US3883700A

US3883700A - System for reproducing four channel signals recorded on a disc record

Info

Publication number: US3883700A
Application number: US465467A
Authority: US
Inventors: Benjamin B Bauer; Daniel W Gravereaux; Emil L Torick
Original assignee: CBS Inc
Current assignee: Sony Music Holdings Inc
Priority date: 1973-12-26
Filing date: 1974-04-30
Publication date: 1975-05-13
Anticipated expiration: 1992-05-13

Abstract

As described herein, sound information comprising four discrete audio signals is recorded on a single groove, two-channel record disc. To this end, two of the four signals are added to produce a sum signal and subtracted to produce a difference signal. The difference signal is employed to amplitude modulate a carrier signal and the upper sideband thereof is added to the sum signal and applied to one input terminal of a recording transducer. The transducer modulates one channel of the record groove in response to the sum signal and the upper sideband signal applied thereto. A similar operation is performed upon the remaining two signals, and the transducer modulates the second channel of the record groove in response to the sum signal and the upper sideband applied thereto. In reproduction, the two composite signals picked up from the two channels of the record groove are each separated into their respective components, the separated modulated carrier signals are demodulated to recover the two difference signals, and the demodulated difference signals are separately matrixed with their corresponding sum signals to recover the four original audio signals for reproduction on respective loudspeakers placed at predetermined positions in a listening area.

Description

' United States Patent Bauer et a1.

[451 May 13, 1975 Assignee: CBS Inc., New York, N.Y.

Filed: Apr. 30, 1974 Appl. No.: 465,467

Related US. Application Data Division of Ser. No. 428,294, Dec. 26, 1973, which is a continuation of Ser. No. 169,219, Aug. 5, 1971, abandoned.

References Cited UNITED STATES PATENTS 2/1959 Dauguet 179/15 BT 12/1962 Minter 179/100.4 ST 3,401,237 9/1968 Takayanagi.. l79/100.4 ST 3,686,471 3/1972 Takahashi l79/l00.4 ST

FOREIGN PATENTS OR APPLICATIONS 1,196,711 7/1965 Germany 179/l00.1 TD

OTHER PUBLICATIONS Burstein, News For The Audiophile, Radio-Electronics, pages 3839, Apr. 58.

Primary Examiner-Raymond F. Cardillo, Jr. Attorney, Agent, or Firm-Spencer E. Olson [57] ABSTRACT As described herein, sound information comprising four discrete audio signals is recorded on a single groove, twochannel record disc. To this end, two of the four signals are added to produce a sum signal and subtracted to produce a difference signal. The difference signal is employed to amplitude modulate a carrier signal and the upper sideband thereof is added to the sum signal and applied to one input terminal of a recording transducer. The transducer modulates one channel of the record groove in response to the sum signal and the upper sideband signal applied thereto. A similar operation is performed upon the remaining two signals, and the transducer modulates the second channel of the record groove in response to the sum signal and the upper sideband applied thereto. In reproduction, the two composite signals picked up from the two channels of the record groove are each separated into their respective components, the separated modulated carrier signals are demodulated to recover the two difference signals, and the demodulated difference signals are separately matrixed with their corresponding sum signals to recover the four original audio signals for reproduction on respective loudspeakers placed at predetermined positions in a listening area.

2 Claims, 3 Drawing Figures Low PHASE DE-EMFHASIS C EpuAuzms cincuir 62 66 lLg+Lgl+ moo. CARRIER I lClRCUlT 1 i J 1 55 5; J PREAMPLlFlER MATRIX -46 5 8 6,4 l 7} men AM DE-EMPHASIS fg nerecron h ciacuir 68 LOW PHASE DE-EMPHASIS 44 45 4B C eounuzms CIRCUIT 62' 1 CIRCUIT I 1 I 53' g, J Rf PREAMPLIFIER MATRIX R 5 4 :7) b (a. R t MOD. CARRIER mg: m DEEMPHASIS HLTER DETECTOR cmcunr 1 SYSTEM FOR REPRODUCING FOUR CHANNEL SIGNALS RECORDED ON A DISC RECORD This is a division of application Ser. No. 428,294, filed Dec. 26, 1973, which, in turn, is a continuation of application Ser. No. 169,2l9, filed Aug. 5, l97l, now abandoned.

BACKGROUND OF THE INVENTION This invention relates to stereophonic disc recording systems and, more particularly, to a new and improved recording system adapted to record four independent channels of audio information on a two-channel disc record.

In systems for recording stereophonic information comprising two signals in a single groove record disc. it is customary to modulate the two channels (left and right) of the record groove in two perpendicular directions. Thus, the groove variations in one channel represent one function of the two signals and those in the other channel represent another function of the two signals. Typically, the groove is cut with modulation in each channel of the groove representing each signal and with lateral modulation representing the sum of the signals and vertical modulation representing the difference between the signals.

Since the commercialization of stereophonic phonograph sound reproduction in about 1958, efforts have been made to record three or more signals on a twochannel record for the obvious reason that the sound distribution of the reproduced information will improve as the number of signal sources increases. For example, three channel disc recording systems have been proposed in which a selected fraction of a third signal corresponding to the sound detected by a centrally disposed microphone is added to the two signals corre sponding to the sound detected by spaced left and right microphones to record a dependent sum, or center, channel on the stereophonic record. Alternatively, it has been proposed to subtract a selected fraction of the third signal from each of the other two signals and to record a dependent difference channel on the stereophonic record. However, such a difference channel is not compatible with monophonic players because the vertical groove modulation cancels for monophonic reproduction, nor with stereophonic players which produce an unnatural sound during reproduction.

SUMMARY OF THE INVENTION It is a principal object of the present invention to pro vide a method and apparatus for recording four discrete channels of audio information on a two-channel disc record, and for reproducing the four discrete channels upon playback of the record.

Another object of the invention is to provide a system for recording four discrete channels of audio information on a two-channel disc record which may be reproduced on a conventional stereophonic phonograph without loss ofany sound contained in the original four signals.

These and other objects of the invention are accomplished by processing four discrete audio signals and modulating each channel of a two-channel disc record of the conventional 45-45 modulation type in accordance with separate functions of two of the signals. For convenience of description. the four discrete signals correspond to the positions in a listening area at which tour loudspeakers would be placed to respectively reproduce the four signals; that is, they may be characterized as left front (L right front (R left back (L and right back (R,,). In particular, selected pairs of the audio signals, for example, L; and L and R, and R,,, are added to produce sum signals and subtracted to produce difference signals. The difference signals are employed to amplitude modulate carrier signals and the resultant carrier signals are combined with their respective sum signals. The combined sum signals and amplitude modulated carrier signals are then recorded on the separate channels of a two-channel stereophonic record.

In a preferred embodiment of the invention, only the upper sidebands of the difference signal modulated carrier signals are combined with their respective sum signals and recorded on the separate channels of the record. To compensate for the envelope distortion of the single sideband carrier signals, envelope processing circuits are provided to process or predistort the difference signals prior to developing the single sideband. In addition, equalizing networks are provided to maximize the modulation in the upper frequency ranges of the difference signals.

In a preferred embodiment of a record reproducing system, the sum signals are separated from the difference signal modulated carrier signals, the difference signals are detected, and the sum and difference signals are again added and subtracted to produce four audio signals corresponding to the original four channels for application to four appropriately positioned loudspeakers.

BRIEF DESCRIPTION OF THE DRAWINGS In the Drawing:

FIG. 1 is a block diagram illustrating a preferred embodiment of a recording system arranged according to the invention for recording four discrete channels of audio information on a two-channel disc record;

FIG. 2 is a diagram illustrating the frequency distribution of the sum and the single sideband carrier signals recorded on one channel of a two-channel stereophonic record; and

FIG. 3 is a block diagram illustrating a preferred embodiment of a reproducing system arranged according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT In a preferred embodiment of a four-channel recording system arranged according to the present invention, as shown in FIG. I, a pair of matrix circuits l0 and 10' receive four audio signals from a suitable four-channel source, such as a master tape, (not shown) via input terminals l2, l4 and l2, 14', respectively. As illustrated, the four input signals are designated left front (L left back (L,,), right front (R and right back (R and each corresponds to the location in a listening area in which they are to be reproduced. The processing circuitry for the L; and L,, signals will be described in detail. As will be apparent hereinafter, the L, and L signals are recorded in the left channel of a sterophonic record and the R, and R signals are recorded in the right channel. The components of the processing circuitry for the R; and R signals corresponding to the components of the L; and L,, signal processing circuitry are designated by prime numbers.

Within the matrix circuit 10, which may be of cnventional construction and, accordingly, need not be described herein, the L, and L signals are added together to supply a sum signal (L,+ Li.) to a channel 16 and subtracted to supply a difference signal (L, w L,,) to a channel [8. As described hereinafter. the sum signal (L,+ L is recorded in the left channel of a stereophonic record and the upper sideband of a carrier signal modulated by the difference signal (L, L is also recorded in the left channel of the stereophonic record. In like manner, the sum signal (R,+ R1,) developed in the matrix It) and the upper sideband of a carrier signal modulated by the difference signal (R, R1,) are recorded in the right channel of the stereophonic recordv The sum signal '(L, L,,) is applied via a phaseequalizing circuit 19 to a buffer amplifier 20, whereas the difference signal (L, L is applied initially to an equalizer circuit 22. The phase equalizer circuit 19 is provided to maintain coincidence between the sum signal and the difference signal, which is phase-altered by reason of the processing to which it is subjected, as will be apparent hereinafter.

The equalizer circuit 22 attenuates the components of the difference signal (L, L,,) ranging in frequency from about 20 Hz to about 1 kHz to compensate both for the fact that some of the lower sideband signal is transmitted to the output of the processing circuitry because of the method employed for producing single sideband. The signal-to-noise ratio of the composite signal is improved by pre-emphasizing the high frequency components of the difference signal (in which the spectral energy is less than for mid-range frequencies) and correspondingly deemphasizing the signal during playback so that a flat frequency response is attained and the record noise subsequently attenuated. The method utilized to generate single sideband is to pass a double sideband signal with carrier through a high-pass filter having a cutoff frequency equal to the carrier frequency. Consequently. the audio level for low frequencies must be attenuated with respect to the high frequencies so that the modulation percentage in the double sideband region is the same as in the single sideband region for constant level input. These signals ranging in frequency from about 17 kHz to 20 kHz will be transmitted, albeit with attenuation.

From the equalizer circuit 22 the difference signal is supplied to an envelope processor circuit 24 which includes an amplitude modulator 26, a highpass filter 28, a full wave detector and an inverter 32. The present invention utilizes a single sideband modulator to record the difference signal (L, L,,) on the same channel (left) of the stereophonic record groove that the sum signal (L,+ L,,) is recorded. As is understood in the art. when a sideband is removed from an amplitude modulated carrier signal to develop a single sideband signal with carrier. the envelope of the single sideband signal is in the shape of a poid" and thus represents a distorted replica ofthe difference audio signal. In order to compensate for such distortion of the envelope of the single sideband signal. it is a feature of the present invention to predistort the difference signal (L, L,,) in the processor 24.

Within the processor 24, therefore. the difference signal (L, L,,) is applied to one input terminal of the amplitude modulator 26 wherein the signal modulates a carrier signal having a frequency of 20 kHz. The developed modulated carrier signal is then applied to the highpass filter 28 having a cutoff frequency of 20 kHz which rejects at rapid rate frequencies below 20 kHz and passes without attenuation the upper sideband of the modulated carrier signal. The transmitted sideband signal is then applied to the full wave detector circuit 30 which comprises, for example, a full wave rectifier and lowpass filter and, accordingly. detects only the envelope of the transmitted sideband signal. As above described. the detected signal constitutes a distorted replica of the difference signal (LI- L1,).

The inverter 32 inverts the detected and distorted difference signal and applies the inverted signal to one input terminal of an amplitude modulator 34. composing the first component in a single sideband modulator 36. In the modulator 34, the inverted and distorted difference signal modulates a 20 kHz carrier signal applied to the other input terminal of the modulator, A highpass filter 38 having a cutoff frequency of 20 kHz transmits only the upper sideband of the difference signal amplitude modulated carrier signal. By virtue of the inherent distortion in the modulation and following process, the envelope of the transmitted upper sideband signal is distorted with the result that the previous distortion is cancelled and the envelope of the resultant signal is an exact replica of the difference signal (L,

From the filter 38, the upper sideband signal with carrier is supplied to a buffer amplifier 40 having its output terminal connected to one input terminal of a summing network to the other input terminal of which the output terminal of buffer amplifier 20 is applied. In the summing network 41, the sum signal (L, L,,) and the (L, L,,) single sideband signal with carrier are added together and supplied to one input terminal of a recording transducer 42. In this case. the transducer 42 is arranged to modulate the left channel of the record groove by means of a stylus 43 in response to the sum signal (L,+ L,,) and the (L, L,,) single sideband signal with carrier supplied thereto.

ln like manner, the sum signal (R, R1,) and the upper sideband signal with carrier having an envelope representative of the difference signal (R, R,,) are added together in a summing network 41' and then applied to the other input terminal of the recording transducer 42. The transducer 42 is arranged to modulate the right channel of the record groove in accordance with the sum signal (R, R and the (R, R single sideband signal with carrier supplied thereto.

Referring now to FIG. 2, there is shown the frequency distribution of the sum signals (L,+ L,,) and the difference signals (L, L,,). The sum signal is recorded as a regular audio signal in the respective channel of the record groove and the difference signal is recorded as single sideband modulation with carrier in the same channel of the record groove. The total bandwidth ex tends from 30 Hz to 35 kHz. The distribution of the frequency components of the right channel signal is iden tical to that shown for the left channel.

ln a preferred embodiment of a four-channel record reproducing system arranged according to the present invention. as shown in FIG, 3, a two-channel disc phonograph pick-up transducer 44 includes a stylus 45 that responds to the modulations in the left and right channels of a record disc recorded in the above-described manner to supply the sum signal and the (L,* L,,) single sideband signal with carrier. and the sum signal (R, R,,), together with the (R,- R single sideband sig nal with carrier, along a pair of

conductors

46 and 48, respectively, to a pair of

preamplifiers

50 and 52, respectively. Like the recording system, the components of the processing circuitry for the left and right channels are identical, and accordingly, only the processing circuitry for the left channel information will be described herein. The corresponding components in the right channel are designated by prime numbers.

The preamplifier 50 amplifies the sum signal (L; L and (L, L single sideband signal with carrier and applies the amplified signal to both a lowpass filter 53 and to a highpass filter 54. The

filters

53 and 54 separate the sum and single sideband signals and supply the separated signals to a phase equalizing circuit 56 and to an AM detector circuit 58, respectively. The phase equalizing circuit 56 is provided to maintain coincidence between the sum signal and the (L,- L,,) single sideband signal with carrier which is altered in phase by virtue of the additional circuits to which it is subjected, as will be apparent hereinbelow. From the circuit 56, the sum signal is applied by way of a deemphasis circuit 60, conventional in stereophonic record reproducing systems, to one input terminal of a matrix 62.

In the AM detector circuit 58, which may comprise, for example, a diode network and a lowpass filter, the envelope of the (L; L single sideband signal with carrier is detected, the detected signal being an exact replica of the (L, L signal produced by the matrix in the recording process (FIG. 1). The difference signal (L, L is then applied by way of a de-emphasis circuit 64 to the other input terminal of the matrix 62. In the matrix 62, the sum signal (1 L is combined with the detected difference signal (L, L in the conventional manner to reproduce at two pairs of

output terminals

66 and 68 the left front (L signal and the left back signal (L,,). The right front (R,) signal and the right back (R signal are reproduced in like manner at the two pairs of output terminals 66 and 68' of the matrix 62. Although not shown, these signals may then be supplied through power amplifiers to four spaced loudspeakers positioned at the left front, right front, left back and right back corners of the listening area, so that the spatial relation of the original four-channel sound is reconstructed during reproduction.

In operation, the matrices l0 and 10' receive signals Ly, L R, and R and convert the signals into sum signals (L;+ L,,) and (R;+ R and difference signals (L, L and (R, R respectively. The sum signal (L, L,,), together with a (L, L,,) single sideband signal with carrier, is recorded in the left channel ofa stereophonic record and the sum signal (R,+ R together with a (R;-- R,,) single sideband carrier, is recorded in the right channel of the record. In the reproduction of the record, the sum and difference signals in each channel are separated and the difference signal detected. Thereafter, the signals are combined in the matrices 62 and 62' and reproduced as the original four audio signals, L L R, and R,,.

Although the invention has been described herein with reference to a specific embodiment, many modifications and variations therein will be readily apparent to those skilled in the art. Accordingly, all such variations and modifications are included within the intended scope of the invention as defined by the following claims.

We claim:

l. A system for reproducing first, second, third and fourth audio signals which are recorded in a single groove cut on a record disc, wherein a signal representative of the combination of a signal representative of the sum of said first and second audio signals and a single sideband signal with carrier of predetermined frequency modulated by a signal representative of the difference of said first and second audio signals is recorded on one wall of the record groove and a signal representative of the combination of a signal representative of the sum of said third and fourth audio signals and a single sideband with carrier of said predetermined frequency modulated by a signal representative of the difference of said third and fourth audio signals is recorded on the other wall of the record groove, and wherein the lower frequency limit of each of said single sideband with carrier signals is higher than the upper frequency limit of said sum signals, said reproducing system comprising:

pickup means for generating a first electrical signal in response to the signals recorded on the one wall of the groove and for generating a second electrical signal in response to the signals recorded on the other wall of the groove, first low pass filter means to which said first electrical signal is applied for deriving the signal representative of the sum of said first and second audio signals and first high pass filter means to which said first electrical signal is applied for deriving the single sideband with carrier which is modulated with the signal representative of the difference of said first and second audio signals, second low pass filter means to which said second electrical signal is applied for deri ing the signal representative of the sum of said third and fourth audio signals and second high pass filter means to which said second electrical signal is applied for deriving the single sideband with carrier which is modulated with the signal representative of the difference of said third and fourth audio signals,

first amplitude detector means for demodulating the single sideband with carrier derived by the first high pass filter means to thereby reproduce the sig' nal representative of the difference of said first and second audio signals,

second amplitude detector means for demodulating the modulated carrier wave derived by the second high pass filter means to thereby reproduce the signal representative of the difference of said third and fourth audio signals,

first matrix means responsive to the sum signal derived by said first low pass filter means and the reproduced signal from said first amplitude detector means for separately reproducing the original first and second audio signals, and

second matrix means responsive to the sum signal de rived by said second low pass filter means and the reproduced signal from said second amplitude detector means for separately reproducing the original third and fourth audio signals.

2. The reproducing system as defined in claim 1 further comprising four loudspeakers for generating sound in response to respective ones of the reproduced first, second. third and fourth audio signals, and

wherein said loudspeakers are arranged with respect to the listener so that the first and second loudaudio signals are placed at the front and on the other side and at the rear and on the other side, respectively.

Claims

1. A system for reproducing first, second, third and fourth audio signals which are recorded in a single groove cut on a record disc, wherein a signal representative of the combination of a signal representative of the sum of said first and second audio signals and a single sideband signal with carrier of predetermined frequency modulated by a signal representative of the difference of said first and second audio signals is recorded on one wall of the record groove and a signal representative of the combination of a signal representative of the sum of said third and fourth audio signals and a single sideband with carrier of said predetermined frequency modulated by a signal representative of the difference of said third and fourth audio signals is recorded on the other wall of the record groove, and wherein the lower frequency limit of each of said single sideband with carrier signals is higher than the upper frequency limit of said sum signals, said reproducing system comprising: pickup means for generating a first electrical signal in response to the signals recorded on the one wall of the groove and for generating a second electrical signal in response to the signals recorded on the other wall of the groove, first low pass filter means to which said first electrical signal is applied for deriving the signal representative of the sum of said first and second audio signals and first high pass filter means to which said first electrical signal is applied for deriving the single sideband with carrier which is modulated with the signal representative of the difference of said first and second audio signals, second low pass filter means to which said second electrical signal is applied for deriving the signal representative of the sum of said third and fourth audio signals and second high pass filter means to which said second electrical signal is applied for deriving the single sideband with carrier which is modulated with the signal representative of the difference of said third and fourth audio signals, first amplitude detector means for demodulating the single sideband with carrier derived by the first high pass filter means to thereby reproduce the signal representative of the difference of said first and second audio signals, second amplitude detector means for demodulating the modulated carrier wave derived by the second high pass filter means to thereby reproduce the signal representative of the difference of said third and fourth audio signals, first matrix means responsive to the sum signal derived by said first low pass filter means and the reproduced signal from said first amplitude detector means for separately reproducing the original first and second audio signals, and second matrix means responsive to the sum signal derived by said second low pass filter means and the reproduced signal from said second amplitude detector means for separately reproducing the original third and fourth audio signals.

2. The reproducing system as defined in claim 1 further comprising four loudspeakers for generating sound in response to respective ones of the reproduced first, second, third and fourth audio signals, and wherein said loudspeakers are arranged with respect to the listener so that the first and second loudspeakers which respectively reproduce the first and second audio signals are placed at the front and on one side and at the rear and on the one side, respectIvely, and the third and fourth loudspeakers which respectively reproduce the third and fourth audio signals are placed at the front and on the other side and at the rear and on the other side, respectively.