US3889061A

US3889061A - Four-channel reproducing system

Info

Publication number: US3889061A
Application number: US428423A
Authority: US
Inventors: Ryosuke Ito; Susumu Takahashi
Original assignee: Sansui Electric Co Ltd
Current assignee: Sansui Electric Co Ltd
Priority date: 1971-12-21
Filing date: 1973-12-26
Publication date: 1975-06-10
Anticipated expiration: 1992-06-10

Abstract

A signal converter is provided for mixing first and second channel signals from a conventional two-channel source to produce first and second composite signals. Phase differences between the first and second composite signals with respect to the first and second channel signals respectively vary as a function of frequency of the first and second channel signals. The first and second channel signals or the first and second composite signals are applied to a second signal converter for producing fourchannel output signals by mixing input signals. The second signal converter is operative to vary mixing ratios of input signals in accordance with the phase relationship between the first and second composite signals.

Description

United States Patent 1191 Ito et al.

1 FOUR-CHANNEL REPRODUCING SYSTEM [75] Inventors: Ryosuke Ito; Susumu Takahashi.

both of Tokyo, Japan [73] Assignee: Sansui Electric Co., Ltd., Tokyo,

Japan [22] Filed: Dec. 26, 1973 [21] Appl. No.: 428,423

Related US. Application Data [63] Continuation-impart of Ser. No. 315,928, Dec. 18,

11] 3,889,061 51 June 10, 1975 3,757,047 9/1973 lshida et a1 179/1 GO Primary Examiner-Kathleen H. Claffy Assistant Examiner-Thomas DAmico Attorney, Agent, or FirmHarris, Kern, Wallen &

Tinsley [57] ABSTRACT A signal converter is provided for mixing first and second channel signals from a conventional two-channel source to produce first and second composite signals. Phase differences between the first and second composite signals with respect to the first and second 52 n 1 I channel signals respectively vary as a function of fre- 1 U S Cl quency of the first and second channel signals. The [51] Int Cl first and second channel signals or the first and second I I i o t I l I s s o u u r a [58] Fleld Ofsearch -179/1GQ11OO'4 verter for producing four-channel output signals by 179/100'1TD-15 BT mixing input signals. The second signal converter is [56] Ref Ci operative to vary mixing ratios of input signals in ac- UNITED STATES PATENTS cordance with the phase relationship between the first 3,170,991 2/1965 Glasgal 179/1 GO and Second composlte Slgnals' 3.691692 10 1972 Hafler 179/1 60 3 Claims, 17 Drawing Figures 3,745,254 7/1973 Ohta 179/1 GQ t 1:5 FL r L C E A 0 09 17 5 5 FR a o 2 Lu 8 :2 RL 1 E i o-eo 2 CC 1- o RR 21 RC (7) E x (I ECi CONTROL UNIT 5C2 1% 6890651 JUN 10 I975 I PATENTED SHEET 4 PATENTEDJUN 1 0 I915 a 8 89,061

SHEET 6 F I G. 10

\(ECZ l I l i F l G. H

H 122 MATRIX J-'- SIGNAL 2-CHANNEL 4-CHANNEL SOURCE R CONVERTER (R, REPRODUCER SHEET fill-R) Er E2 R, I-DISCRIMINATOR TOR iTi

DISCRIMINA LI FR PHASE PATENTEDJUH I 0 I975 ECi - CONTROL F l G. 13

PATENTED SHEET FIG. 15

FREQUENCY FIG. l6

FIG. 17

Odb

FREQUENCY F OUR-CHANNEL REPRODUCING SYSTEM CROSS-REFERENCE TO RELATED APPLICATION This is a Continuation-impart of U.S. Patent application Ser. No. 315,928, filed Dec. 18, 1972.

This invention relates to a signal converting system which is utilized to reproduce two-channel signals from a conventional two-channel source by a four-channel reproduction system and capable of enhancing the separation between signals in a reproduced sound field.

Recently, a matrix four-channel sound reproduction system has been used wherein four-channel original signals are converted into two-channel signals, the twochannel signals are recorded on such recording medium as a phonograph record or a magnetic tape, the two-channel signals reproduced from the recording medium are converted into four-channel signals corresponding to the original signals and the four-channel signals are reproduced by four loudspeakers arranged around a listener.

The matrix four-channel reproducing system, however, involves a serious problem that the crosstalk be tween reproducing channels is extremely large. Specifically, in one type of the matrix four-channel reproducing systems the separation between channels disposed in a diagonal direction is infinity whereas that between adjacent channels equals 3 db.

Although the matrix four-channel reproducing system has been successfully developed as above pointed out, the number of matrix four-channel stereo records now on the market is far smaller than that of twochannel stereo records. The matrix four-channel reproducing system is compatible with conventional twochannel stereo records so that it is possible to enjoy a four-channel playback of a conventional two-channel stereo record.

However, owing to the inherently poor separation characteristic of the matrix four-channel reproducing system, when reproducing a conventional two-channel stereo record by a four-channel system, the rear side location of the listening area of a sound image presents a problem. More particularly, where a two-channel stereo record is reproduced by a four-channel system, when only a left signal is reproduced from the record it is desirable to locate the sound image based on this signal at the rear-left side of the listening room for the purpose of providing a satisfactory four-channel reproduction. However, with the system described above, the sound image will be located at an intermediate point between the front-left side and the rear-left side. This means a poor separation between the front and rear channels.

It is an object of this invention to provide an improved signal converting system capable of enhancing the separation between the channels when reproducing sound signals from a conventional two-channel stereo recording medium by a four-channel reproduction system.

According to another aspect of this invention there is provided a signal converting system for reproducing two channel stereo signals from a two channel signal source by means of a four-channel stereo reproducing system, said system comprising a combination of: first signal converting means connected to receive the first and second channel signals from said two channel signal source for combining the first and second channel signals at a relative amplitude ratio and in a phase relationship to produce first and second composite signals; a control unit responsive to the phase relationship between the first and second composite signals for producing first and second control outputs; and second signal converting means connected to receive the two channel signals for producing four-channel output signals, said second signal converting means including means for combining the two channel signals at relative amplitude ratios and in phase relationships therebetween to produce the four-channel output signals and means responsive to the first and second control outputs from said control unit for controlling at least relative amplitude ratios between the two channel signals contained in the four-channel output signals.

According to still another aspect of this invention there is provided a signal converting system for reproducing two channel stereo signals from a two channel signal source by means of a four-channel stereo repro ducing system comprising a combination of: first signal converting means connected to receive the first and second channel signals from said two channel signal source for combining the first and second channel signals at a relative amplitude ratio and in a phase relationship to produce first and second composite signals; a control unit responsive to the phase relationship between the first and second composite signals for producing first and second control outputs; and second signal converting means connected to receive the first and second composite signals for producing four-channel output signals, said second signal converting means including means for combining the first and second composite signals at relative amplitude ratios and in phase relationships therebetween to produce the fourchannel output signals and means responsive to the first and second control outputs from said control unit for controlling at least relative amplitude ratios between the first and second composite signals contained in the four-channel output signals.

This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which:

FIG. 1 shows asblock diagram of a signal converting system embodying the invention;

FIG. 2 shows a'connection diagram of a matrix circuit shown in FIG. 1;

FIG. 3 shows a block diagram of a portion of a modified embodiment of this invention;

FIG. 4 shows a modification of the embodiment shown in FIG. 3;

FIGS. 5, 6 and 7 show connection diagrams of different signal converters utilized in this invention;

FIG. 8 shows a connection diagram of one example of a control unit utilized in the embodiments shown in FIGS. 3 and 4;

FIG. 9 shows a connection diagram of one example of a variable matrix circuit utilized in the embodiments shown in FIGS. 3 and 4;

FIG. 10 is a graph showing the output characteristic of the control unit shown in FIG. 8;

FIG. 11 is a simplified block diagram of still another embodiment of the invention;

FIGS. 12, 13 and 14 show block diagrams of other variable matrix circuits;

FIG. 15 is a circuit diagram of a signal converter according to another embodiment of this invention; and

FIGS. 16 and 17 are characteristic diagrams of the signal converter shown in FIG. 15.

With reference now to FIG. 1 of the accompanying drawing illustrating a preferred embodiment of this invention, reference numeral designates a suitable two-channel source which may be a conventional stereo phonograph record, a stereo recorded magnetic tape or an FM stereo receiver. The first and second audio signals L and R produced by the two-channel source 10 are applied to a matrix circuit 12 via a signal converter 11 to be described later. The matrix circuit 12 may be constructed as shown in FIG. 2, for example. The circuit shown in FIG. 2 is constructed to convert the first and second audio signals L and R into fourchannel signals consisting of FL (front-left), FR (frontright), RL (rear-left) and RR (rear-right), which are expressed by the following equations:

where each A represents a matrix coefficient having a value of about 0.4. In the four-channel reproducing system, it is usual to install four loudspeakers SFL, SFR, SRL and SRR about a listener 13 in a listening room 14.

The output FL from matrix circuit 12 is applied to the corresponding loudspeaker SFL through a phase shifter 15 and a power amplifier 16 while the output FR to loudspeaker SFR through a phase shifter 17 and a power amplifier 18. Similarly, the outputs RL and RR are supplied to corresponding loudspeakers SRL and SRR respectively through

phase shifters

19 and 21 and

power amplifiers

20 and 22. The purpose of the

phase shifters

15, 17, 19 and 21 is to maintain front signals FL and FR at the in-phase relationship throughout the entire range of audio frequencies and to bring the rear signals RL and RR into in-phase relationship which have been 180 out-of-phase.

Where only a left signal is impressed upon the matrix circuit 12, both signals FL and RL are designated by L. Accordingly, under these conditions, although it is desirable to locate the sound image of this left signal L at the position of the loudspeaker SRL, actually the sound image is located at a mid-point between the loudspeakers SFL and SRL. As will be described later, the signal converter 11 is constructed to form difference signals L (L-aR) and R (R-aL) in response to the left and right signals. Accordingly, responsive to the signals L-aR and R-aL, the matrix circuit 12 operates to form the following signals:

Accordingly, where only the left signal L is impressed upon signal converter 11, the outputs FL and RL from the matrix circuit 12 are shown by L(1Aa) and L(1+Aa), respectively. This means that the sound image corresponding to the left signal L is located at a position closer to the loudspeaker SRL.

One example of the signal converter 11 shown in FIG. 5 is provided with

input terminals

21 and 22 connected to receive the left and right signals L and R, respectively, and a pair of

output terminals

23 and 24.

The first input terminal 21 is connected to the input terminal of a first inverter 25, and a first potentiometer resistor 26 is connected between the output terminal of the first inverter 25 and the first input terminal 21. The sliding arm 27 of the first potentiometer resistor 26 is connected to the second input terminal 22 through serially connected

resistors

28 and 29, and the junction between these

resistors

28 and 29 is connected to the second output terminal 24. The second input terminal 22 is connected to the input terminal of a second inverter 30, and a second potentiometer resistor 31 is connected between the output terminal of the second inverter 30 and the second input terminal 22. The sliding arm 32 of the second potentiometer resistor 31 is connected to the first input terminal 21 through serially connected

resistors

33 and 34, the junction therebetween being connected to the first output terminal 23. The sliding

arms

27 and 32 of the first and second potentiometer resistors 26 and 31 are mechanically interlocked as shown by dotted lines.

When the sliding

arms

27 and 32 are positioned at the centers of first and second potentiometers 26 and 31 the left and right signals L and R are produced at the first and

second output terminals

23 and 24, respectively.

When the sliding

arms

27 and 32 are moved in the direction of arrows a along the potentiometer resistors 26 and 31, respectively, there are respectively derived from the

output terminals

23 and 24 two difference signals L-ozR and R-aL each having varying relative amplitude ratio between the signals L and R, whereas when the sliding

arms

27 and 32 are moved in the direction of arrows b there are obtained two sum signals L+BR and R-l-BL each having varying relative amplitude ratio. In the embodiment shown in FIG. 1 it is advantageous to use the potentiometer resistors 26 and 31 such that their sliding arms are positioned to the right of their mid-points to produce the two difference signals.

In a modified signal converter shown in FIG. 6, each of the collector-emitter paths of first and second transistors Q and Q is connected across a source indicated by +B and the ground, and the base electrodes of these transistors are connected to input

terminals

21 and 22 respectively through coupling capacitors. In parallel with the collector-emitter paths of the transistors Q and Q, are connected first and second potentiometer resistors 36 and 37 provided with sliding

arms

38 and 39, respectively. The sliding arm 38 of the first potentiometer resistor 36 is connected to the emitter electrode of transistor Q through serially connected

resistors

40 and 41, whereas the sliding arm 39 of the other potentiometer resistor 37 is connected to the emitter electrode of transistor Q through serially connected

resistors

42 and 43. Junctions between

resistors

42 and 43 and between 40 and 41 are connected to

output terminals

23 and 24, respectively. The sliding

arms

38 and 39 of two potentiometer resistors 36 and 37 are mechanically interlocked each other as shown by dotted lines.

In this embodiment, the

collector resistors

44 and 45 and the

emitter resistors

46 and 47 of transistors Q, and Q are made to have an equal value. Again, when sliding

arms

38 and 39 are moved in the direction of dotted arrows a two difference signals L-aR and RaL are produced at the

output terminals

23 and 24 respectively whereas when these sliding arms are moved in the direction of solid line arrows b two sum signals L-l-BR and R+BL are produced.

In another embodiment of the signal converter shown in FIG. 7, each of the collector-emitter paths of transis tors Q and O is connected across the source. The base electrodes of these transistors are connected to input

terminals

21 and 22 respectively through coupling capacitors while the collector electrodes are connected to

output terminals

23 and 24 respectively. The emitter electrodes of transistors Q and Q, are inerconnected through a resistor R. In this signal converter two difference signals each having a predetermined fixed amplitude ratio between the signals L and R are derived out from

output terminals

23 and 24.

A modified embodiment of this invention shown in FIG. 3 comprises a variable matrix circuit 48 and a control unit 49. This modification illustrates a decoder capable of reproducing with satisfactory channel separation sound signals recorded on a matrix four-channel recording medium by a four-channel system. In such a decoder the phase relationship between the twochannel signals, for example, L LF+AFR+jRL+jARR and R FR+AFLjRRjARL which are reproduced from the matrix four-channel recording medium is detected by the control unit 49 constituted by a phase discriminator or a level comparator, and the matrix coefficients of the matrix circuit 48 are controlled by the outputs ECl and EC2 from the control unit 49. When the two-channel signals reproduced from a conventional two-channel recording medium are applied to such a decoder system the control unit 49 can not control the matrix circuit 48 because the two-channel signals are generally in phase. For this reason, in such a case, it is desirable to provide signal converter 11 as shown in FIG. 5, 6 or 7 on the input side of the control unit 49 so that when only one signal is reproduced from the two-channel recording medium, the input signals to the control unit 49 will have opposite phases thereby enabling the control unit 49 to control the variable matrix circuit 48. For example, where only L signal presents, it is possible to locate the sound image of the signal at the position of loudspeaker SRL by the operation of the variable matrix circuit 48, thus improving the separation between the front channels and the rear channels.

To aid the understanding of the invention, the constructions and operations of variable matrix circuit 48 and control unit 49 will be described briefly hereunder.

FIG. 8 shows a circuit diagram of a phase discriminator which comprises a first limiter 50 including

transistors

51 and 52 connected to receive the L signal and a second limiter 53 including transistors 54 and 55 connected to receive the R signal. The first and second limiters 50 and 53 have large amplification gains and operate to transform the signals L and R into rectangular wave signals. Two output signals of opposite polarities produced by the second limiter 53 are amplified by first and

second amplifiers

56 and 58 including

transistors

57 and 59 respectively. The outputs from the first and

second amplifiers

56 and 58 are supplied to a first switching circuit 60 and a second switching circuit 61 respectively including bridge connected diodes D, to D and diodes D to D thereby causing these switching circuits ON and OFF alternately. The output from the first limiter 50 is coupled to the common input of the first and

second switching circuits

60 and 61, while the output terminals of these switching

circuits

60 and 61 are grounded through

capacitors

62 and 63 respectively, and are connected to a point of reference voltage (in this case, +B/2 volts) through

potentiometers

64 and 65, respectively. The slidable arms of the

potentiometers

64 and 65 supply the first and second control outputs ECl and EC2.

The phase discriminator constructed as above described operates to switch the left signal L by alternately rendering ON and OFF the first and

second switching circuits

60 and 61 in response to the right signal R thereby discriminating the phase difference between the right and left signals R and L. FIG. 10 shows the operating characteristic of the phase discriminator showing that the first and second control outputs ECl and EC2 vary symmetrically but in opposite directions about the reference level, which is equal to about +B/2 volts in the phase discriminator shown in FIG. 8.

FIG. 9 illustrates an example of the variable matrix circuit 48 wherein a first matrix circuit associated with the front channels comprises a first differential amplifier 91 including

transistors

92 and 93. The left signal L is coupled to the base electrode of transistor 92 while the right signal R is coupled to the base electrode of transistor 93 through an inverter 94 including a transistor 95. The collector electrode of transistor 92 is connected to the first output terminal of the matrix circuit while the collector electrode of transistor 93 is connected to the second output terminal of the matrix circuit through an inverter 96 comprising a transistor 97. A first control circuit 99 including a field effect transistor 100 is capacitively connected in parallel with a common emitter resistor 98 of

transistors

92 and 93 which constitute the differential amplifier 91. The gate electrode of the field effect transistor 100 is connected to a control input terminal so that it acts as a variable resistor. The first control circuit 99 operates to vary the AC impedance of the emitter circuits of

transistors

92 and 93 in accordance with the magnitude of the control input ECl so as to control the common mode gain of the differential amplifier 91.

The second matrix circuit associated with the rear channels comprises a second differential amplifier 106 including

transistors

107 and 108. The left signal L is coupled to the base electrode of transistor 107, whereas the right signal R is coupled to the base electrode of transistor 108. The collector electrodes of

transistors

107 and 108 are respectively connected to the third and fourth output terminals of the matrix circuit. A second control circuit including a field effect transistor 111 is capacitively connected in parallel with a common emitter resistor 109 for

transistors

107 and 108. The gate electrode of field effect transistor 111 is connected to a control input terminal. The second control circuit 110 operates in the same manner as the first control circuit 99 so as to control the common mode gain of the second differential amplifier in accordance with the magnitude of the control input EC2.

The operation of the variable matrix circuit shown in FIG. 9 will be briefly described as follows: Where the composite signals L and R are substantially in phase, the control input ECl is large and the control input EC2 is small. Consequently, the AC impedance of the emitter circuits of

transistors

92 and 93 is decreased whereby the gain of the first differential amplifier 91 is increased, whereas that of the second differential amplifier 106 is decreased. Increase in the gain of the first differential amplifier 91 results in the increase in the level of the left signal L which is derived out from the collector electrode of transistor 92 and in the decrease in the level of the right signal R contributing to increasing the crosstalk. On the other hand, the level of the right signal R derived out from the collector electrode of transistor 93 is increased and the level of the left signal L contributing to increasing the cross-talk is decreased. Accordingly, the separation between the front channels is improved with the increase in the signal level. In the rear channels, as the gain of the second differential amplifier 106 decreases, the separation degrades with the decrease in the signal level.

In FIG. 3, when the signal converter 1 1 produces two difference signals L-aR and R-aL in in-phase relationship, the outputs EC] and EC2 of the control unit 49 have a large level and a small level, respectively. Then, the gains of the first and second

differential amplifiers

91 and 106 are respectively increased and decreased, and the first output FL and second output FR of the first matrix circuit 90 are formed mostly of signal L and signal R, respectively. Thus, signal L is localized at the loudspeaker SFL, and signal R at the loudspeaker SFR.

If supplied with only signal L or signal R, the signal converter 11 produces two output signals L and AL or two output signals R and AR both in reverse-phase relationship. As a result, the outputs ECI and EC2 of the control unit 49 have a small level and a large level, respectively, thereby to decrease the gain of the first differential amplifier 91 and to increase the gain of the second differential amplifier 106. Consequently, the outputs RL and RR of the second matrix circuit 105 come to be filled mostly with signal L and signal R, respectively. That'is, if only signal L is supplied to the converter 11, it' is localized at the loudspeaker SRL. Similarly, if only signal R is supplied to the converter 11, it is localized at the loudspeaker SRR.

To generalize the above, signals to be localized somewhere to the left and to the right of midway between left and right loudspeakers for 2-channel reproduction are localized at the loudspeaker SFL and at the loudspeaker SFR, respectively. Signals to be localized at the left and right loudspeakers are localized at the loudspeakers SRL and SRR, respectively.

When the signal converter 11 produces two sum signals L+BR and R+BL, the outputs ECl and EC2 of the control unit 49 have a large level and a small level, respectively. As a result, the in-phase components in the input signals L and R are localized in front of the reproduction sound field, and the reverse-phase components, e.g. reverberation components, of these signals are localized at the back of the reproduction sound field.

The detail of the construction and operation of the variable matrix circuit 48 and the control unit 49 and their modifications are fully described in the copending US. Patent application Ser. No. 298,933, filed Oct. 19, 1972, of the title Decoder for use in 4-2-4 matrix playback system now US. Pat. No. 3,825,684.

FIG. 4 shows a modification of the circuit shown in FIG. 3.

In this modification, the variable matrix circuit 48 and control unit 49 are connected to receive the two channel signals L and R via the signal converter 11. When the two channel signals L and R are in-phase and include crosstalk components therebetween, the difference signals produced by the signal converter 11 are caused to be decreased in level as well as enhanced in separation. Accordingly, the arrangement of FIG. 4 in which the separation previously enhanced two-channel signals are supplied to the variable matrix circuit 48 can provide somewhat better separation characteristics than the arrangement of FIG. 3.

FIG. 12 is a block diagram of a variable matrix circuit according to another embodiment used in the arrangement of FIG. 4. With reference first to the front channels, there are provided a first matrix circuit adapted to produce sum signals (L'+R') and (L'+R) of opposite polarities from the composite signals L and R produced by the signal converter 11, and a second matrix circuit 131 adapted to produce a difference signal (L'R'). The difference signal (L'R') is applied to a third matrix circuit 133 via a variable gain amplifier 132 to be added therein to the outputs from the first matrix circuit 130. The first variable gain amplifier 132 is controlled by the first control output ECl from the control unit 49 and has an amplification gain f which varies from 0 to 2.41 with respect to the gain of the first matrix circuit 130. The third matrix circuit 133 functions to produce a first output expressed by (l+f)L (1f)R and a second output expressed by (lf)L' (1+f)R which is phase inverted by an inverter 134.

Associated with the rear channels there are provided a fourth matrix circuit 135 adapted to produce difference signals (L'R') and (L'R) of the opposite polarities and a fifth matrix circuit 136 adapted to produce a sum signal (L'+R'). The sum signal (L'+R') is applied through a second variable gain amplifier 137 to a sixth matrix circuit 138 where it is added to the outputs (L'R') and (L'R) from the fourth matrix circuit 135.

The second variable gain amplifier 137 has an amplification gain b which varies from O to 2.41 with respect to the gain of the fourth matrix circuit 135. Accordingly, the sixth matrix circuit 138 produces a third output expressed by (1+b)L (1b)R' and a fourth output expressed by (1+b)R (1-b)L. The gains of the first and second

variable gain amplifiers

132 and 137 are varied in the opposite directions by the control outputs ECl and EC2 from the control unit 49.

FIG. 13 shows a block diagram of another embodiment of the variable matrix circuit. FIG. 13 is different from FIG. 12 in that it is incorporated with the following circuit components. More particularly, there are provided a 0 phase shifter and a 45 phase shifter 171 which introduce a phase difference of 45 between the composite signal L and the composite signal R. Responsive to the outputs from

phase shifters

170 and 171 an adder 172 provides an output (L'+R +45), whereas a subtractor 173 provides an output (LR +45). A phase discriminator 174 for controlling the left and right channels operates to detect the phase difference between the output signals (L+R +45) and (LR +45) to produce control outputs Er and El. A matrix circuit 175 is connected to receive the signal L and the signal R through a variable gain amplifier 176 to produce outputs FL3 (=L'+IR) and RL3(=LIR). Similarly, a matrix circuit 177 is connected to receive the signal L through a variable gain amplifier 178, and the signal R to produce outputs FR3(=R+rL) and RR3(=R'-RL') where l and r represent the gains of the

variable gain amplifiers

176 and 178 respectively. These gains are controlled in the opposite directions in a range of from 0 to 3.414 by the outputs Er and El from the phase discriminator 174. The gains of the

variable gain amplifiers

132 and 137 are controlled in the opposite directions in a range of from 0 to 3.414 by the outputs Ef and Eb of phase discriminator as the control unit 49 for controlling rear and front channels by detecting the phase difference between the signals L and R. The output FL1 from the matrix circuit 133 is coupled to one input of an adder 179 via a l/ 2 attenuator 180 and the output FL3 from the matrix circuit 175 is applied to the other input of adder 179. Similarly, the output FRl from the matrix circuit 133 is applied to one input of an adder 181 through a l/ 2 attenuator 182 whereas the output FR3 from the matrix circuit 177 is coupled to the other input of adder 181. Likewise, the output RL1 of matrix circuit 138 is applied to one input of an adder 183 through a l/ \f2 attenuator 184 while the output RL3 of matrix circuit 175 is applied to the other input of adder 183. The output RRl of matrix circuit 138 is coupled to one input of adder 185 through a l V2 attenuator 186 and the output RR3 of the matrix circuit 177 is applied to the other input of adder 185.

It will be clear that the four channel signals FL, FR, RL and RR are expressed by the following equations.

FIG. 14 shows a block diagram of a modification of the variable matrix circuit shown in FIG. 13. In FIG. 14,

variable gain amplifiers

132 and 137 are controlled respectively by the outputs Ef and Eb from a comparator as the first control unit 49 for the front-rear control which detects the difference in the levels of the sum signal (L+R) and the difference signal (L--R), and

variable gain amplifiers

176 and 178 are controlled respectively by the outputs El and Er from a comparator 190 as the second control unit for the left and right control which detects the difference in the levels of the left signal L and the right signal R. In this manner, it is possible to obtain the same effect as the variable matrix circuit shown in FIG. 13 by detecting the difference in the signal levels.

A conventional 2-channel stereophonic disc is so designed that the reproduced sound image of a low-tone instrument may be localized midway between the left and right loudspeakers. To use such a 2-channel stereophonic disc for 4-channel sound reproduction, the sound images of low-tone and high-tone instruments may better be localized respectively at the front and back of the reproduced sound field, thereby to effect a desirable 4-channel sound reproduction. To this end, the characteristics of the converter 11 should be such that two composite signals L and R produced by the converter will possess a phase relationship which varies in accordance with the frequency of the input signals. In other words, the phase difference between the composite signals L and R should approach 180 as the frequencies of input signals L and R grow higher. Shown in FIG. 15 is an embodiment of a preferable signal converter having such phasefrequency characteristics and preferably combined with a variable matrix circuit.

In FIG. 15, a transistor Q has its base connected to an input terminal 21, and a capacitor C and a resistor R are connected in series between the collector and emitter of the transistor Q with their junction connected to an output terminal 23. A collector resistor R and an emitter resistor R of the same value are provided for the transistor Q The transistor Q the resistors R R and R and the capacitor C constitute a phase shifter of well-known type. Similarly, a transistor Q, has its base connected to an input terminal 22, and a capacitor C and a resistor R are connected in series between the collector and emitter of the transistor Q6 with their junction connected to an output terminal 24. A collector resistor R and an emitter resistor R of the same value are provided for the transistor Q Another resistor R is used to connect the emitters of the transistors Q 5 and Q The signal converter of the above-mentioned construction functions in the following manner. Between input signal L supplied to the input terminal 21 and output signal L appearing at the output terminal 23 there is observed a phase difference having such a frequency dependency as represented by curve I in FIG. 16. If input signal L has a frequency f defined by the capacitor C and the resistor R (e.g. when C 0.01 microfarad and R 22 kiloohms, f /zrrC R 723 Hz), the resultant phase difference is 90. Thus the phase difference goes near --l as the frequency of the input signal L grows higher. As proved by curve II in FIG. 17, however, the gain of the phase shifter including the transistor Q, is constant with respect to signal L regardless of the changes of frequency of signal L.

Signal L coming out of the emitter of the transistor Q is attenuated by an attenuator constituted by the re sistors R and R and appears at the output terminal 24 through a high-pass filter constituted by the capacitor C and the resistor R Signal L undergoes, at the highpass filter, such a phase variation as indiated by curve II in FIG. 16 and, at the attenuator and high-pass filter, such a gain variation as depicted by curve IV in FIG. 17. The phase of signal L is varied by 45 at the highpass filter if the frequency of signal L is f The abovementioned operation of the signal converter is applicable also to the input signal R.

As mentioned above, the phase difference between the output signals L and R from the signal converter is about when input signal L has a frequency of low frequency band. However, level amplitude lelvel of output signal R can be negligibly small compared with output signal L. If input signal L has a frequency of high frequency band, output signals L and R are opposite in phase to each other. Accordingly, so long as input signals L and R are of frequencies of low frequency band, the phase relationship between output signals L and R can never be reversed even in only one of input signals is supplied to the input terminal, and signals having frequencies of low frequency band are never localized at the back of the reproduced sound field. If input signals L and R are of an equal frequency of low frequency band, that is, if they are to be. localized midway between the left and right loudspeakers, they will be localized at the front of the reproduced sound field. If only input signal L is of a frequency of high frequency band, it will be localized at the rear-left loudspeaker. And if only signal R is of a frequency of high frequency band, it will be localized at the rearright loudspeaker.

FIG. 11 shows another embodiment of this invention in which a signal converter is used for the two-channel playback of a matrix four-channel source. The first and second audio signals L and R reproduced from the matrix four-channel source which may be a matrix fourchannel stereo record, a matrix four-channel magnetic tape or a matrix four-channel FM stereo signal source are expressed by the following equation;

L =FL AFR +jRL +jARR R= FR AFL jRR jARL Where only the rear-left signal RL presents, L RL and R ARL. Where signal converter 11 shown in FIG. 5 or 6 is adjusted to provide two sum signals L L Br and R R BL which are applied to a conventional Z-channel reproducer 122, the signal converter described above will produce outputs L RL ABRL and R RL BRL. Accordingly, outputs L (lA )RL and R can be produced by a proper adjustment of B. This means that it is possible to make infinity the separation between rear signals. Thus, it will be seen that incorporation of a signal converter which produces sum signals into a combination of a matrix four-channel source containing rear signals of an especially large level and a two-channel reproducing system is advantageous in the reproduction of a matrix fourchannel sound source by a two-channel system.

Where only a front left signal FL presents, L FL and R AFL. When the potentiometer resistors of the signal converter shown in FIGS. and 6 are adjusted to provide difference signals L L OR and R R aL respectively, the outputs will become L' FL AaFL and R AFL aFL. Thus, it is possible to obtain outputs L (1A )FL and R 0 by a proper adjustment of a. This shows that it is possible to make infinity the separation between front signals. Accordingly, when a signal converter 11 as shown in FIG. 5 or 6 is used in the circuit shown in FIG. 11 it is possible to control the separation between rear signals or front signals in accordance with the content recorded on a matrix four-channel recording medium. In the case of a jazz, the level of the rear signal is relatively high whereas in the case of a classical music the level of the rear signal is low.

What we claim is:

1. A sound reproducing system for reproducing stereophonically related first and second channel signals from a two-channel signal source by means of a fourchannel reproducing system, said system comprising a combination of:

first means connected to receive the first and second channel signals for producing first and second composite signals by combining the first and second channel signals with such amplitude and phase relationships that the first composite signal contains a greater proportion of the first channel signal and a smaller proportion of the second channel signal, the second composite signal contains a greater proportion of the second channel signal and a smaller proportion of the first channel signal, and for producing phase differences between the first and second composite signals with respect to the first and second channel signals respectively which vary as a function of frequency of the first and second channel signals;

control means connected to receive the first and second composite signals for producing control outputs the levels of which vary depending on the phase relationship between the first and second composite signals; and

second means connected to receive, as first and second input signals, the first and second composite signals or the first and second channel signals for producing four-channel output signals by combining the first and second input signals, said second means being operative to produce the four-channel output siganls varying at least mixing ratios of the first and second input signals depending on the levels of the control outputs of said control means.

2. A sound reproducing system according to claim 1 wherein the phase differences between the first and second composite signals with respect to the first and second channel signals represent substantially at a low frequency and substantially at a high frequency, respectively.

3. A sound reproducing system according to claim 1 wherein the amplitude levels of the second and first channel signals contained in the first and second composite signals vary as a function of frequency, respectively.

4. A sound reproducing system according to claim 1 wherein said first means comprises first and second phase shifter means each including a transistor having a collector, and emitter and a base, and a resistor and a capacitor connected in series across the collector and emitter of the transistor; and a resistor means connecting the emitter of transistor in said first phase shifter means with the emitter of transistor in said second phase shifter means, the bases of transistors in said first and second phase shifter means being connected to receive the first and second channel signals, respectively, the resistor-capacitor junctions in said first and second phase shifter means being adapted to derive the first and second composite signals, respectively.

5. A sound reproducing system according to claim 1 wherein said second means comprises a first differential amplifier having first and second output terminals deriving a pair of output signals and first and second input terminals, the first input terminal being connected to receive the first input signal; phase reversing means for reversing the phase of the second input signal; means for coupling the output of said phase reversing means to the second input terminal of said first differential amplifier; a second differential amplifier having first and second output terminals deriving another pair of output signals and first and second input terminals connected to receive the first and second input signals, respectively; and means for controlling the gains of said first and second differential amplifiers in opposite relationship in accordance with the levels of control outputs from said control means.

6. A second reproducing system according to claim 1 wherein said second means comprises means connected to receive the first and second input signals A and B for producing a first output signal substantially proportional to (1+j)A (lf)B where f represents a first variable matrix coefficient; means connected to receive the first and second input signals A and B for producing second output signal substantially proportional to (1+f)B l-f)A; means connected to receive the first and second input signals A and B for producing a third output signal substantially proportional to (l+b)A (1-b)B where b represents a second variable matrix coefficient; means connected to receive the first and second input signals A and B for producing a fourth output signal substantially proportional to (1+b)B (lb)A; and means for varying the matrix coefficients f and b in opposite relationship depending on the levels of control outputs from said control means.

7. A sound reproducing system according to claim 1 wherein said control means includes a phase discriminator for detecting the phase difference between the first and second composite signals.

8. A sound reproducing system according to claim 1 wherein said control means includes a level comparator for comparing the levels of sum and difference of the first and second composite signals.

9. A sound reproducing system according to claim 1 further comprising further control means for producing control outputs in response to the first and second composite signals; and wherein said second means comprises means connected to receive the first and second input signals A and B for producing a first output signal substantially proportional to (l+f+ VZA (1f 51 )8, where f and I represent variable matrix coefficients; means connected to receive the first and second input signals A and B for producing a second output signal substantially proportional to (1+f+ E)B (lf+ 2 r)A, where r represent a variable matrix coefficient; means connected to receive the first and second input signals A and B for producing a third output signal substantially proportional to (l+b+ 2 )A (lb+ VEDB, where b represent a variable matrix coefficient; means connected to receive the first and second input signals A and B for producing a fourth output signal substantially proportional to (l+b+ )B (1-b+ V 2 r)A; and means for varying the matrix coefficients f and b in opposite relationship depending on the levels of control outputs from said control means; and means for varying the matrix coefficients l and r in opposite relationship depending on the levels of control outputs from said further control means.

10. A sound reproducing system according to claim 9 wherein said further control means includes a phase discriminator for detecting phase difference between sum and difference of the first and second composite signals.

11. A sound reproducing system according to claim 9 wherein said further control means includes a level comparator for comparing the levels of the first and second composite signals.

12. A sound reproducing system for reproducing stereophonically related first and second channel signals from a two-channel signal source by means of a fourchannel reproducing system, said system comprising a combination of:

first means connected to receive the first and second channel signals for producing first and second composite signals by combining the first and second channel signals with such amplitude and phase relationships that the first composite signal contains a greater proportion of the first channel signal and a smaller proportion of the second channel signal, the second composite signal contains a greater pro portion of the second channel signal and a smaller proportion of the first channel signal, and for producing phase differences between the first and second composite signals with respect to the first and second channel signals respectively vary as a function of frequency of the first and second channel signals;

first control means connected to receive the first and second composite signals for producing control outputs the levels of which vary depending on the phase relationship between the first and second composite signals;

second control means connected to receive the first and second composite signals for producing control outputs the levels of which vary depending on the phase relationship between sum and difference of the first and second composite signals;

second means connected to receive, as first and second input signals, the first and second composite signals or the first and second channel signals for producing four-channel output signals by combining the first and second input signals, said second means including means for varying the mixing ratios of and polarity relationship between the first and second input signals in producing the output signals depending on the levels of control outputs from said first control means; and means for varying the mixing ratios of the first and second input signals in producing the output signals depending on the levels of control outputs from said second control means.

13. A sound reproducing system for reproducing stereophonically related first and second channel signals from a two-channel signal source by means of a fourchannel reproducing system, said system comprising a combination of:

first means connected to receive the first and second channel signals for producing first and second composite signals by combining the first and second channel signals with such amplitude and phase relationships that the first composite signal contains a greater proportion of the first channel signal and a smaller proportion of the second channel signal, the second composite signal contains a greater proportionof the second channel signal and a smaller proportion of the first channel signal, and for producing phase differences between the first and second composite signals with respect to the first and second channel signals respectively vary as a function of frequency of the first and second channel signals;

second means connected to receive, as first and second input signals, the first and second composite signals or the first and second channel signals for producing four-channel output signals by combining the first and second input signals, said second means including means for varying the mixing ratios of and polarity relationship between the first and second input signals in producing the output signals depending on the levels of control outputs from said control means.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3,889,061 DATED June 10, 1975 INVENTOR(S) Ryosuke Ito and Susumu Takahashi It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Add on first page of patent:

[30] Foreign Application Priority Data December 21, 1971 Japan 103970/71 Signed and Scaled this A tres r:

RUTH C. MASON C. MARSHALL DANN Arresting Officer (ommissinncr uj'lalenrs and Trademarks

Claims

1. A sound reproducing system for reproducing stereophonically related first and second channel signals from a two-channel signal source by means of a four-channel reproducing system, said system comprising a combination of: first means connected to receive the first and second channel signals for producing first and second composite signals by combining the first and second channel signals with such amplitude and phase relationships that the first composite signal contains a greater proportion of the first channel signal and a smaller proportion of the second channel signal, the second composite signal contains a greater proportion of the second channel signal and a smaller proportion of the first channel signal, and for producing phase differences between the first and second composite signals with respect to the first and second channel signals respectively which vary as a function of frequency of the first and second channel signals; control means connected to receive the first and second composite signals for producing control outputs the levels of which vary depending on the phase relationship between the first and second composite signals; and second means connected to receive, as first and second input signals, the first and second composite signals or the first and second channel signals for producing four-channel output signals by combining the first and second input signals, said second means being operative to produce the four-channel output siganls varying at least mixing ratios of the first and second input signals depending on the levels of the control outputs of said control means.

2. A sound reproducing system according to claim 1 wherein the phase differences between the first and second composite signals with respect to the first and second channel signals represent substantially 90* at a low frequency and substantially 180* at a high frequency, respectively.

6. A second reproducing system according to claim 1 wherein said second means comprises means connected to receive the first and second input signals A and B for producing a first output signal substantially proportional to (1+f)A + (1-f)B where f represents a first variable matrix coefficient; means connected to receive the first and second input signals A and B for producing second output signal substantially proportional to (1+f)B + (1-f)A; means connected to receive the first and second input signals A and B for producing a third output signal substantially proportional to (1+b)A - (1-b)B where b represents a second variable matrix coefficient; means connected to receive the first and second input signals A and B for producing a fourth output signal substantially proportional to (1+b)B - (1-b)A; and means for varying the matrix coefficients f and b in opposite relationship depending on the levels of control outputs from said control means.

9. A sound reproducing system according to claim 1 further comprising further control means for producing control outputs in response to the first and second composite signals; and wherein said second means comprises means connected to receive the first and second input signals A and B for producing a first output signal substantially proportional to (1+f+ Square Root 2)A + (1-f + Square Root 2l)B, where f and l represent variable matrix coefficients; means connected to receive the first and second input signals A and B for producing a second output signal substantially proportional to (1+f+ Square Root 2)B + (1-f+ Square Root 2r)A, where r represent a variable matrix coefficient; means connected to receive the first and second input signals A and B for producing a third output signal substantially proportional to (1+b+ Square Root 2)A - (1-b+ Square Root 2l)B, where b represent a variable matrix coefficient; means connected to receive the first and second input signals A and B for producing a fourth output signal substantially proportional to (1+b+ Square Root 2)B - (1-b+ Square Root 2r)A; and means for varying the matrix coefficients f and b in opposite relationship depending on the levels of control outputs from said control means; and means for varying the matrix coefficients l and r in opposite relationship depending on the levels of control outputs from said further control means.

12. A sound reproducing system for reproducing stereophonically related first and second channel signals from a two-channel signal source by means of a four-channel reproducing system, said system comprising a combination of: first means connected to receive the first and second channel signals for producing first and second composite signals by combining the first and second channel signals with such amplitude and phase relationships that the first composite signal contains a greater proportion of the first channel signal and a smaller proportion of the second channel signal, the second composite signal contains a greater proportion of the second channel signal and a smaller proportion of the first channel signal, and for producing phase differences between the first and second composite signals with respect to the first and second channel signals respectively vary as a function of frequency of the first and second channel signals; first control means connected to receive the first and second composite signals for producing control outputs the levels of which vary depending on the phase relationship between the first and second composite signals; second control means connected to receive the first and second composite signals for producing control outputs the levels of which vary depending on the phase relationship between sum and difference of the first and second composite signals; second means connected to receive, as first and second input signals, the first and second composite signals or the first and second channel signals for producing four-channel output signals by combining the first and second input signals, said second means including means for varying the mixing ratios of and polarity relationship between the first and second input signals in producing the output signals depending on the levels of control outputs from said first control means; and means for varying the mixing ratios of the first and second input signals in producing the output signals depending on the levels of control outputs from said second control means.

13. A sound reproducing system for reproducing stereophonically related first and second channel signals from a two-channel signal source by means of a four-channel reproducing system, said system comprising a combination of: first means connected to receive the first and second channel signals for producing first and second composite signals by combining the first and second channel signals with such amplitude and phase relationships that the first composite signal contains a greater proportion of the first channel signal and a smaller proportion of the second channel signal, the second composite signal contains a greater proportion of the second channel signal and a smaller proportion of the first channel signal, and for producing phase differences between the first and second composite signals with respect to the first and second channel signals respectively vary as a function of frequency of the first and second channel signals; control means connected to receive the first and second composite signals for producing control outputs the levels of which vary depending on the phase relationship between the first and second composite signals; and second means connected to receive, as first and second input signals, the first and second composite signals or the first and second channel signals for producing four-channel output signals by combining the first and second input signals, said second means including means for varying the mixing ratios of and polarity relationship between the first and second input signals in producing the output signals depending on the levels of control outputs from said control means.