CA1258120A - Tv stereo adapter - Google Patents

Abstract

TV STEREO ADAPTER

ABSTRACT OF THE DISCLOSURE
A television stereo adapter is provided for use with a television receiver to detect and decode stereophonic audio signals that may be transmitted with the composite television signals. The adapter includes an IF pick-up that is physically separate from the television receiver for picking up spurious IF audio signals that normally are generated by the television receiver. A stereo detector is coupled to the IF pick-up for detecting from the IF audio signals monophonic and stereophonic components that are transmitted with the composite television signals. The detected components are decoded into left-channel and right-channel audio signals; and these left-channel and right-channel signals are adapted to drive sound transducers, such as loudspeakers. Optionally, the left-channel and right-channel audio signals are amplified by gain-controlled amplifiers whose gains are a function of the output sound level of the television receiver.

Description

PATENT

l~S812(~

2 This invention relates to ~ television stereo 8 adapter and, more particularly, to such an adapter which is ~ physically separate from the conventional television 6 receiver with which it is used to detect and decode 6 stereophonic audio signals that may be transmitted but which 7 are not otherwise detectable by the elevision receiver.
8 Advantageously, no physical or electrical li. e. no wired) 9 I connections to the television receiver are needed for the 10 I successful operation of the stereo adapter. Optionally, 11 I however, enhanced results may be attained by connecting the 12 adapter to the usual multiplex output (or MPX) jack of the 13 I television receiver.
14 I Recently, the transmission of stereophonic audio 15 I signals for television broadcasting has been approved. The 16 ! format for such stereophonic audio broadcasting is the 17 so-called~Zenith system described in, for example, Audio 18 I Magazine, June 1984, pages 47-51, and also in Engineering 19 , Report GL-84091, National Semiconductor, September 1984, "A
20 ¦I Stereo TV Decoder for Zenith System". As described in the 21 ~' aforementioned publications, the Zenith system of 22 Il stereophonic audio broadcasting includes a monophonic 23 ¦ (L + R) component whose base band ranges from 0 to 15 KHz, a4 this monophonic component being frequency modulated onto the 26 usual audio (or sound) frequency carrier fS that normally 26 accompanies composite television signals, as is 27 conventional, a stereophonic (L - R) component that is 28 I amplitude modulated onto a subcarrier of frequency 2fH
29 ¦ (wherein f~ is the usual horizontal synchronizing frequency ~0 I of 15.734 ~Hz), the amplitude modulated stereo component 81 being of the double sideband suppressed carrier type, a æ ~ pilot signal of frequency fH, a so-called "second audio PA~ENT

l'~ ZO

1 program", referred to by those of ordinary skill in the art 2 as SAP, frequency modulated onto a subcarrier whose 8 frequency is 5fH, and an auxiliary channel, referred to as a ~ professional channel in which voice or data information may fi be transmitted, this auxiliary channel being frequency 6 modulated onto a subcarrier of frequency 6.5 f~. The 7 monophonic component, the amplitude modulated double 8 sideband suppressed carrier stereophonic component, the 9 I pilot signal, the second audio program and the auxiliary 0 ~ channel all frequency modulate the usual audio carrier fs~
11 , Since there is substantially no change in the monophonic 12 ~ (L + R) component, and since this monophonic component 13 , serves to frequency modulate the usual audio carrier fs~
14 ¦ conventional television receivers that are not equipped to 15 I detect or decode the multiplexed stereo signals nevertheless 16 can detect and demodulate the monophonic (L + R) component 17 ' in the conventional manner~ However, those television 18 ' receivers that are equipped with a stereo detector/decoder 19 I compatible with the~Zenith system will detect not only the 20 ~ monophonic (L + R) component but also the stereophonic 21 1 (L - R) component and, by conventional matrix decoding, 22 derive from the monophonic and stereophonic components 23 separate left-channel (L) and right-channel (R) audio 24 ~ signals. Hence, separate left-channel and right-channel 2~ loudspeakers may be driven to reproduce stereophonic sounds.
26 The aforementioned stereo-equipped television 27 receivers also may detect and decode the second audio 28 1 program which, typically, may provide bi-lingual (or 29 ~I translational) information associated with the broadcasted 80 1I television program. It also is contemplated that the 81 ~¦ information transmitted in the auxiliary channel will be æ 1 detected and utilized either by the television receiver or ~s e `Y\ ~ ~\~

.

PATENT

l~S~lZO

1 by other means connected to the television receiver and 2 supplied with the detected informatiQn.
8 To improve and enhance the signal-to-noi~e ratio 4 of the stereophonic (L - R) component, a noise reduction 6 companding system is used. More particularly, the 6 stereophonic component is subjected to noise reduction 7 compression prior to transmission and to expansion upon 8 reception. The particular noise reduction compression 9 I techni~ue that has been adopted for use with the ~enith 10 I system is the so-called dbx companding system which has been 11 ~ developed by DBX Inc. of Waltham, Massachusetts. It is, of 12 i course, expected that the stereo detector/decoder included lB ~ in stereo-compatible television receivers will be provided 14 I with a noise reduction expansion circuit that is compatible 15 ~ with the dbx compression circuit.
6¦l Although the monophonic (L + ~) component of the 17l transmitted stereo audio signals still will be detected by 18 ll conventional television receivers that are not equipped with 19l', stereo detectors/decoders, stereophonic sound reproduction, or the sensation (or simulation) of stereophonic sound will 21 not be reproduced. It has been proposed, heretofore, that 22j such conventional television receivers be modified (or 23 retrofitted) in order to detect and reproduce stereophonic 24 ' sound from TV stereo transmissions. Alternatively, separate 2S receivers capable of receiving the audio portion of 26 television transmissions, that is, the audio carrier fs~ and 27 detecting and decoding the stereo signals therefrom, would 28 have to be purchased.
~9 I The aforementioned proposals of retrofitting conventional television receivers to include a stereo $1~l detector/decoder or purchasing special TV stereo sound æ ~I receivers not only are expensive but, in many instances, ! 1

-3- !

PATENT

l;~lZO

1 will not be preferred by the several million consumers who 2 now own conventional television receivers that are not 8 compatible with stereo TV transmissions. Consequently, it

4 is believed that, once stereo TV transmissions become more ~ prevalent, there will be a need for a relatively ~imple 6 adapter that can be used without significant modifications 7 to conventional television receivers. Advantageously, such 8 a stereo TV adapter should be physically and electrically 9 independent of the television receiver, thus requiring no internal wiring or soldering which otherwise would be 11 ' necessary for the adapter to be compatible and usable with 12 , the television receiver. ln particular, it is desirable to 13 I permit the adapter to pick up the received audio portion of 14 I the television program without providing a wired circuit 15 I connection between the adapter and the conventional 16 I components of the television receiver.
~q j OBJECTS OF THE INVENTION
18 l, Therefore, it is an object of the present 19 l invention to provide a TV stereo adapter for use with a 20 1' television receiver for the purpose of detecting and 21 l' decoding stereophonic audio signals that are transmitted z with composite television signals without requiring 23 ! hard-wired interconnections between the adapter and the 24 , television receiver.
2$ Another object of this invention is to provide a 26 TV stereo adapter of the aforementioned type having a 27 pick-up for picking up the audio portion of a received 28 television program, the pick-up being physically independent 29 of the television receiver.
80 ~ A further object of the invention is to provide a 81 I TV stereo adapter of the aforementioned type which need not ll l 1 -4- , PATENT
qO7020-2050 1~5~0 1 be re-tuned or re-adjusted as the television receiver is 2 tuned from one broadcast channel to another.
8 Another object of this invention is to provide a 4 TV stereo adapter of the aforementioned type in which the ~ pick-up is an IF pick-up which senses spurious IF audio 6 signals that normally leak from the IF section of 7 conventional television receivers.
8 A still further object of this inven~ion is to 9 provide a TV stereo adapter of the afor*mentioned type in 10 ¦ which the spurious IF audio signals are picked up and, if a 11 ', stereophonic component is present, left-channel and 12 I right-channel audio signals are decoded from the picked up lB I IF audio signals.
14 ¦ Yet another object of this invention is to provide 15 i a TV stereo adapter of the aforementioned type in which the 16 decoded left-channel and right-channel audio signals are lq 1 subjected to automatic gain control such that the level of 18 ~' these signals is adjusted as a function of the sound volume 19 1 aajustment control le. g. the remote sound level control) 20 j' normally provided with a conventional television receiver.
21 1 A still further objèct of this invention is to z 1 provide a TV stereo adapter of the aforementioned type in 23 1I which the sound level of the decoded left-channel and 24 ri~ht-channel audio signals is controlled as a function of 26 the output sound level of the television receiver.
26 Various other objects, advantages and features of 27 the present invention will become readily apparent from the 28 ensuing det~iled description, and the novel features will be 29 1I particularly pointed out in the appended claims.

81 ~1 In accordance with this invention, a TV stereo æ adapter is provided for use with a ~elevision receiver to .

' -5- 1 PATENT

1~581ZO

1 detect and decode 6tereophonic audio signals that are 2 transmitted with the composite television signals of a 8 stereo television program. The adapter includes an IF
4 pick-up that is physically separate from the television 6 receiver fcr picking up spurious IF audio signals that 6 normally are generated, or leaked, by the television 7 receiver. A stereo detector detects monophonic and 8 stereophonic components from the picked up IF signals, and 9 left-channel and right-channel audio signals are decoded 10 ¦ from the detected components. The decoded left-channel and 11 ¦ right-channel signals are utilized to drive loudspeakers, 12 headphones, or the like.
13 In accordance with one aspect of this invention, 14 1 the IF pick-up comprises a pick-up probe that is 15 I positionable proximate the IF section normally provided in 16 ¦ the television receiver. Preferably, this pick-up probe is 17 i an inductive probe tuned to the audio IF subcarrier 18 I frequency for inducing the flow therein of IF audio signals.
19 , In accordance with another aspect of this 20 11 invention, the decoded left-channel and right-channel 21 '' signals are subjected to automatic gain control in response, 2~ at least in part, to the output sound level of the 23 1 television receiver. As one feature, the output sound level 24 of the television receiver is derived from a microphone 26 which picks up the sounds produced by the loudspeaker 26 normally provided in or with the television receiver.
27 Alternatively, the output sound level of the television 28 I receiver may be detected by connecting a plug to the usual 29 ¦I headphone jack provided with most television receivers In 80 ¦l either embodiment, the signal representing the output sound 81 ¦ level of the television receiver is compared to a decoded ~2 ~' signal, and the difference between the compared signals is PATENT

i~ l20 1 utilized to adjust the gain of the left-channel and 2 right-channel signals. As another feature, the decoded 8 signal to which the sound-representing signal is compared is 4 selectively supplied to the comparator by a ~witch that is ~ closed when the detected monophonic (L + R) component 6 exceeds a predetermined threshold level and is opened when 7 the monophonic component is less than that threshold level.
8 In accordance with yet another aspect of this 9 invention, the adapter is provided with a spatial/stereo/
10 I pseudo-stereo change-over circuit that is coupled to the 11 I stereo decoder and is responsive to the detection of the 12 stereophonic (L - R~ component. If this stereophonic 13 I component is present, the decoded left-channel and 14 l, right-channel signals simply are amplified. If the 15 , stereophonic component is not present, resulting in 16 ¦ substantially identical output signals produced by the , decoder, one of those output signals is filtered and used to 18 I simulate the other channel, thus resulting in pseudo 19 1l left-channel and right-channel signals.
20 ' As yet another aspect of the adapter, the detected 21 1, stereophonic (L - R) component is subjected to noise 22 reduction expansion compatible with the noise reduction 23 ¦~ compression applied to the stereophonic component prior to 24 its transmission. The adapter thus is compatible with the 26 noise reduction technique utilized by the broadcaster of the 26 composite television signal.
Z7 ~ BRIEF DESCRIPTION OF THE DRPWINGS
28 The following d~tailed description, given by way 29 ¦! of example, and not intended to limit the present invention ao~I solely to the embodiments shown and described herein, will 81 I best be understood in conjunction with the accompanying æ ¦ drawings in which:
I

i !

PATENT
4b7020-2050 lZ581Z~

1 FIG. 1 is a schematic/block representation of the 2 manner in whi~h the present invention is used;
8 FIG. 2 is a block diagram of a preferred ~ embodiment of this invention;
6 FIG. 3 is a schematic diagram of one embodiment of 6 the pick-up probe that may be used with this invention;
7 FIG. 4 is a partial block, partial schematic 8 diagram of the gain control circuit that may be used with 9 I this invention;
10 I FIG. 5 is a graphical representation of the video 11 1l and audio transmission spectra; and 12 I FIG. 6 is a graphical representation of the stereo 18 I spectrum for the audio portion that accompanies the ~4 television program.
DETAILED DESCRIPTION OF A CERTAIN PREFERRED EMBODIMENT

16 Referring now to the drawings, wherein like 17 , reference numerals are used throughout, and in particular to 18 I FIG. 1, a representation of the manner in which stereo 19 adapter 10 of the present invention is used with a 20 I conventional television receiver 12 is depicted. The 21 I television receiver is assumed, for the purpose of the z Ij present description, to be of the type which does not 23 1 include stereo detecting/decoding circuitry and, thus, is a 24 I television receiver with which the present invention finds 26 ready application. It will, nevertheless, be appreciated 26 that even if television receiver 12 includes a stereo 27 detector/decoder, adapter 10 may be utilized therewith.
28 The illustrated television receiver includes a 29 cabinet in which the usual circuitry, television picture 90 ¦ tube, loudspea~ers, manual selectors and manual controls are 81 ~ housed. As depicted, the manual selectors may comprise 82 conventional channel selecting push-buttons; although the I

PATENT

l;ZS8~LZV

1 rotary turret-type channel selector may be provided.
2 Television receiver 12 also includes a sound volume 8 adjustment 14 which, as is conventional, is operated by the 4 user to increase or decrease the volume of sound emanating ~ from loudspeaker 16. It is expected that other controls, 6 such as picture brightness, picture contrast and color 7 controls, are included in television receiver 12. In 8 addition, a headphone jack 18 may be provided in the 9 cabinet, this headphone jack being conventional and 0 ! connectable to a headphone plug to enable the user to listen 11 I to the audio portion of broadcasted television programs 12 I without disturbing others. If this headphone jack is 13 ~ provided, it is appreciated that sound volume adjustment 14 14 ¦ operates to adiust the sound level of the audio signals 1~ I reproduced from the headphone jack.
16 ¦ Stereo adapter 10 includes, broadly, an IF probe 17 20 and a stereo detector/decoder 22, the latter operating in 18 I conjunction with the IF probe to produce left-channel (L~
19 , and right-channel (R) audio signals. Stereo 20 ¦ detector/decoder 22 is coupled to a pair of loudspeakers 24 21 1 and 26 for reproducing left-channel and right-channel 22 ¦1 audible sounds. As an optional feature, a microphone 28 may 2% ~I be connected to stereo detector/decoder 22 for supplying 24 I thereto electrical signals representing the output sound 2~ level emanating from loudspeaker 16. As an alternative, 26 microphone 28 may be replaced by a suitable plug that mates 2~ with headphone jack 18. The purpose of supplying ~tereo 28 detector/decoder 22 with signals representing the output 29 I sound level of television receiver 12 will be discussed 80 I below.
In both conventional audio transmissions and æ stereo transmissions that accompany television broadcasting, 1, _ 9_ PATENT

l;~lZO
1 the audio signals are modulated onto an audio carrier that 2 is separate from the carrier onto wh~ch the video signals 8 are modulated. As used herein, the expression ~composite 4 television signals~ means both the video and audio signals ~ which are transmitted by a television broadcaster for 6 reception at various ones of television receivers. Th~
7 frequency fs of the audio carrier is 4.5 MHz greater than 8 the frequency fv of the video carrier. This frequency 9 difference of 4.5 MHz remains substantially constant even 10 I though different carriers are used for different television 11 ~ channels. Some of these frequencies are as follows:
12 I Channel _v MHz f5 MHz 13 2 55.2~ 59.75 14 1 4 67.25 71.75 ~5 ~ 5 77.25 81.75 16 7 175.25 179.75 17 9 187.25 191.75 18 1l 11 199.25 203.75 ~9 ~j 13 211.25 215.75 2d 1 It is seen/ therefore, that althou~h the video and audio 21 I carriers fv and f5 vary from one channel to the next, the z ¦l audio carrier for each channel remains 4.5 MHz greater than 23 ~ the video carrier.
24 Television receiver 12 includes the usual IF
2~ section which, as is known, converts the different video 26 carriers fv tc a common video IF carrier whose frequency is 27 45.75 MHz, and converts the various audio carriers fs to a 28 I common audio IF carrier of freq~ency 41.25 MHz. Of course, 29 ~ the video IF carrier contains the various video components 80 ¦I modulated ~hereon and the audio IF carrier contains the 81 ¦¦ audio signals modulated thereon~ Since the video and audio 82 IF carriers remain separated by the aforementioned 4.5 MHz, 1, - 1 0- ~

PATENT

1~8i~0 1 simple filtering may be used to separate the video and audio 2 information and to process and reproduce that information 8 independently. As is also conventional, the IF section of 4 television receiver 12 further converts the audio IF carrier 6 to 4.5 MHz, and qenerates leakage audio IF signals. Such 6 leakage, or spurious, IF audio signals are detected by IF
7 probe 20 which, as will be described below, is tuned to the 8 audio IF carrier frequency of ~.5 M~z. In the preferred 9 embodiment, the IF prob~ is an inductive probe comprised of a tuned transformer. It is contemplated, however, that 11 other devices may be used, such as a capacitive pick-up 12 I probe.
13 1l An advantageous feature of utilizing IF probe 20 14 ! is the omission of any physical or electrical connections 15 ~ between that probe and the internal circuitry of television 16 , receiver 12. The signal strength of the leakage, or 17 I spurious, IP audio signals normally generated by the IF
18 1 section of the television receiver is sufficient to be 19 l detected and to induce within probe 20 the flow of IF audio 20 1 signals. Probe 20 merely need be placed on, around or under 21 1 the cabinet of television receiver 12. Preferably, the Z l, probe should be located proximate the physical p~sition of 23 !I the IF section of the television receiver. If accessible, 24 therefore, probe 20 can be positioned within the cabinet at 26 that location. Satisfactoxy results are obtained, 26 nevertheless, by disposing probe 20 externally of the 27 cabinet. As an exampie, the probe may comprise a suitable 28 inductance pick-up, such as a cup core having a transformer 29 1I wound thereon, the probe assembly being secured to a suction ~ cup or other means by which the probe may be mounted or 332 otherwise adhered to the cabinet oi television receiver 12.
!

~' -11-PATENT

:l~ZS81'~0 1 In operation, the IF audio signals induced in 2 probe 20 are supplied to stereo detectQr/decoder 22 whereat 8 the IF audio signal is demodulated, and the pxesence of a stereophonic component is detected and decoded into separate ~ left-channel and right-channel signals which, for 6 convenience, are identified hereinafter as the L and R
7 signals. In the event that the transmitted audio signals 8 which accompany the program to which television receiver 12 9 is tuned merely are monophonic signals and, thus, do not include a stereophonic component, stereo detector/decoder 22 11 may produce simulated left and right channel signals, such 12 simulated signals being referred to hereinafter as L' and R' lS signals.
14 I Thus, even if television receiver 12 is not 15 I equipped to detect and decode transmitted stereo audio 16 I signals, adapter 10 may be used therewith to recover such 17 1I stereo signals. Adapter 10 provides the further advantage 18 I of producing simulated L' and R' signals even if the 19 I transmitted audio signals merely are monophonic signals.
Optionally connected microphone (headphone plug) 21 ~ 28 is used to control the output level of the L and R (or L' 22 1l and R') signals produced by adapter 10. Typically, sound 23 ¦I volume adjustment 14 of television receiver 12 is operated 24 I to increase or decrease the gain of the audio amplifier 26 which drives loudspeaker 16. This gain adjustment usually 26 is set manually and is not varied as a function of the 27 magnitude of the demodulated audio signal. Since the user 28 I of television receiver 12 will be accustomed to adjusting 29 I the sound output level thereof by means of this sound volume 80 ~ adjustment, it is preferable that adapter 10 not be provided 81 I wi~h its own independent volume adjustment. Rather, the 82 audible volume emanating from loudspeakers 24 and 26 l -12-1 ~

PATENT

1;~58~ZO
1 preferably should be established by $he user's operation of 2 sound volume adjustment 14. Microphone 28 operates to sense 8 the output sourd level generated by loudspeaker 16 and set ~ the gain of the L and R (or L' and R') signals produced by 6 stereo detector/decoder 22 as a function of that sensed 6 output sound level~ Hence, if the user has established a relatively low volume output level, the L and R signals are 8 subjected to a relatively lower gain. Conversely, if the 9 ~ user has established a higher output volume, the L and R
10 l signals are subjected to a relatively higher gain.
11 I Microphone 28 may be disposed in a housing separate and 12 , apart from IF probe 20 or, alternatively, both devices may 13 ~ be provided in a common housing. As a further alternative, 14 the output sound level to which television receiver 12 has 15 j been set by sound volume adjustment 14 may be sensed by 16 ll connecting a plug to headphone jack 18, thus replacing 17 1I microphone 28 with that plug.
18 I Notwithstanding the optional use of a microphone, 19 plug or other means to sense the output sound level of television receiver 12, adapter 10 may be provided with a 21 I manually adjustable sound level control, if desired.
22 Turning now to FIG. 2, there is illustrated a 23 I block diagram of a preferred embodiment of adapter 10. The 24 adapter, as illustrated in greater detail herein, is 2$ comprised of IF probe 20, a stereo detector 38, a stereo 26 decoder 50, noise reduction circuitry 48 and S4, and output 27 terminals for supplying the decoded L and R signals to, for 28 example, loudspeakers or other suitable transducers. IF
29 I probe 20 is comprised of a tuned transformer 30, the primary 80 I of which bein~ connected to a capacitor 32 such that the LC
81 I resonant frequency is tuned to the audio IF frequency. A
82 I tuned transformer 34 is coupled to IF probe 20 and a tuned I

j -13-PATENT

i'~ S 8 ~Z O

1 amplifier 36 is coupled to this tuned transformer. The 2 combination of tuned transformers 30 and 34 and tuned 8 amplifier 36 effectively eliminate extraneous signals from 4 the IF audio signal.
~ Stereo detector 38 is adapted to detect the usual 6 monophonic component and the stereophonic component, if the 7 latter is transmitted from the IF audio signal. Using 8 conventional notation, the monophonic component is 9 I identified as the (L ~ R) component and the stereophonic 10 ~ component is identified as the (L R) component. The 11 I monophonic and stereophonic components are frequency 12 modulated onto the audio IF carrier and, in accordance with 13 I the Zenith system mentioned above, the IF audio carrier also 14 I is frequency modulated with a pilot signal of frequency f~
15 I (the horizontal synchronizing frequency), with a second 16 li audio program (modulated onto a subcarrier of frequency 5 17 I fH) and with an auxiliary channel modulated onto a 18 ¦I subcarrier of frequency 6.5 fH. The spectrum of the audio 19 1~l signals in the Zenith system is illustrated in FIG. 6. The 20 I purpose of stereo detector 38 is to separate the monophonic 21 11 (L + R) and stereophonic (L - R) components while Z 1 suppressing the second audio program and auxiliary channel 23 from interfering with the separated monophonic and 24 stereophonic components. As illustrated in FIG. 2, the a6 stereo detector may be comprised of a limiter 40, a 26 quadrature detector 42, a trap 44 and a stereo demultiplexer 27 46, all of these components being conventional and, thus, 28 I not being described in detail herein.
29 I Limiter 40 is coupled to tuned amplifier 36 and is 80 I adapted to eliminate or at least minimize amplitude 81 fluctuations in the frequency modulated audio IF si~nal.
82 I The quadrature detector is coupled to limiter 40 and ll j` -14-PATENT

1i~81~U
1 functions in a manner known to those of ordinary skill in 2 the frequency modulator/demodulator art to recover the 8 monophonic (L + R) and ætereophonic (L - R) components from 4 the frequency modulated IF audio signal. The quadrature ~ detector includes means to recover the pilot signal and to 6 utilize that pilot signal ~o detect the stereophonic ~ component (L - R) which, as is conventional, is modulated 8 onto a subcarrier of twice the pilot frequency, or 2 fH.
g Quadrature detector 42 also may include circuitry for recovering the second audio program and the auxiliary 11 channel, each being modulated onto a separate subcarrier 12 that is a multiple of the pilot frequency. However, such 13 I further detection or demodulation of the second audio 14 program and auxiliary channel are not necessary ~or the successful operation of the adapter in accordance with the 16 present invention.
17 ~ The output of quadrature detector 42 is coupled 18 I through trap circuit 44 to stereo demultiplexer 46. The 19 I trap circuit may comprise, for example, a filter adapted to 20 ¦I block higher frequency components such as the second audio 21 , program and auxiliary channel which are modulated onto 22 I higher frequency subcarriers. Stereo demultiplexer 46 thus 23 ~ is supplied with the monophonic and stereophonic components 24 ~ and serves to separate and supply these components to 26 separate outputs, as illustrated. Stereo demultiplexer 46 26 thus may comprise a frequency demultiplexer which 27 discriminates between the relatively lower frequencies 28 ~ occupied by the monophonic component (L ~ R) and the 29 ¦ relatively higher frequencies occupied by the stereophonic 80 I component (L - ~) which, as mentioned above (and shown in 81 1I FIG. 6), is modulated onto a subcarrier that is twice the $2 1I pilot frequency. The monophonic component (L ~ R) is I

Il -15-PATENT

1'~5~f,0 1 coupled to stereo decoder 50 and the stereophonic component 2 (L - R), is demodulated from its subcarrier and then is 8 supplied to decoder 50 via a noise reduction expander 48.
~ The individual components illustrated as 6 comprising stereo detector 38 may be formed as, for example, 6 an FM detector ~chip~ such as a Sanyo Model LA1235, coupled 7 to a multiplex de~oder ~chip", such as Model ~M1884 8 manufactured by Nati~nal Semiconductor 9 The adapter of the present invention is compatible 10 I with the Zenith system of stereophonic TV transmission, as 11 I mentioned above. Since the Zenith system employs noise 12 ¦ reduction compression at the transmitter, the adapter of the 18 ~ present invention is provided with a compatible noise 14 I reduction expander 48. The noise reduction system utilized 15 I by the Zenith system has been developed by DBX Inc. of 16 ~ Waltham, Massachusetts. Accordingly, noise reduction 17 expander 48 comprises a dhx expander which is described in, 18 ¦I for example, Audio Magazine, June 1984, pages 47-51.
19 ¦' Expander 48 restores the stereophonic component (L - R) to 20 11 its original form (i. e. prior to its being subjected to 21 I noise reduction compression), and the restored component is æ . supplied to decoder 50.
23 1I Decoder 50 may comprise a conventional matrix 24 decoder which, as is conventional, functions to sum the 26 monophonic and stereophonic components and to subtract these 26 components from each other. Decoder 50 thus produces 27 respective channel-separated output signals identified in 28 FIG. 2 as 2L and 2R. These decoded signals are supplied to 29 I a spatial/stereo/pseudo-stereo soun~ circui~ 52.
90 I Circuit 52 prefera~ly is a Model TDA 3810 circuit 81 ~ manufactured by Signetics Corporation of Sunnyvale, 82 Calif~rnia. Although not shown in FIG. 2, circuit 52 PATENT

i~81~0 1 operates in response to the actuation of selector 2 pushbuttons connected thereto. One of these pushbuttons is 8 designated the ~stereo synthesizer" selector which, when 4 actuated, closes a switch to utilize one of the output 6 signals produced by decoder 50, such as the left-channel 6 output signal 2L, for the pulpose of simulating the other 7 channel signal (i. e. to simulate the right-channel signal).
8 It is appreciated that, in the absence of the stereophonic 9 component lL - R), as when a stereophonic transmission is not present, decoder 50 supplies at its outputs two 11 substantially similar signals. Circuit S2 synthesizes 12 I left-channel and right-channel signals from one of those 13 output signals. In the presence of the stereophonic 14 j component, the "stereo synthesizer" selector is disabled, 15 ¦ and the output signals produced by decoder 50 merely are 16 I amplified by circuit 52. Another of the selector 17 ~ pushbuttons is designated the "expander" selector which, 18 I when actuated, closes another switch to increase the spatial 19 I separation effect of the left-channel and right-channel stereo signals. Further details of the construction and 21 !j operation of circuit 52 are set out in literature published æ I by Signetics Corporation in January 1984. Circuit 52 may be 23 !1 thought of as producing actual or simulated left-channel and 24 right-channel output signals.
The respective output signals produced by circuit 2~ 52 are coupled to a conventional dynamic noise reduction 27 circuit 54 which is adapted to provide~ at it~ respective 28 outputs, L and R signals. These outputs of noise reduction 29 1 circuit 54 ~ay be connected directly to output terminals or, 80 ~ as indicated in FIG. 2, may be connected to those output ~1 I terminals by means of gain controlled amplifiers 56 and 58.
82 I It will be appreciated that, in acrordance with ~he broader ll PATENT

1'~81f~0 1 aspects of the present invention, such gain controlled 2 amplifiers may be thought of as being optional.
8 Gain controlled amplifiers 56 and 58 are similar, 4 and each preferably is comprised of a transconductance 6 amplifier whose gain is set or adjusted by means of a DC
6 gain controlled signal supplied thereto. The gain, or 7 magnitude, of the L and R siynals thus is adjusted or varied 8 as a function of the DC gain control signal.
9 I A gain control signal generator 60 is coupled to 0 I gain controlled amplifiers 56 and 58 to supply the 1 aforementioned DC gain control signal thereto. Control 12 ' signal generator 60 is comprised of a comparator 62 having 18 ~ one input connected to a sound pick-up 64 and another input 14 I connected to receive one of the left-channel and 15 ¦ right-channel output signals produced by decoder 50. In the 16 I embodiment illustrated herein, the right-channel output 17 ¦I signal 2R is supplied from decoder 50 to comparator 62. It ~8 I will be appreciated, however, that the left-channel output 19 l~ signal 2L may be supplied to the comparator, if desired.
20 1`~ Sound pick-up 64 comprises microphone 28 (PIG. 1) 21 1 or, alternatively, the plug which may be inserted into 22 1I headphone jack 18 of television receiver 12. It is 28 I appreciated, therefore, that sound pick-up 64 supplies to 24 comparator 62 a signal representing the output sound level 26 of the television receiver. This signal varies not only as 2S a function of the ampli~ude of the audio signal that is 27 received by television receiver 12 but also as a function of 2~ i the setting of sound volume adjustment 14.
29 i; Comparator 62 functions to compare the level of ~0 i the signal representing the output sound level of the 81 I television receiver to the signal level of the decoded 82 light- ~or left~ channel ~ignal produced by decoder 50. The ¦ PATENT

l;~S~ 0 1 difference between these compared signals controls the gain 2 of gain controlled amplifiers 56 and 58. For the purpose of ~ the present description, it is assumed that comparator 62 4 includes inverting and non-inverting inputs so as to produce ~ a DC gain control signal that is a function of the 6 difference between the signals supplied thereto. Sound q pick-up 64 is illustrated as being coupled to the B non-inverting input of comparator 62 and the right-channel 9 output signal produced by decoder 50 is illustrated as being 10 ¦ coupled to the inverting input of the comparator. As the 11 ¦ difference between the signal representing the output sound 12 I level of the television receiver and ~he decoded lS ~ right-channel signal increases, the gain of the gain ¦ controlled amplifiers likewise increases. Conversely, as 1~ ¦ this difference decreases, the gain of ~mplifiers 56 and 58 16 ~ is reduced. Comparator 60 preferably comprises a 17 I differential integrator, described in greater detail below.
18 To prevent comparator 62 from inadvertently 19 I changing the DC gain control signal produced thereby during 20 1 pauses or periods of silence in the audio signal transmitted 21 ~ to the television receiver, it is preferred that the æ I right-channel output signal produced by decoder 50 be 23 1! uncoupled from the comparator during such pauses or periods 24 of silence. To this effect, a switch 66 couples the 2S right-channel output signal from decoder 50 to comparator 2æ 62, this switch being ~opened~ and ~closed" as a func$ion of 27 the signal level of the monophonic component (L + R). As 28 I illustrated~ the monophonic component produced by s~ereo 29 I demultiplexer 46 is coupled to a level detector 68 which ao i senses when this monophonic component is above or below a 81 I predet~rmined threshold. When the monophonic component ~2 l PATENT

1;~581;~0 1 (L + R) is below that threshold, level detector 68 supplies 2 a signal to switch 66 to effect the "opening" of that 8 switch, thereby decoupling the right-channel output signal 4 ~rom comparator 62. Conversely, when the monophonic 6 component (L + R) exceeds the aforementioned threshold 6 level, switch 66 is ~closed" to couple the right-channel 7 output signal to the comparator.
8 The manner in which the adapter illustrated in 9 I FIG. 2 operates now will be briefly described. IF probe 20 10 I picks up the audio IF signal that is produced as a leakage 11 I signal by the usual IF section included in television 1~ ¦ receiver 12. By reason of tuned transformer 30, tuned lB transformer 34 and tuned amplifier 36, the picked up IF
14 I signal is suitably amplified and also is isolated from most 16 ~ electrical noise. The IF audio signal supplied ts stereo 16 I detector 38 is comprised of a 4.5 MHz carrier that is 17 I frequency modulated with the monophonic component (L I R), 18 I the stereophonic component (L - R) which, in turn, is 19 ¦ modulated onto a subcarrier whose frequency is twice the frequency of the pilot signal, the pilot signal itself, a 21 ¦I second audio program which is modulated onto a subcarrier 22 ! whose frequency is five times the frequency of the pilot 23 ~ signal, and an auxiliary channel that is modulated onto yet 24 another subcarrier whose frequency is 6.S times the 26 frequency of the pilot signal. Stereo detector 38 functions 26 to "trap" or block the second audio program and the 27 auxiliary channel, and to demodulate and demultiplex the 28 I monophonic and stereophonic components which are produced at 29 I the respective outputs of the detector.
80 ~ ~he detected stereophonic component (L - R) is 81 I subjected to noise reduction expansion compatible with the æ noise reduction compression to which it had been subjected l -20-PATENT

1;~i81~) 1 at the transmitter. Thus returned to its original form, the 2 stereophonic component (L - R) is supp.lied to decoder 50 8 along with the detected monophonic component (L + ~). As is conventional, decoder 50 decodes the left-channel and ~ right-channel signals fxom the monophonic and stereophonic 6 components supplied thereto, and these decoded signals are 7 supplied to spatial/stereo/pseudo-stereo circuit 52.
8 Circuit 52 may be omitted, if desired. In the event that a 9 stereophonic transmission has been received, actual L and R
output signals are provided at the outputs of circuit 52.
11 However, if a stereophonic transmission is not present, 12 ! circuit 52 produces simulated L' and R' output signals.
1~ ~ The output signals, either L and R or L' and R' 14 ~ signals, provided by circuit 52 are subjected to dynamic 1~ ~ noise reduction and then the gains of the L and R (or L' and 16 I R') signals are adjusted accordingly by gain controlled 17 amplifiers 56 and 58.
18 I Gain controlled amplifiers 56 and 58 respond to 19 ~ the DC gain control signals supplied thereto by gain control 20 1~ signal generator 60 so as to amplify the L and R audio 21 I signals as a function, at least in part, of the setting of 22 sound volume adjustment 14 of the television receiver.
23 During a pause or period of silence, the level of the 24 demultiplexed monophonic component (L ~ R) falls below the 26 threshold level established by (or supplied to3 level 26 detector 68, thereby "opening~ switch 66. This uncouples 27 the right-channel signal produced by decoder S0 from being 28 supplied to comparator 62. At the same time, it is expected 2g I that the output sound level of the television receiver is 80 I very low, and a correspondingly small (or zero) level signal 81 ¦ is supplied to the non-inverting input of the comparator 82 j from sound pick-up 64. As a result, during such pauses or ' PATENT

1;~

1 periods of silence, no change is imparted to the DC gain 2 control signal supplied to the gain controlled amplifiers.
8 Upon the next occurrence of an audio 6ignal, the previously established gain of these amplifiers results in ~ amplification of the L and R signals corresponding to the 6 setting of sound volume adjustment 14. Hence, there are no 7 sudden increases in the L and R signal levels which would 8 otherwise result in sudden loud sounds emanating from 9 loudspeakers 24 and 26. Likewise, there is no undesired 10 I negative chanqe in the DC gain control signal during such Il I pauses or periods of silence which otherwise would result in 12 I very low level sounds emanating from loudspeakers 24 and 26 18 I upon the succeeding presence of a transmitted audio signal.
14 It is appreciated that, if desired, television 15 I receiver 12 may be used with a so-called remote control 16 I device by which the user may adjust the sound level (among 17 I other parameters) from a remote location. Gain controlled 18 I amplifiers 56 and 58 together with gain control signal 19 I generator 60 are adapted to function in the manner described above to amplify the L and ~ audio signals as a function, at 21 l' least in part, of the setting of such a remote sound 22 I adjustment control device for that television receiver.
23 I Turning now to FIG. 3, there is illustrated a 24 schematic diagram of IF probe 20, tuned transformer 34 and 26 tuned amplifier 36. IF probe 20 has been described above 26 with respect to FIG. 2, and is illustrated o~ce again as 2~ being comprised of a transformer 30 whose primary winding is 28 I coupled to a capacitor 32, the values of the capacitance of 29 I capacitor 32 and the inductance of the transformer being such that the resonant frequency thereof is equal to 4.5 81 ~I MHz, the audio IF frequency. The secondary winding of a2 1~ transformer 30 is coupled to the primary winding of tuned PATENT

1'~81ZO

1 transformer 34; and the secondary winding of this latter ~ transformer is coupled to the input (ox gate) electrode of 8 an FET 70. It is seen that the secondary winding of 4 transformer 34 also is connected in parallel with an RC
6 circuit whose values are selected, in combination with the 6 inductance of transformer 34 to provide a resonant frequency 7 equal to 4.5 MHz.
8 Tuned amplifier 36 is comprised of FET 70, a tuned 9 transformer 72, another FET 74 and another tuned transformer 10 ~ 76. Although FET's are illustrated as being the amplifying 11 transistors, it is appreciated that other equivalent 12 I amplifying transistors may be used, if desired.
lB I The input, or gate, electrode of FET 70 is coupled 14 ~ to tuned transformer 34, as mentioned above, and a bias voltage is generated at the source electrode by means of the ~6 'I illustrat d RC circuit connected thereto. Tuned transformer 17 l 72 is connected in the drain circuit of FET 70, and the 18 I primary of this transformer is connected to another RC
19 ¦' circuit, resulting in a resonant fre~uency equal to 4.5 MHz.
The secondary winding of tuned transformer 72 is connected 21 I to the gate electrode of FET 74 whose source electrode is 22 I supplied with a bias voltage by reason of the illustrated RC
23 circuit connected thereto.
24 1 Finally, the drain electrode of FET 74 is 25 ¦ connected to tuned transformer 76 whose primary winding is 26 connected in parallel with the illustrated RC circuit to 27 I resonate at 4.5 MHz. The secondary winding of transformer 28 , 76 supplies an amplified IF audio signal to limiter 40 of 29 stereo detector 38.
80 I Thus, it is seen that the multiple stages of 81 amplification provide adequate gain, or boosting, to the IF
æ 1 audio signals which are picked up by IF probe 20.

PATENT
407~20-2050 l;~S81ZO

1 Furthermore, the use of successive stages of tuned 2 transformers substantially eliminates extraneous noise and 8 interference from the picked up IF audio sig~als.
4 A preferred embodiment of gain control signal B generator 60 now will be described in conjunction with the 6 partial block, partial schematic diagram of FIG. 4.
7 Comparator 62 is comprised of a differential integrator 84 8 having a non-inverting input coupled to sound pick-up 64 by 9 ¦ means of a rectifying circuit formed of amplifier 78, low 10 ¦ pass filter 80 and rectifier 82. The differential 11 I integrator also includes an inverting input coupled to a 12 ~ rectifying circuit comprised of low pass filter 90 and 13 ¦ rectifier 92, the output of this rectifying circuit being 14 ¦! connected through an FE~ switch 94 which, as is appreciated, ~ is a preferred embodiment of aforementioned switch 66 (FIG.
16 1 2). The output of differential integrator 84 is coupled to 17 I a capacitor 86 which is connected in the resistive feedback 18 I circuit thereof, and the output of the differential 19 I integrator also is connected, by means of a resistor, to a 20 1I storage capacitor 88.
21 j FET switch 94 has its gate electrode connected to 22 1l monophonic level detector 68, which, as illustrated in FIG.
23 1 4, is comprised of an amplifying transistor 100 whose base 24 electrode is supplied with the monophonic component (L + R) 2~ from stereo detector 38 (PIG. 2) and whose collector 26 electrode is connected to a threshold comparator comprised 27 of a rectifying diode 102 and a zener diode 104. The 28 emitter electrode of transistor 100 is supplied with a bias 29 I ~oltage by means of the illustrated RC circuit connected 80 I thereto.
81 The cathode of diode 102 included in the threshold 82 I comparator is connected to the zener diode, thus limiting ,! -24-PATENT
4~7020-2050 .~ ~S8120 1 the maximum voltage level that may be present at the aJJode 2 of diode 102. As illustrated, this anode is connected to 8 the gate electrode of FET switch 94 which, preferably, 4 comprises a p-type FET.
~ The DC voltage produced across capacitor 88 at the 6 output of differential integrator 84 is supplied in common 7 to the aforementioned gain controlled amplifiers which, as 8 described above and as illustrated herein, preferably ~ comprise conventional transconductance amplifiers 96 and 98, 10 I the gains of these amplifiers being controlled as a function 11 of the DC voltage applied thereto. The outputs of 12 transconductance amplifiers 96 and 9B are connected to 15 I loudspeakers 24 and 26 to drive these loudspeakers to 14 I reproduce the left-channel and right-channel audible sounds.
15 ~ In operation, sound pick-up 64 produces a signal 16 I representing the sound level of television receiver 12, as 17 ~ described above. This signal is amplified and rectified to 18 I supply a corresponding DC signal to the non-inverting input 1~ I of differential integrator 84. It is appreciated that the level of this DC signal varies as a function of the output 21 1I sound level of the television receiver.
22 I The amplified R signal produced by 23 ! transconductance amplifier 98 is fed back and rectified to a 24 DC signal, this DC signal being supplied via FET switch 94 26 to the inverting input of the differential integrator.
2S Although the R signal is seen to be fed back to the 27 differential integrator, it is readily appreciated that, if 28 desired, the L signal may be fed back thereto. The DC
29 signal thus derived from the fed back R (or L) signal represents the magnitude of the audio signal picked up by IF
81 probe 20, detected by stereo detector 38 and decoded by æ stereo decoder 50. AIthough not shown in FIG. 4, it is I' PATENT

1 f~ 581~0 1 appreciated that the L and R signals supplied to 2 transconductance amplifiers 96 and 98 are the same L and R
8 signals that are produced by dynamic noise reduction circuit ~ 54 (FIG. 2). Thus, it will be seen that the level of the DC
S signal supplied to the non-inverting input of differential 6 integrator 84 is a function, at least in part, of the 7 setting of sound volume adjustment 14; and the level of the 8 DC signal supplied to the inverting input of the 9 differential integrator is a function Df the volume of the 10 , sound produced by loudspeakers 24 and 26. As the sound 11 ~ volume adjustment level increases, that is, as the DC signal 12 I supplied to the non-inverting input of differential 13 I integrator 84 increases, the gain of transconductance 14 ¦ amplifiers 96 and 98 increases to increase the volume of the 15 I sound produced by loudspeakers 24 and 26. Conversely, as 16 . the level of the signal produced by sound pick-up 64 17 ¦I decreases, the transconductance amplifier gains likewise 18 ~I decrease. Hence, loud output sounds will not be produced in 19 ¦I the event that a low level has been set by sound volume 20 1 adjustment 14. In this manner, the level of the output 21 , sounds is controlled by the sound volume adjustment of the 22 1 television receiver.
23 1 During pauses or periods of silence, the 24 I monophonic component (L + R) is at a relatively low level, as is appreciated from the foregoing discussion of the 26 adapter shown in FIG. 2. Accordin~ly, the (L I R) sisnal 27 supplied to transistor 100 from stereo detector 38 renders 28 1, the transistor non-conductive~ As a result, the voltage 2g 11 level at the collector electrode of this transistor ao 11 increases to the operating potential level (+V), thereby 81 ¦I forward biasing rectifying diode 102. ~owever, the maximum 8~ voltage that may be present at the anode of diode 102 is ,, -26-PATENT

~58~Z~) 1 limited by the zener breakdown voltage of zener diode 104.
2 This reference voltage, which in the described example is a 8 positive voltage, is supplied from the anode of diode 102 to 4 the gate electrode of FET switch 94 to render that switch ~ non-conductive. Consequently, the DC signal derived from 6 transconductance amplifier 98 is inhibited from being 7 supplied to the inverting input of differential integrator 8 84. At this time, therefore, the differential integrator 9 does not vary the voltage across capacitor 88, thus maintalning the same gain at the transconductance amplifiers 11 I during such pauses or periods of silence.
12 I When sound is present, the monophonic component 13 I (L + R) increases in magnitude, thereby rendering transistor 14 ¦ 100 conductive. The voltage at the anode of diode 102 thus 15 I falls below the reference voltage level supplied by zener 16 I diode 104 to turn on FET switch 94. Consequently, the DC
17 voltage derived from the R output of transconductance 18 ll amplifier 98 is supplied by the FET switch tc the inverting 19 ,l input of differential integrator B4. The difference between 20 I this DC level and the DC level derived from sound pick-up 64 21 ~ is integrated and varies the voltage across capacitor 88.
22 I Accordingly, the gain of transconductance amplifiers 96 and 23 1 98 is adjusted accordingly.
24 Thus, it is seen that the volume of the sound 2$ produced by loudspeakers 24 and 26 is controlled, at least 26 in part, as a function of the output sound level of the 27 television receiver and as a function of the magnit~de of 28 the audio signal which is received at that television 2~ ! receiver.
80 ~ FIG. 5 is a graphical representation of the video ~1 I and audio spectra transmi~ted to the television receiver.
$2 The video bandwidth 106 is centered on the video carrier l -27-¦ ~ PATENT

~ S8~ZO

1 ¦ frequency fv, and the video bandwidth contains the usual 2 ¦ video signals, including the horizontal ~ynchronizing 8 ¦ signal~ the video information signal~ the chrominance 4 ¦ information signal~ and the like. The audio bandwidth 108 6 ¦ is seen to be centered on the audio carrier frequency f5 6 ¦ which, in accordance with the convention adopted in the q ¦ United States, is separated from the video frequency carrier 8 ¦ by 4.5 MHz. The audio bandwidth 108 contains the audio 9 ¦ signals that are transmitted from the broadcasting source, ~0 ¦ including the usual monophonic component and, if 11 ¦ stereophonic audio transmissions are effected, the 12 I stereophonic component, as well as the second audio program 13 ll and the auxiliary channel.
14 ¦ FIG. 6 is a graphical representation of the ~5 I spectrum of those audio signals, prior to frequency 16 ¦ modulation onto the audio carrier, normally transmitted in 17 ~ accordance with the Zenith system of stereo TV broadcasting.
18 Thus, the graphical representation of the spectrum shown in ~ FIG. 6 is the base band spectrum containing the monophonic 20 I component (L + R) 110, the pilot signal 112 ~of frequency 21 ¦~ fH)~ the stereophonic component (L - R) 114 which is 22 1 modulated onto a subcarrier whose frequency is 2 fH, the 23 ! second audio program 116 which is modulated onto another 24 subcarrier of frequency 5 fH and the auxiliary information spectrum 118 which is modulated onto yet another subcarrier 26 of frequency 6.5 fH. It will be appreciated that the 27 spectrum illustrated in FIG. 6 is provided at the output of 2~ I quadrature detector 42 of stereo detector 3B (FIG. 2). It 29 I will be recognized that the spectrum illustrated in FIG. 6 80 I is the typical freguency ~pectrum of the Zenith system.
81 ! While the present invention has been particularly 82 ~I shown and described with reference to a preferred I

' PATENT

.~ 0 1 embodiment, it will be readily apparent to those of ordinary 2 skill in the art that various changes ~nd modifications may 8 be made without departing from the spirit and scope of the 4 invention. For example, the use of gain controlled ~ amplifiers 56 and 5R, together with the gain control ~ignal 6 generator therefor shown at FIG. 4, may be omitted, if ~ desired. Likewise, spatial/stereo/pseudo-stereo circuit 52 8 may be replaced by conventional amplifiers which merely 9 ! amplify the left-channel and right-channel output signals provided by decoder 50. Other changes and optional 11 j omissions or substitutions are contemplated.
12 ! It is intended that the appended claims be 13 1l interpreted as including the foregoing as well as other 14 I changes and modifications.

19~, 21 ' æ

Z7 l ~o ~l !

I`

Claims (17)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A TV stereo adapted for use with a television receiver to detect and decode stereophonic audio signals that are transmitted with composite television signals, said adapter comprising: IF pick-up means physically separate from said television receiver for picking up spurious IF audio signals normally generated by said television receiver; stereo detecting means for detecting from said IF audio signals monophonic and stereophonic components that are transmitted with said composite television signals; coupling means tuned to the IF carrier frequency of the picked up IF audio signals and connected between said IF pick-up means and said stereo detecting means for supplying said picked up IF audio signals at said IF carrier frequency to said stereo detecting means;
stereo decoding means responsive to the detected components for decoding left-channel and right-channel audio signals therefrom; and output means for providing left-channel and right-channel signals adapted to drive sound transducing means.

2. The adapter of Claim 1 wherein said IF pick-up means comprises a pick-up probe positionable proximate the IF stage normally provided in said television receiver.

3. The adapter of Claim 2 wherein said pick-up probe comprises an inductive probe tuned to said audio IF frequency for inducing the flow wherein of IF audio signals.

4. The adapter of Claim 1 wherein the transmitted stereophonic audio signals include a monophonic component (L
+R) and a stereophonic component (L - R), with said stereophonic component (L - R) being subjected to noise reduction compression prior to transmission, and wherein said stereo decoding means includes noise reduction expansion means for subjecting said stereophonic component (L - R) to noise reduction expansion that is compatible with said noise reduction compression.

5. The adapter of Claim 4 wherein said stereo decoding means includes a decoder coupled to said noise reduction expansion means to receive the noise reduced expanded stereophonic component (L - R), and means for supplying the detected monophonic component (L + R) to said decoder, said decoder being operative to produce left-channel (L) and right-channel (R) signals from the components supplied thereto.

6. The adapter of Claim 5 wherein said decoder comprises a matrix decoder for summing the components supplied thereto ((L + R) + (L - R)) and for subtracting the components supplied thereto ((L + R) - (L - R)).

7. The adapter of Claim 5 wherein said stereo decoding means further includes stereo/pseudo-stereo change-over means coupled to said decoder and operable when a stereophonic component (L - R) is detected to supply the left-channel (L) and right-channel (R) signals produced by said decoder to said output means, said change-over means being operable when a stereophonic component (L - R) is not detected to simulate from one of the channel signals (L or R) produced by said decoder the other channel signal (R or L) and to supply to said output means said one channel signal and the simulated channel signal.

8. The adapter of Claim 1 wherein said output means includes automatic gain control means for controlling the gain of said left-channel and right-channel signals in response to a gain controlling signal applied thereto; and generating means responsive, at least in part, to the output sound level of said television receiver to generate said gain controlling signal.

9. The adapter of Claim 8 wherein said generating means comprises a sound signal source for providing a signal representing said output sound level of said television receiver; and comparator means for comparing the sound-representing signal provided by said sound signal source to at least one of said left-channel and right-channel signals for generating a gain controlling signal whose magnitude varies with the difference between the signals compared by said comparator means.

10. The adapter of Claim 9 further comprising switch means for coupling said at least one of said left-channel and right-channel signals to said comparator means; and switch control means for opening said switch means to inhibit said at least one of said left-channel and right-channel signals from being coupled to said comparator means when the magnitude of the detected monophonic component is below a predetermined threshold.

11. The adapter of Claim 10 wherein said switch control means comprises level detecting means, and means for supplying said detected monophonic component to said level detecting means.

12. The adapter of Claim 10 wherein said comparator means comprises differential integrating means having a first input connected to receive said sound-representing signal, a second input connected to receive said at least one of said left-channel and right-channel signals coupled by said switch means, and an output for providing a signal representing the integral of the difference between the signals applied to said first and second inputs.

13. The adapter of Claim 9 wherein said television receiver is provided with at least one loudspeaker; and said sound signal source comprises a microphone for picking up sounds produced by the loudspeaker of said television receiver to provide said sound-representing signal.

14. A TV stereo adapter for use with a television receiver to detect and decode stereophonic audio signals that are transmitted with composite television signals, said adapter comprising: IF pick-up means physically separate from said television receiver for picking up spurious IF audio signals normally generated by said television receiver; stereo detecting means coupled to said IF pick-up means for detecting from said IF audio signals monophonic and stereophonic components that are transmitted with said composite television signals; stereo decoding means responsive to the detected components for decoding left-channel and right-channel audio signals therefrom; output means for providing left-channel and right-channel signals adapted to drive sound transducing means and including automatic gain control means for controlling the gain of said left-channel and right-channel signals in response to a gain controlling signal applied thereto; a sound signal source for providing a signal representing the output sound level of said television receiver; comparator means for comparing the sound-representing signal provided by said sound signal source to at least one of said left-channel and right-channel signals for generating a gain controlling signal whose magnitude varies with the difference between the signals compared by said comparator means; said television receiver being provided with a headphone jack, and said sound signal source comprising a plug for connection with said headphone jack for coupling sound-representing signals normally provided at said headphone jack.

15. A TV stereo adapter for detecting and decoding stereophonic audio signals that are received by a television receiver and comprising: IF pick-up means positionable proximate the usual IF converter section normally provided in said television receiver for picking up leakage audio IF
signals at an IF carrier frequency as produced by said IF
converter section; stereo detecting means for detecting monophonic (L + R) and stereophonic (L - R) components that may be included in said picked up audio IF signals; means coupled to said IF pick-up means for supplying said picked up audio IF
signals at the produced IF carrier frequency to said stereo detecting means; noise reduction means coupled to said stereo detecting means for subjecting said stereophonic (L - R) component to a noise reducing operation; and stereo decoding means responsive to the monophonic (L + R) component from said stereo detecting means and to the stereophonic (L - R) component from said noise reduction means to produce left-channel (L) and right-channel (R) audio signals for driving sound transducing means.

16. The adapter of Claim 15 wherein said stereophonic (L - R) component is subjected to noise reduction compression prior to being received by said television receiver, and said noise reduction means includes expansion means compatible with said noise reduction compression.

17. The adapter of Claim 15 further comprising gain controlled amplifying means for amplifying said left-channel (L) and right-channel (R) audio signals as a function, at least in part, of the setting of sound adjustment means normally included in said television receiver.