be defined as parameters in a yet to be adopted stereophonlc te1evision sound standard.

.
. . .
. . - 39 -It should be understood that the curves of Figure 16 are idealizations, in that they represent the case for sinusoidal signals oE prescribed amplitude plotted one fre-quency at a time. By contrast, audio programs originate as S complex signals of constantly changing amplitude and frequency.
Modulation envelopes of complex signals tend to blend as a continuum and to thereby obscure details of the underlying signals such as the 8 K~z gap which separates the L~R and L-R components ln Figure 16. For this reason, observed complex 10 . signal modulated spectra Lor the stereophonic tel~vision sound transmission system appear somewhat as shown in the expanded television sound channel portrayed in Figure 5. Referring to that figure, modulation of the sound carrier with a com-posite signal, such as developed in the basic (non-enhanced) `transmission system of this invention, results in the genera-tion .of an RF signal 180 at the upper end of the television channel (Figure 2) having a maximum overall bandwidth of S0 K~z during monophonic (L+R only~ transmissions, and an RF
signa.l.181 having a maximum overall bandwi~th of 106 KHz during stereophonic transmissions. Mod~latio~ of the sound carrier Z6 with the composite signal developed when the basic transmission system.has bee~ modifi.ed to incor~orate .the~
circuitry o~ Figure 17A, so as to provide 9.6 db enhancement of the L-R component, results in generation of an RF signal 182 having shoulders above those of the envelope of RF
.signal 181.

. _ _ . . ... . .

3æ

--- The conditions depicted in Figures 3 and 16 - represent extreme modulation limits for the L~R and L-R
. :.:. .
components, those limits being mutually exclusive in the - .
sense of being attainable in only one channeI at a time, `: 5 and only under the uncommon circumstance that the other :.......... .
channel equates to 0 at that instant. For this reason, most .
--- of the audio content of the program is developed at levels .*
-- below these limits, being susceptible therefore to environ-, .......................... .
mental ~lectrical`noise to a still greater degree. One way . , .
to furt~er improve noise rejection for`the low level signals .;.` . .
` is to process the program content before transmission in a - m~nner that raises the average modulation of the composite ~.................. . .
- signa~ closer- to 100%, and to then compensate at the receiver with reciprocally matched circuitry to restore the original conditions. One such system, the Dolby Type B noise reduction system, is findlng increasing commerclal application in FM
stereophonic radio broadcasting for reducing high frequency over-modulation and raising the average modulation level of ~` the transmitted program.
Basically, the tran~er chaIacteristic of the Dolby B system i5 such as to enhance low-level high frequency ` signals, the degree of low level enhancement increasing as : .
a functio~ Q~ frequency. Since the Dolby B system has been -- detailed promlnently in the literature, only the relevant, `- 25 qualitative features are noted herein. Tnese are summarized in Table 2, wherein various configurations, consisting of , .

~L~4~3;~ ( -various transmisslon pre-emphasis and reception de-emphasis time constants are compared with and without Dolby Type B
transmission and reception units with respect to maximum modulation level for high fidelity transmission (0 db = 100 modulation),.high fidelity capability, and relative signal-to-noise performance.

i~aximum Configuration Relative Net System Transmitter Receiver l~lodulation High Relative ~ ~o. Pre-Emphasis* De-Emphasis* Level Fidelitv S~N
1 75 75 -8.3 db Yes -0 9 dh 2 25 75 0 db No ~2.7 db 3 25 25 0 db Yes -~2.7 db

4 Dolby & 75 75-8.~ db No ---Dolby ~ Z5 75 0 db No 15 : 6 Dolby & 25 25 0 db No ---7 Dolby & 25 25 & Dolby 0 db Yes ~12.3 db * = microseconds As.can he seen in Table 2, the canfigurations are listed ln order o~ increasing signal-to-noise (S/N) ratio, 20 . althou~hj for configurations 4, 5 and 6 the improvement has dubious merit~ since the result ls distortion of the audio signal due to over-emphacis o~ high frequencies. Only. configu-rations 1, 3, and 7, for which the de-emphasis is truly complementary, are capable of high fidelity reproduction; and of these, only configurations 3 and 7 can transmit.all of ~he program material at essentially full modulation. Configuration 3 . . . .. .
_ _ .

3~ ~ -was described ~arlier as the preferred 25 microsecond pre-emphasis/de-emphasis system for the basic transmission/reception system of the invention. Configuration 7, which incorporates complementary Dolby noise-reduction, may be implemented in the system of configuration 3 without modification, resulting in a 13.2 db S/N improvement as compared with the complementary 75 microsecond pre-emphasis standard for FM broadcasts.
Furthermore, without system modification, Dolby Type B noise reduction may be combined with the previously described L-R
io enhancement and composite signal pre-emphasis techniques for further noise reduction to the extent permitted by whatever frequency deviatlon limits are ultimately adopted as a standard for stereophonic television sound transmission~
Figure l9A shows the application of Dolby Type B
noise reduction to the ~asic transmission system of the inven-tion, wherein two Dolby B processors 185 and 1~6 at the transmitter encode the pre-emphasized L and R audio signals prior to application to the system stereo generator 25. At the receiver, as shown in Figure 19B, the L and R audio outputs of the rece~r stereo demodulator stage 55 are applied through respective de-emphasis net~orks 187 and 188, which i~ accord-ance with the previous discussion have a time constant of 25 microseconds, to respective Dolby B processors 189 and 1~0, which decode the L and R audio signals. The de-emphasis networks and processors together provide compensation to restore the de-emphasized L and R audio signals from demodulator 55 to their original relationship.

.

33~

., .
The encoding and decoding processors may be basically identical in construction, differing only in the manner in whlch the input signal is routed, as shown in Figure l9C.
The input signal traverses two paths; a main path through a combining network 191 and an lnverter 192, and a secondary path through a voltage-responsive variable-frequency filter 193, a signal amplifier 194, and an overshoot suppressor 195.
The main path passes the input signal essentially unchanged_ ` The secondary path is essentially a filter which passes only low-level, high-frequency components of the input~signal During encoding, the output of the secondary path is added to the main path, boosting the low-level, high-frequency por-tions of the input signal. During decoding, the output of - - the secondary path is subtracted from the main signal path output, a result of the secondary path input belng sensed as -the inverted output of the processor. Decoding thus removes the same inormation to the same degree as was inserted during encoding.
The characteristics of the secondary path variable filter 1~ are determined by a feedback loop consisting of a contro~ amp~fier 196 and recti~ier/integrator 197. For signal amplitudes which do not exceed a fixed threshold, no feedback signaL is generated, and the transfer function of the filter is simply that of a fixed, 500 Hz high pass filter.
The threshold level is fixed at approximately 40 db below Dolby level, an internationally standardized reference 3æ ~ -corresponding to a ~requency deviation of + 37.5 K~lz for FM
broadcasting. A similar reference would be established for television sound at 50% of the maximum frequency deviation allowed for the television sound carrier. The gain of control amplifler 196 is a non-linear function of frequency, so that as signal amplitudes increase above the threshold level nega-tive feedback raises the.variable filter cut~off frequency in a non-linear manner, reaching a constant, maximally.
restricted ~andwidth for input signals near Dolby level. The 10. overall effect is negligible at low frequencles an~ at levels approachLng full modulation, but increases with increasing frequency and decreasing amplitude.
. . In another en~odlment of the invention, enhancement of low level signals is accomplished in a manner similar to 15 that of .the ~olby Type B system, in that the degree of enhance-ment also lncreases as a function of f~equency. However, unlike Dolby Type B, enhancement of low level signals occurs only above 2Q K~zr an~ i5 applied to the composite signal, being provided immediately following the system stereo multi-plex generator.25, as with the pre-emphasis netwark 1~l provided for the composite signal in Figure 18A~ Complementaxy circuitry i5` installed at the receiver just prior to the s.tereo demodulator SS, as with the composite signal de-emphasis circuits 172 in Figure 18B. Applied in this way, only the .25 L-R channel is affected, the effect being to raise the modu-lation level (and hence the noise tolerance) of low level L-~

. . .

S~3;2 signals without increasing the modulation level of high-level signals.
Since neither commercial stereophonic television . broadcasts nor suitable con~ercial receivers for receiving

5 such broadcasts are in existence, no problem of obsolescence of existing equipment exists in adopting the proposed compatible transmission system. Receivers ~r adapters to reproduce television stereophonic sound in accordance with the trans-mission/reception system of the invention may be manufactured with the preferred 25 microsecond time constant, and may in fact immediately incorporate Dolby Type B receptIon circuitry, since such circuitry is already commercially availabIe in economical ~C packaging. As for compatibIlity with existing monophonic tele~ision receivers which have 75 microsecond sound de-emphasis, it is doubtful that any unfavorable change in tAe repraduced sound signal could be perceived, sInce very few television receivers are capable of high fidelity receptlon At any rate, F~ broadcasts using the previously discussed.configuration.5 system indicate that many monophonic TV listenersl would actually perceive the sound quality as . `improved bècause ofi its -increased high frequency con~ent ~ S~32 To better enable Dolby-B processor-equipped receivers tq receive non-Doiby transmissions a remote switching signal may be added to the composite signal to control the decoding processors within the receivers. This switching signal may take the form of a sub-sonic fixed-frequency signal in the 10 to 25 Hz range which may be selectively added to either the composite signal output of the stereo generator 25 ~Figure 1) or generated within the stereo generator whenever the transmission includes Dolby-B process-ing. The subsonic tone would be detected by a c~ntrol tone sensing circuit 198 (Figure l9c~ ln the receiver and utilized to control the Dolby-B processing stage(s) therein.
-For example, a subsonic 20 Hz signal could be generated at a fixed amplitude corresponding to 25 Hz fre-quency deviation o~ the sound carrier The corresponding modulation index (1 25) of this inaudible component would be 60 db below the 100~ modulation level (25 XHz), being there-fore innocuous to other intelligence and functions At the receiver, a narrow-band frequency detector would respond to this 20 Hz component by generating a DC control signal suitable for switching or otherwlse conditioning the receiver for Dolby-~ decoding.
The above-described subsonic switching signal requires no additional bandwidth and, combined with the con-trol possibilities of the 19 Khz pilot component constitutes a flexible means for conveying conditions of transmission.

3~
, . .
:::
-'- This is shown in Table 3 which illustrates the four trans-- mission format conditions possible with the pilot and -.
- subsonic signals.

. . .
-. TABLE 3 .:.
Control Signals .
--' Pilot Subsonic Signal Transmission Format ~- Yes - No Stereophonic, 25 Microsecond --- preemphasis :........ . .
-- No No Monophonic, 75 Microsecond -' - preemphasis -:. 10 -.` No Yes Monophonic, Dolby, 25 Micro--. second preemphasis ~.~ Yes Yes ' Stereophonic, Dolby with 25 -`~' Microsecond preemphasis . It will be appreciated that more than one subsonic switching i .
-- signal may'be transmLtted to accomplish additional control . 15 - functions, and that the aboue-described subsonic switching -~ technique is also applicable to conventlonal FM stereo broad-~ cast transmissions.
. .
' The system of the present invention enables .. . .
. stereophonic'sound to be broadcast over commercial television : 20 `~ channels and' faithfully and compatably reproduced in cQn-.`~ ' junctio~ wit~ conventional existing television receivers.
~ The system requires a minimal amount of additicnal transmittin~
~ equipment an~ minimal modification of èxisting receiving `` equip~ent. With modification the system may also pro~ide `- for compatable broadcast and reception of bilingual television .
` programming.
.
'l'elevision sound transmissions in accordance with the lnvention may be received on converters, either'of 3~:

the type wherein channel selection is accomplished independ-ently of the associated television receiver on which video information is being reproduced, or by means of the tuner contained in the receiver. Such converter may provide a low .level audio signal for amplific.ation on an external stereo amplifier/speaker system, or may provide high level audio and/or speakers for direct sound reproduction. A variation of the converter allows reception by means of a conventional stereo FM broadcast receiver. Alternatively, adapters may be integrally installed in existing-or newly constructed television rece;vers to achieve stereophonic sound reception.
. Improvements in the signal-to-noise ratio of the basic t~a~smission system are possible by . 1) Adopting a 25 microsecond preemphasis/deemphasis .15 time constant, .2) Enhancing the L-R component of the composite .signal, 3) Applying preemphasis and deemphasi~ to the composite signal, `~ . 4~ -Applying Dolby ~ype- signal processing to the L and ~ au~io si~nals, and~o~
5) Applyin~ Dolby Type-~ s~gnal pxocessing to the L-R component a~ the co~posite signal.
Further improvement is contemplated by selecting a pilot component having a frequency equal to 5/4 F~l to reduce the . _ : . . . . . . .

number o~ audible interference bands which result from hori-zontal scanning frequency harmonics within the L-R component sidebands.

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Claims (9)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A receiver for receiving stereophonic sound transmissions included on a television broadcast channel of defined frequency limits, wherein said sound transmissions comprise a sound carrier frequency-modulated by a composite signal including a first component representative of the sum of the left and right source signals, a second amplitude modulated subcarrier component representative of the difference between the left and right signals, said subcarrier component having upper and lower sidebands centered about a suppressed sub-carrier, and a pilot component having a frequency one-half the frequency of said suppressed carrier; said receiver comprising, in combination:
tuner means for converting said television broadcast channel to an intermediate frequency channel including a sound signal;
means for separating said sound signal from said inter-mediate frequency channel;
sound detector means for deriving from said sound signal a composite signal including said first, second and third components; and stereo demodulator means for deriving said left and right source signals from said composite signal.

2. A receiver as defined in Claim 1 wherein said sound signal is centered at 10.7 MHz.

3. A receiver as defined in Claim 1 wherein said tuner means comprise the tuner of an associated television receiver.

4. A receiver as defined in Claim 1 which includes second conversion means comprising a mixer stage and a con-tinuous wave oscillator, and wherein said sound signal is con-verted from a first intermediate frequency by said second conversion means to a second intermediate frequency and said second inter-mediate frequency signal is applied to said detector.

5. A receiver as defined in Claim 4 wherein said first intermediate frequency is 41.25 MHz and said second intermediate frequency is 10.7 MHz.

6. A receiver as defined in Claim 1 wherein said intermediate frequency channel includes an intermediate fre-quency video signal; and which further includes:
video bandpass filter means for separating said video signal from said intermediate frequency channel;
FM detector means for deriving from said intermediate frequency video signal an output signal representative of fre-quency shift in said video signal; and matrixing means for combining said output signal with said first component of said composite signal to compensate for frequency shift in said intermediate frequency channel.

7. A receiver as defined in Claim 6 which includes first and second mixer stages and wherein said sound signal is a first intermediate frequency and is applied to said first mixer stage to develop a sound signal at a second intermediate frequency, said sound signal being applied to said sound de-tector, and wherein the output of said video bandpass filter means is applied to said second mixer stage to develop an inter-mediate frequency video signal, and said intermediate frequency video signal is applied to said FM detector means.

8. A receiver as defined in Claim 7 which includes a continuous wave oscillator, the output of said oscillator being applied to said first and second mixer stages to develop said second intermediate frequency sound and video signals.

9. A receiver as defined in Claim 8 wherein said first intermediate frequency video signal is centered at 45.75 MHz, said first intermediate frequency audio signal is centered at 41.25 MHz, said second intermediate frequency video signal is centered at 6.2 MHz, said second intermediate frequency sound signal is centered at 10.7 MHz, and the frequency of said oscillator is 51.95 MHz.