US3152224A - F. m. stereophonic multiplex receiver having a single volume control for adjusting the magnitude of the signals presented to the stereo detector and the output materixing means - Google Patents

Description

Oct. 6, 1964 A. COTSWORTH 3,

F.M. STEREOPHONIC MULTIPLEX RECEIVER HAVING A SINGLE VOLUME CONTROL FOR ADJUSTING THE MAGNITUDE OF THE SIGNALS PRESENTED TO THE STEREO DETECTOR AND THE OUTPUT MATRIXING MEANS Filed Aug. 1, 1962 Filter (A-B)S //5l Phase f I "JV- I E) I United States Patent 0 3,152,224 EM. STEREQPHQNTC MULTIPLEX PECEIVER HAVING A SINGLE VGLUME CONTROL FGR ADEUSTING THE MAGNHTUDE Gir THE SIGNALS PRESENTED Til THE STAB-tilt) DETECTQR AND THE OUTPUT MATPIG lviEANS Aihert Cotsworth, Oak Park, Ill., assignor to Zenith Radio Corporation, Chicago, 111., a corporation of Delaware Filed Aug. 1, 1962, Ser. No. 213,919 1 Claim. (ill. 17915) The present invention relates to radio receivers which may be employed in the reception of stereophonic broadcast programs. More particularly, it concerns itself with loudness control circuitry for varying the sound level of the reproduced program.

To achieve stereophonic reproduction, it is essential that the reproducing system comprise two separate audio channels driving appropriately spaced loudspeaker systerns. In the usual case, the receiver for the reproduction of stereo programs is constructed with a principal chassis containing one of the speakers and all of the receiving and amplification circuitry necessary to produce the stereophonic program signals. There is an associated remote speaker positioned for optimum stereo effect and cable connected to the principal channel to be driven therefrom. In keeping with true stereophonic reproduction it is necessary that the balance or relative sound level between the two speakers be kept constant when the program volume is varied. It is customary to employ concentric, simultaneously actuated potentiometers to adjust the gain of the two amplifier channels of the receiver to vary the level of the reproduced sound but a tracking error may be encountered. This error, which may be attributed to the potentiometer construction and to tolerance variations impairs the balance of the reproduced programs and is especially noticeable to discriminating listeners.

In one prior arrangement concentrically mounted p0- tentiometers me coupled between the stereo detector and the audio amplifiers of the signal channels with their actuating shafts coupled together by way of a clutch mechanism. Two concentric control knobs are provided with the outer knob causing both potentiometers to track as a loudness adjustment. Adjustment of the balance of the two audio channels is achieved by rotating the inner control knob and holding the outer knob fixed. However, because of tracking errors associated with the concentric otentiometers, the balance may vary when a loudness adjustment is subsequently made.

In another arrangement, the stereo detector supplies left and right stereo signals to separate audio amplifiers by way of individual potentiometers which are concentrically mounted but, in contrast to the first arrangement, their control shafts are rigidly coupled together. Between these potentiometers and the audio amplifiers are individual balance potentiometers which are so arranged that when actuated, the signal supplied to one amplifier varies inversely to the signal applied to the other. However, after the channels have been balanced, error may be introduced when the loudness control is varied, again, because of tracking dificulty.

Still another arrangement, which also employs concentrically mounted and simultaneously actuated loudness potentiometers for adjusting the magnitude of the left and right stereo signals, has a single potentiometer, poistioned between the stereo detector and amplifier in one channel, to vary the balance. Here again, tracking errors may introduce unbalance when a loudness adjustment is made.

3,l52,224 Patented Get. 6, 1964 It is, therefore, an object of this invention to overcome the aforenc-ted deficiencies of prior stereo loudness controls.

It is another object of this invention to provide a novel loudness control for use in stereophonic receivers.

It is a further object of this invention to provide a loudness control arrangement which eliminates tracking error associated with simultaneously actuated loudness controls.

It is an additional object to provide a new and improved loudness control for stereo receivers which is inexpensive contrasted to prior loudness controls.

The invention is used in a receiver for receiving stereo broadcast signals comprising a single carrier concurrently frequency modulated with the sum of A and B program signals, a suppressed-carrier amplitude-modulated subcarrier signal carrying the difference of the A and B signals, and a pilot signal related in frequency to the subcarr-ier signal. The receiver comprises a frequencymodulation detector responsive to the received carrier for developing a composite signal corresponding to the modulation components and means responsive to the pilot signal for deriving a modulation signal having a frequency corresponding to that of the subcarrier. Additionally, the receiver comprises a synchronous stereo detector network and a potentiometer coupled between the frequencymodulation detector and the stereo detector network for applying one polarity of e composite signal to the stereo detector network. Furthermore, the receiver includes means for applying the demodulation signal to the stereo detector network and a phase invertor network, coupled between the potentiometer and the stereo detector network, for applying the other polarity of the composite signal to the stereo detector network in phase opposition to the output signals to the stereo detector network to effect matrixing and develop cleanly separated A and B audio signals.

The features of this invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood, however, 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 schematic diagram of a stereo broad cast receiver embodying the loudness control apparatus of the invention; and

FIGURE 2 is a schematic diagram of an alternative embodiment of the apparatus of FIGURE 1.

The invention will be described with reference to the recently approved standards of the Federal Communications Commission for stereophonic broadcasting. In accordance with the standards a single carrier is concurrently frequency-modulated with: (1) the sum of the left and right program signals, hereinafter designated as A and B; (2) the fundamental components of a suppressed-carrier amplitude-modulated sub-carrier signal carrying the difference of the A and B signals, where the expression fundamental components means the first order modulation sidebands which attend the fundamental of the sub-carrier and excludes the higher order sidebands attendant the harmonics of the sub-carrier signal; and (3) a pilot signal of a frequency corresponding to one-half the frequency of that sub-carrier. Since the subject invention concerns itself more particularly with the receiver, the preferred forms of the transmitter for developing such a signal need not be illustrated nor described; however, illustrative transmitters are disclosed in detail in the copending application of Robert Adler et 211., Serial No. 22,926, filed April 18, 1960 and also in the copending application of Carl G. Eilers, Serial No.

23,030, filed April 18, 1969, both assigned to the same assignee as the present application. 'Moreover, while the receiver is to be described in conjunction with the aforementioned FM stereophonic transmission system, the invention is broadly applicable to any single carrier stereophonic transmission system irrespective of the type of modulation. For example, the stereo transmission may employ amplitude-modulated sub-carriers, phase-modulated sub-carriers, an amplitudeand phase-modulated main carrier, or any well known multiplex technique compatible with a single main carrier transmission system.

Referring now more particularly to FIGURE 1, the receiver there shown may be employed for the reception of either monaural or stereophonic broadcasts. The receiver adjusts itself between monophonic and stereophonic FM reproduction automatically in accordance with the type of broadcast that is received. In presenting the structure and operation of this receiver, it is convenient to consider its use in the reproduction of stereophonic frequency-modulated broadcast signals of the type transmitted by apparatus described in the above-identified Adler et al. application.

The arrangement comprises receiver circuits which up to the second signal detector are conventional. They include a tunable radio-frequency amplifier of any desired number of stages and a heterodyning stage or first detector, these being collectively represented by block 10. The input of the amplifying portion connects with a wave signal antenna 11. The output of block connects with a unit 12 which will be understood to include any desired number of stages of intermediate-frequency amplification and one or more amplitude limiters.

The receiver is to have certain characteristics which are superior to those found in conventional monophonic FM receivers. More specifically, it is preferred that the receiver have a high sensitivity so that the signal-to-noise ratio, particularly on stereo operation, will be acceptable in fringe areas. Both automatic gain control for the RF and IF stages and automatic frequency control for the heterodyne oscillator of unit 19 are desired and may be considered to have been included in the block showing. The intermediate-frequency bandwidth of the usual monaural FM receiver is 150 to 180 kilocycles (kc.) Wide at the minus 6 decibel point but the bandwidth for the receiver under consideration should be wider to prevent intermodulation or crosstalk between the several services that may be simultaneously accommodated on a single radiation. A bandwidth of 230 kilocycles is adequate if automatic gain control retains the level of signal through the RF and IF amplifiers at a substantially constant value in spite of variations in intensity of the received signal.

Following the IF amplifier and limiter 12 is a frequency-modulation detector 15 responsive to the amplitude-limited intermediate-frequency signal for demodulating a selected carrier signal to derive the modulation components thereof. Since effective amplitude limiting is highly desirable in this receiver, it is convenient to follow the limiter of unit 12 with the ratio detector 15 which also performs an amplitude limiting function. The composite signal detected in frequency-modulation detector 15 represents the complex modulation of the selected carrier Wave and is available at output terminals which supply the composite signal to various portions of the receiver. One set of output terminals P and P supply the composite signal to a pilot amplifier network 15.

It is necessary for stereo reproduction to have a further demodulation of the sub-carrier which conveys the difference information of the A and B signals. Since the sub-carrier is transmitted with no carrier component, the receiver has means responsive to the pilot signal of the received transmission for deriving a demodulation signal having a fixed frequency and phase relation to the carrier component of the suppressed-carrier amplitudemodulated sub-carrier. This means includes amplifier 16 which is tuned to the pilot frequency.

Pilot amplifier 16 in turn is coupled to a frequency doubler 17. The frequency doubler, when excited by the pilot signal, produces a signal of the same frequency as, and with a fixed phase relation to, the fundamental component of the carrier conveying the difierence information. Suificient control of this phase may be provided by variable tuning of one or more of the tuned circuits included in the pilot amplifier and frequency doubler chain.

A carrier amplifier 18 receives the signal from doubler 17 and amplifies it to a magnitude sufiicient to energize a synchronous stereo detector 19 which is coupled to the output of amplifier 18. The stereo detector 19 serves as stereo-program-signal-developing means for producing the A and B audio program signals The composite signal developed in the ratio detector is fed from an output terminal D by Way of a potentiometer 30 to synchronous detector 19. The potentiometer 30 is coupled between terminal D and ground and its wiper serves as adjustable means, coupled between detector 15 and stereo-program-signal-developing means 19, for varying the magnitude of the signal presented to the stereoprograrn-signal-developing means. The demodulation signal obtained from amplifier 18 and the composite signal obtained from detector 15 control the diodes of detector 19 to the end that one diode develops predominantly the A audio signal with a slight contribution of the B audio signal while the remaining diode develops principally the B audio signal with a slight contribution of the A audio signal.

A clean separation of the A and B audio signals is obtained by matrixing in network 29 with the composite signal obtained from detector 15 by way of potentiometer 3i and a phase inverter 15a. The addition of this opposed phase signal and proper adjustment of the matrixing network produce a substantially pure A signal at terminal output a of the matrix and a substantially .pure B output terminal b. The matrixing network 29 concurrently serves as a low pass filter for removing super-audible components from the detector output also perfiorms the desired function of de-emphasis. The detecting and matrixing operation is explained more fully in the copending application of Adrian DeVries, Serial No. 118,- 009, filed June 16, 1961, and assigned to the same assignee as the present application.

A potentiometer 31 forming a portion of a balance control, is connected between terminal a and ground while its Wiper translates the A signal to an A audio amplifier 32 having a loudspeaker system 33 coupled thereto. Similarly, a potentiometer 35, forming the remaining portion of the balance control, is coupled between terminal b and ground with its wiper connected to a B audio amplifier 36 which in turn drives a loudspeaker system 37. The balance control potentiometers 31 and 35 have their Wiper contacts respectively coupled together so that when actuated an increased audio signal is presented to one audio amplifier while a correspondingly decreased audio signal is applied to the remaining audio amplifier. This construction and operation is well known in the art. Alternatively, one potentiometer may be eliminated and balance achieved by adjustment of the remaining potentiometer but this is undesirable in that the loudness control must be adjusted after a balance adjustment is made.

Loudness is adjusted by means of potentiometer 30. The setting of potentiometer 39 determines the magnitude of the composite signal presented to synchronous detector 19. As the magnitude of this signal is varied, the magnitude of the A and B audio signals is likewise varied. Varying the magnitude of the composite signal presented to stereo detector 19 affects the A and B modulation components equally and, consequently, tracking errors experienced with previous loudness controls are eliminated.

Once the balance level of the reproduced program has been set with balance controls 31, 35, loudness control 30 may be adjusted at will without disturbing the balance.

FIGURE 2 shows an alternative embodiment of the invention wherein the wiper of potentiometer 30 is coupled to a filter network 40 which separates at least the A+B signal and the (AB)S signal. The (AB)S signal is applied to a demodulator 41 of conventional construction and a demodulation signal which may be derived from the received pilot is also delivered to demodulator 41 which develops an AB output signal. A matrix network 42 is coupled to both filter 46 and to demodulator 41 and receives the A-l-B and A-B signals respectively therefrom. The matrix combines these signals in conventional fashion and provides A and B output signals at terminals a and b which correspond to terminals a and b of FIGURE 1. As the single control 30 is used to adjust the magnitude of the A and B components equally, the correct relationship between the A and B signals at the output of matrix 42 will be maintained.

Thus the invention provides a new and improved loudness control for stereophonic broadcast receivers. The

customary concentrically mounted, simultaneously actuated potentiometers normally employed as a loudness control have been eliminated and replaced by an inexpensive loudness control which does not have tracking errors and does not adversely afiect balance.

While particular embodiments of the present invention have been shown and described, it is apparent that changes and modifications may be made therein without departing irom the invention in its broader aspects. The aim of the appended claim, therefore, is to cover all such changes and modifications as fall within the true spirit and scope of the invention.

I claim:

A receiver for a stereophonic frequency-modulation system for utilizing a transmitted signal comprising a single carrier concurrently frequency-modulated with the sum of A and B program signals, a suppressed-carrier amplitude-modulated subcarrier signal carrying the difierence of the A and B signals, and a pilot signal related in frequency to the subcarrier signal, said receiver comprising:

a frequency-modulation detector responsive to said received carrier for developing a composite signal corresponding to said modulation components;

means responsive to said pilot signal for deriving a demodulation signal having a frequency corresponding to that of said subcarrier;

a synchronous stereo detector network;

a potentiometer coupled between said frequency modulation-detector and said stereo detector network for applying one polarity of said composite signal to said stereo detector network;

means for applying said demodulation signal to said stereo detector network;

and a phase inverter network, coupled between said potentiometer and said stero detector network, for applying the other polarity of said composite signal to said stereo detector network in phase opposition to the output signals of said stereo detector network to effect matrixing and develop cleanly separated A and B audio signals.

References Cited in the file of this patent UNITED STATES PATENTS 2,515,619 Weyers July 18, 1950 3,070,662 Eilers Dec. 25, 1962 3,087,994 Schutte Apr. 30, 1963

Claims (1)

1. A RECEIVER FOR A STEREOPHONIC FREQUENCY-MODULATION SYSTEM FOR UTILIZING A TRANSMITTED SIGNAL COMPRISING A SINGLE CARRIER CONCURRENTLY FREQUENCY-MODULATED WITH THE SUM OF A AND B PROGRAM SIGNALS, A SUPPRESSED-CARRIER AMPLITUDE-MODULATED SUBCARRIER SIGNAL CARRYING THE DIFFERENCE OF THE A AND B SIGNALS, AND A PILOT SIGNAL RELATED IN FREQUENCY TO THE SUBCARRIER SIGNAL, SAID RECEIVER COMPRISING: A FREQUENCY-MODULATION DETECTOR RESPONSIVE TO SAID RECEIVED CARRIER FOR DEVELOPING A COMPOSITE SIGNAL CORRESPONDING TO SAID MODULATION COMPONENTS; MEANS RESPONSIVE TO SAID PILOT SIGNAL FOR DERIVING A DEMODULATION SIGNAL HAVING A FREQUENCY CORRESPONDING TO THAT OF SAID SUBCARRIER; A SYNCHRONOUS STERO DETECTOR NETWORK; A POTENTIOMETER COUPLED BETWEEN SAID FREQUENCY MODULATION-DETECTOR AND SAID STEREO DETECTOR NETWORK FOR APPLYING ONE POLARITY OF SAID COMPOSITE SIGNAL TO SAID STEREO DETECTOR NETWORK;

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