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Publication numberUS3885101 A
Publication typeGrant
Publication dateMay 20, 1975
Filing dateDec 18, 1972
Priority dateDec 21, 1971
Also published asDE2262761A1, DE2262761B2, DE2262761C3
Publication numberUS 3885101 A, US 3885101A, US-A-3885101, US3885101 A, US3885101A
InventorsRyosuke Ito, Susumu Takahashi
Original AssigneeSansui Electric Co
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Signal converting systems for use in stereo reproducing systems
US 3885101 A
Abstract
A signal converter is provided for intermixing first and second audio signals reproduced from a conventional 2-channel recording medium to produce two difference signals which are supplied to a 2 to 4-channel converter for improving the separation between the front and rear signals. The signal converter may be constructed to mix together first and second audio signals at a variable relative amplitude ratio therebetween and with a variable polarity relationship to form two sum signals or two difference signals.
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Description  (OCR text may contain errors)

United States Patent Ito et al. 1 May 20, 1975 SIGNAL CONVERTING SYSTEMS FOR USE 3,697,692 10/1912 l-c-ilafler 179/1 00 3,710,023 l/l973 reuzard [N STEREO REPRODUCING SYSTEMS 3,718,773 2/1973 Berkovitz [75] Inventors: Ryosuke Ito; Susumu Takahashl, 3,725,586 4/1973 lida both of Tokyo, Japan 3,745,254 7/1973 Ohta 7 7 91973 Ih'da 179i [73] Assignee: Sansui Electric Co., Ltd., Tokyo, 57 04 l s I I Go Japan Primary Examiner-Kathleen H. Claffy [22] Filed: Dec. 18, 1972 Assistant Examiner-Thomas D Amico [211 App] No: 315,928 gnome Agent, or Firm-Hams, Kern, Wallen &

msley [30] Foreign Application Priority Data [57] ABSTRACT Dec. 21, I971 Japan 46-[03970 A signal convene], is provided for intermixing first and second audio signals reproduced from a conventional [52] US. Cl. 179/1 GQ; 179/ 100.4 ST; Lchaune] recording medium to Produce two differ 179/100" TD ence signals which are supplied to a 2 to 4-channel Ill. converter f i p i g the separation between the [58] Field of Search "179/1 1 front and rear signals. The signal converter may be 179/1004 loo'l 15 BT constructed to mix together first and second audio signals at a variable relative amplitude ratio therebe- [56] References Cited tween and with a variable polarity relationship to form UNITED STATES PATENTS two sum signals or two difference signals. 3,170,99I 2/1965 Glasgal l79/l G 3,329,712 4/1967 Farrell 20 Claims 14 Drawing Figures 3,684,835 8/1972 179/! G FL s 0-0" 5 FR m D w o c: 5 3 6 RL 19 0 CD X s E 1m 2 m lo 2i 5 2 RR o+ 9o 1 ECi CONTROL UNIT EC2 SHEET 2 OF 8 CONTROL UNIT FIG. 3

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SIGNAL CONVERTING SYSTEMS FOR USE IN STEREO REPRODUCING SYSTEMS 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 or to reproduce two-channel signals from a matrix four-channel source by a two 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 between reproducing channels is extremely large. Specifcally, 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-channnel 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.

Matrix four-channel stereo records are also compatible with the conventional two-channel reproducing system. However, since two-channel signals recorded on a matrix four-channel stereo record usually contain four-channel signals, the separation between the reproduced two-channel signals is extremely poor.

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 or from a matrix four-channel recording medium by a two-channel reproduction system.

According to one aspect of this invention there is provided a signal converting system for reproducing two-channel stereo signals from a source of twochannel signals by means of a four-channel stereo reproducing system. said signal converting system comprising a combination of first signal converting means including means converted to receive the first and second channel signals from the two-channel signal source for combining the first channel signal with a 180 outof-phase portion of the second channel signal and means for combining the second channel signal with a 180 out-of-phase portion of the first channel signal; and second signal converting means connected to receive two output signals from the first signal converting means for converting the two output signals into fourchannel signals.

According to another aspect of this invention there is provided a signal converting system for reproducing two-channel stereo signals from a source of matrix four-channel signals by means of a two-channel stereo reproducing system, said signal converting system comprising means for combining the first channel signal with the second channel signal at a variable relative amplitude ratio therebetween and with a variable polarity relationship and means for combining the second channel signal with the first channel signal at a variable relative amplitude ratio and with a variable polarity relationship.

The present invention can be more fully understood from the following detailed description when taken in connection with reference to the accompanying drawings, in which:

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

FIG. 2 shows a connectiondiagram 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 embodiment 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. I0 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;

FIG. 12 shows a block diagram of another variable matrix circuit;

FIG. 13 shows a block diagram of another variable matrix circuit; and

FIG. 14 shows a block diagram of a modification of the variable matrix circuit of FIG. 13.

With reference now to FIG. 1 of the accompanying drawing illustrating a preferred embodiment of this invention, reference numeral 10 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 H0. 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 l5, l7, l9 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' (LXaR) and R (RXaL) 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( l-Aa) 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 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 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 0 along the potentiometer resistors 26 and 31, respectively, there are respectively derived from the output terminals 23 and 24 two difference signals LaR annd 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-i-BR and R+BL each having varying relative amplitude ratio. In the embodiment shown in FIG. I 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 O is connected across a source indicated by +8 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.

ln 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 at two difference signals L-aR and R-aL 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-t-BR and R-l-BL are produced.

In another embodiment of the signal converter shown in FIG. 7, each of the collector-emitter paths of transistors Q and Q 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 interconnected 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+ RL+jARR and R=FR+AFL-jRR-jARL 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 FIGS. 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, this 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 6S supply the first and second control outputs EC 1 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. shows the operating characteristic of the phase discriminator showing that the first and second control outputs EC 1 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 EC] 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 cross-talk. 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 channel 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 11 produces two difference signals L-ozR and R-aL in in-phase relationship, the outputs ECl 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 ECl 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 the 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 sumsignals 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. revibration components, of these signals are localized at the back of the sound reproduction 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 Pat. application Ser. No. 298,933, filed Oct. 19, 1971, of the title Decoder for use in 4-2-4 matrix playback system now US. Pat. No. 3825684.

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 differ ence 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 130 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 13] 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 ECI from the control unit 49 and has an amplification gain f which varies from 0 to 2.41 with resepct to the gain of the first matrix circuit 130. The third matrix circuit I23 functions to produce a first output expressed by 1+1) L'+ l-f) R and a second output expressed by lj) L l-l-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 0 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( 1-b) R and a fourth output expressed by (1+b) R'-( lb)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 ECZ 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'+lR') and RL3 (=L'lR'). 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 PR3 (=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 3.4l4 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 ELl from the matrix circuit 133 is coupled to one input of an adder 179 via a l/ 2 attenuator I80 and the output FL3 from the matrix circuit is applied to the other input of adder 179. Similarly, the output FRI from the matrix circuit 133 is applied to one input of an adder 181 through a 1/ V7 attenuator 182 whereas the output PR3 from the matrix circuit 177 is coupled to the other input of adder 181. Likewise, the output RLl of matrix circuit 138 is applied to one input of an adder 183 through a 1/ W2 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 I38 is coupled to one input of adder 185 through a l 2 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.

F 1G. 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 signakL' 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.

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 equations:

R=FR+AFLjRR-jARL Where only the rear-left signal RL is present, L=RL and R=ARL. Where signal converter 11 shown in FIG. 5 or 6\is aadjusted to provide two sum signals L'=L+BR and R'=R+BL which are applied to a conventional two-channel reproducer 122, the signal converter described above will produce outputs L'=RL-ABRL and R'=ARL+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 betweeen 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 four-channel 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-aR and R'=RaL respectively, the outputs will become L'=FL-AaFL and R'=AFLaFL. Thus, it is possible to obtain outputs L'"-( l-A) 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 fourchannel recording medium. In the case of a jazz music. the level of the rear signal is relatively high whereas in the case of classical music the level of the rear signal is low.

What we claim is:

1. A signal converting system for reproducing stereophonically related first and second channel 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 for producing first and second composite signals by combining the first and second channel signals;

a control unit responsive to the phase relationship between the first and second composite signals for producing first and second control outputs, the levels of which vary in opposite relationship; and

second signal converting means connected to receive the first and second channel signals for producing four-channel output signals by combining the first and second channel signals, said second signal converting means including means for controlling at least a relative amplitude ratio between the first and second channel signals contained in each of a pair of output signals in accordance with the level of said first control output and means for controlling at least a relative amplitude ratio between the first and second channel signals contained in each of another pair of output signals in accordance with the level of said second control output.

2. A signal converting system according to claim 1 wherein the first composite signal is a sum signal of the first and second channel signals which contains the first channel signal at a larger amplitude level and the second channel signal at a smaller amplitude level, and the second composite signal is a sum signal of the first and second channel signals which contains the first channel signal at a smaller amplitude level and the second channel signal at a larger amplitude level.

3. A signal converting system according to claim 1 wherein the first composite signal is a difference signal of the first and second channel signals which contains the first channel signal at a larger amplitude level and the second channel signal at a smaller amplitude level, and the second composite signal is a difference signal of the first and second channel signals which contains the first channel signal at a smaller amplitude level and the second channel signal at a larger amplitude level.

4. A signal converting system according to claim 1 wherein said second signal converting means comprises:

a first differential amplifier having first and second output terminals deriving a pair of output signals and first and second input terminals, said first input terminal being connected to receive the first channel signal;

phase reversing means for reversing the phase of the second channel signal;

means for coupling the output of said phase reversing means to said second input terminal of said first differential amplifier;

means for controlling the gain of said first differential amplifier in accordance with the level of said first control output;

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 said first and second channel signals respectively; and

means for controlling the gain of said second differential amplifier in accordance with the level of said control output.

5. A signal converting system according to claim 1 wherein said second signal converting means comprises:

means connected to receive the first and second channel signals L and R for producing a first output signal substantially proportional to l+f)L+( lj)R where f represents a first variable matrix coefficient;

means connected to receive the first and second channel signals L and R for producing a second output signal substantially proportional to (1+j)R+(lflL;

means connected to receive the first and second channel signals L and R for producing a third output signal substantially proportional to (l+b)L-(- l-b) R where b represents a second variable matrix coefficient;

means connected to receive the first and second channel signals L and R for producing a third output signal substantially proportional to (l+b)R means for varying the first variable matrix coefficient f in accordance with the level of said first control output; and

means for varying the second variable matrix coetficient b in accordance with the level of said second control output.

6. A signal converting system according to claim 1 which further comprises a further control unit for producing third and fourth control outputs, the levels of which vary in opposite relationship in response to the first and second composite signals, and

wherein said second signal converting means comprises:

means connected to receive the first and second channel signals L and R for producing a first output signal substantially proportional to (l+f+ {ilLH l-f-l- /51 )L; wherefand l represent variable matrix coefficients;

means connected to receive the first and second channel signals L and R for producing a second output signal substantially proportional to (1+f+ /)R+( lf+ 1/ 2 r)L, where r represents a variable matrix coefficient;

means connected to receive the first and second chaanel signals L and R for producing a third output signal substantially proportional to (l+b+ {2 )L( lb+ film, where b represents a variable matrix coefficient;

means connected to receive the first and second channel signals L and R for producing a fourth out- 12 put signal substantially (l+b+ 4'2')R- 1b+ 421.; means for varying the coefficient f in accordance with the level of said first control output means for varying the coefficient b in accordance with the level of said second control output;

means for varying the coefficient 1 in accordance with the level of said third control output; and

means for varying the coefficient r in accordance with the level of said fourth control output.

7. A signal converting system according to claim I wherein said control unit comprises a phase discriminator for detecting the phase difference between the first and second composite signals.

8. A signal converting system according to claim 1 wherein said control unit comprises a level comparator for comparing levels of the sum and difference signals of the first and second composite signals.

9. A signal converting system according to claim 6 wherein said further control unit comprises a phase discriminator for detecting phase difference between the sum and difference signals of the first and second composite signals.

10. A signal converting system according to claim 6 wherein said further control unit comprises a level comparator for comparing the levels of the first and second composite signals.

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

first signal converting 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;

a control unit responsive to the phase relationship between the first and second composite signals for producing first and second control outputs, the lev els of which vary in opposite relationship; and

second signal converting means connected to receive the first and second composite signals for producing four-channel output signals by combining the first and second composite signals, said second signal converting means including means for controlling at least a relative amplitude ratio between the first and second composite signals contained in each of a pair of output signals in accordance with the level of said first control output and means for controlling at least a relative amplitude ratio between the first and second composite signals contained in each of another pair of output signals in accordance with the level of said second control output.

12. A signal converting system according to claim 11 wherein the first composite signal is a sum signal of the first and second channel signals which contains the first channel signal at a larger amplitude level and the second channel signal at a smaller amplitude level, and the second composite signal is a sum signal of the first and second channel signals which contains the first channel signal at a smaller amplitude level and the second channel signal at a larger amplitude level.

13. A signal converting system according to claim ll wherein the first composite signal is a difference signal of the first and second channel signals which contains the first channel signal at a larger amplitude level and proportional to the second channel signal at a smaller amplitude level, and the second composite signal is a difference signal of the first and second channel signals which contains the first channel signal at a smaller amplitude level and the second channel signal at a larger amplitude level.

14. A signal converting system according to claim 11 wherein said second signal converting means comprises:

a first differential amplifier having first and second output terminals deriving a pair of output signals and first and second input terminals, said first input terminal being connected to receive the first composite signal;

phase reversing means for reversing the phase of the second composite signal;

means for coupling the output of said phase reversing means to said second input terminal of said first different'ial amplifier;

means for controlling the gain of said first differential amplifier in accordance with the level of said first control output;

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 said first and second composite signals respectively; and

means for controlling the gain of said second differential amplifier in accordance with the level of said second control output.

15. A signal converting system according to claim 11 wherein said second signal converting means comprises:

means connected to receive the first and second composite signals L' and R for producing a first output signal substantially proportional to (l+j)L-H-( lf)R' where f represents a first variable matrix coefficient;

means connected to receive the first and second composite signals L and R for producing a second output signal substantially proportional to (1+f)R'+( l-f)L';

means connected to receive the first and second composite signals L and R for producing a third output signal substantially proportional to (l+b)L'-( lb)R where b represents a second variable matrix coefficient;

means connected to receive the first and second composite signals L and R for producing a third output signal substantially proportional to (l+b)R'-( lb)L;

means for varying the first variable matrix coefficient f in accordance with the level of said first control output; and

means for varying the second variable matrix coefficient b in accordance with the level of said second control output.

16. A signal converting system according to claim 11 which further comprises a further control unit for producing third and fourth control outputs, the levels of which vary in opposite relationship in response to the first and second composite signals, and

wherein said second signal converting means comprises: means connected to receive the first and second composite signals L and R for producing a first output signal substantially proportional to (l+f+ {2 L'+( l-f+ 450R. where fand I represent variable matrix coefficients; means connected to receive the first and second composite signals L and R' for producing a second output signal substantially proportional to (l+f+ fi)R'+( lf+ EHL'. where r represents a variable matrix coefficient;

means connected to receive the first and second composite signals L and R' for producing a third output si nal substantially proportional to (H-b+ 2)L(lb+ {27)R', where h represents a variable matrix coefficient;

means connected to receive the first and second composite signals L and R for producing a fourth output signal substantially proportional to (l+b+ 47 R'( lb+ Jim;

means for varying the coefficient f in accordance with the level of said first control output;

means for varying the coefficient b in accordance with the level of said second control output; means for varying the coefficient l in accordance with the level of said third control output; and means for varying the coefficient r in accordance with the level of said fourth control output.

17. A signal converting system according to claim 11 wherein said control unit comprises a phase discriminator for detecting the phase difference between the first and second composite signals.

18. A signal converting system according to claim 11 wherein said control unit comprises a level comparator for comparing levels of the sum and difference signals of the first and second composite signals.

19. A signal converting system according to claim 16 wherein said further control unit comprises a phase discriminator for detecting phase difference between the sum and difference signals of the first and second composite signals.

20. A signal converting system according to claim 16 wherein said further control unit comprises a level comparator for comparing the levels of the first and second composite signals.

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Classifications
U.S. Classification381/18, 369/90
International ClassificationH04S3/00, H04H20/89, H04S3/02
Cooperative ClassificationH04S3/02, H04H20/89
European ClassificationH04S3/02, H04H20/89