Title:
MULTISOUND REPRODUCING APPARATUS FOR DERIVING FOUR SOUND SIGNALS FROM TWO SOUND SOURCES
United States Patent 3725586
Abstract:
Left and right sound signals applied to two separate input circuits are each shifted in phase by a predetermined amount by phase shifters and are then supplied to separate output circuits. The left sound signal is also fed through a low pass filter to be combined with the phase shifted right sound signal and this combined signal is supplied to a separate output circuit. Likewise the right sound signal is fed through a low pass filter to be combined with the phase shifted left sound signal and this combined signal is also supplied to a separate output circuit. The resultant four outputs give the listener the illusion of naturalness.
US Patent References:
FOUR CHANNEL STEREO SYNTHESIZER
Orban - August 1972 - 3684835
SOUND SYSTEM WITH ENHANCED LOW FREQUENCY DISTRIBUTION
Brickner - June 1971 - 3586783
METHOD AND APPARATUS FOR SIMULATING LOCATION AND MOVEMENT OF SOUND
Chowning - May 1972 - 3665105
COMPATIBLE STEREO GENERATOR
Holzer - February 1972 - 3646574
TWO-CHANNEL, FOUR-COMPONENT STEREOPHONIC SYSTEM
Hafler - October 1972 - 3697692
FOUR CHANNEL STEREO SYNTHESIZER
Orban - August 1972 - 3684835
SOUND SYSTEM WITH ENHANCED LOW FREQUENCY DISTRIBUTION
Brickner - June 1971 - 3586783
METHOD AND APPARATUS FOR SIMULATING LOCATION AND MOVEMENT OF SOUND
Chowning - May 1972 - 3665105
COMPATIBLE STEREO GENERATOR
Holzer - February 1972 - 3646574
TWO-CHANNEL, FOUR-COMPONENT STEREOPHONIC SYSTEM
Hafler - October 1972 - 3697692
Application Number:
05/242945
Publication Date:
04/03/1973
Filing Date:
04/11/1972
View Patent Images:
Export Citation:
Assignee:
Sony Corporation (Tokyo, JA)
Primary Class:
International Classes:
H04S3/02; H04S5/02; H04S3/00; H04S5/00; H04R5/04
Field of Search:
179/1G,1GP,1GQ,15BT,1.4ST,1.1TD
Primary Examiner:
Claffy, Kathleen H.
Assistant Examiner:
D'amico, Thomas
Claims:
What is claimed is
1. A multisound reproducing system comprising a first input circuit for receiving a first sound signal, a second input circuit for receiving a second sound signal, a first phase shifting circuit connected to the first input circuit for shifting the phase of the first sound signal by a predetermined phase angle, a second phase shifting circuit connected to the second input circuit for shifting the phase of the second input signal by a predetermined phase angle, means responsive to the phase shifted first input signal for producing a first output, means responsive to the phase shifted second input signal for producing a second output, a first filter circuit for filtering the first input signal, a second filter circuit for filtering the second input signal, means for combining the filtered first input signal with the phase shifted second input signal and for producing a third signal, means for combining the filtered second input signal with the phase shifted first input signal to produce a fourth signal, means responsive to the third signal for producing a third output and means responsive to the fourth signal for producing a fourth output.
2. A multisound reproducing system as recited in claim 1 wherein each of the filtering circuits comprises a low pass filter.
3. A multisound reproducing system as recited in claim 1 wherein the phase shift provided by each of the first and the second phase shifting circuits varies in substantially direct proportion to the logarithm of the frequency of the signal being phase shifted within a band range centered about a predetermined frequency, the phase shift provided at the predetermined frequency being 180° .
4. A multisound reproducing system as recited in claim 1 wherein the first and the second filter circuits and the first and the second phase shifting circuits are designed such that when the first and the second input signals have the same phase and are at a predetermined frequency the output signals from the first and the second filter circuits each have a phase difference of 180° with respect to the outputs from the first and the second phase shifting circuits.
5. A multisound reproducing system as recited in claim 4 wherein the predetermined frequency is selected to be within the range of 650 to 700 Hz.
6. A multisound reproducing system as recited in claim 1 further comprising a third phase shifting circuit connected to the first input circuit for shifting the phase of the first input signal by a predetermined angle with respect to the output signal from the first phase shifting circuit and a fourth phase shifting circuit connected to the second input circuit for shifting the phase of the second input signal by a predetermined angle with respect to the output signal from the second phase shifting circuit.
7. A multisound reproducing system as recited in claim 6 wherein the predetermined angle of phase shift provided by the third and the fourth shifting circuits is 90° .
8. A multisound reproducing system as recited in claim 6 further comprising switching means connected to the outputs of the third and the fourth phase shifting circuits and to the outputs of the means for producing the third signal and the means for producing the fourth signal for selectively transmitting the third and the fourth signals or the phase shifted signals from the third and the fourth phase shifting circuits to the means for producing the third and the fourth outputs.
1. A multisound reproducing system comprising a first input circuit for receiving a first sound signal, a second input circuit for receiving a second sound signal, a first phase shifting circuit connected to the first input circuit for shifting the phase of the first sound signal by a predetermined phase angle, a second phase shifting circuit connected to the second input circuit for shifting the phase of the second input signal by a predetermined phase angle, means responsive to the phase shifted first input signal for producing a first output, means responsive to the phase shifted second input signal for producing a second output, a first filter circuit for filtering the first input signal, a second filter circuit for filtering the second input signal, means for combining the filtered first input signal with the phase shifted second input signal and for producing a third signal, means for combining the filtered second input signal with the phase shifted first input signal to produce a fourth signal, means responsive to the third signal for producing a third output and means responsive to the fourth signal for producing a fourth output.
2. A multisound reproducing system as recited in claim 1 wherein each of the filtering circuits comprises a low pass filter.
3. A multisound reproducing system as recited in claim 1 wherein the phase shift provided by each of the first and the second phase shifting circuits varies in substantially direct proportion to the logarithm of the frequency of the signal being phase shifted within a band range centered about a predetermined frequency, the phase shift provided at the predetermined frequency being 180° .
4. A multisound reproducing system as recited in claim 1 wherein the first and the second filter circuits and the first and the second phase shifting circuits are designed such that when the first and the second input signals have the same phase and are at a predetermined frequency the output signals from the first and the second filter circuits each have a phase difference of 180° with respect to the outputs from the first and the second phase shifting circuits.
5. A multisound reproducing system as recited in claim 4 wherein the predetermined frequency is selected to be within the range of 650 to 700 Hz.
6. A multisound reproducing system as recited in claim 1 further comprising a third phase shifting circuit connected to the first input circuit for shifting the phase of the first input signal by a predetermined angle with respect to the output signal from the first phase shifting circuit and a fourth phase shifting circuit connected to the second input circuit for shifting the phase of the second input signal by a predetermined angle with respect to the output signal from the second phase shifting circuit.
7. A multisound reproducing system as recited in claim 6 wherein the predetermined angle of phase shift provided by the third and the fourth shifting circuits is 90° .
8. A multisound reproducing system as recited in claim 6 further comprising switching means connected to the outputs of the third and the fourth phase shifting circuits and to the outputs of the means for producing the third signal and the means for producing the fourth signal for selectively transmitting the third and the fourth signals or the phase shifted signals from the third and the fourth phase shifting circuits to the means for producing the third and the fourth outputs.
Description:
BACKGROUND OF THE INVENTION
This invention relates to a multisound decoding apparatus and more particularly to apparatus for reproducing a monaural audio information signal or stereophonic audio information signal with four separate loudspeakers without accompanying unnaturalness.
There is increasing interest in multiple channel recording and reproducing systems because of the variety of sounds and music forms which such systems reproduce. It is a well known phenomenon that the quality of music reproduction is enhanced when the number of reproduction channels is increased. In the early days of the phonograph, only a single channel or monophonic recording was used. Investigators soon realized the value, however, of recording and transmitting two separate channels of information, which in modern parlance is known as binaural or stereophonic sound.
However, even two channels of information are not considered sufficient for a full illusion of reality. For example, when a listener is placed in front of a symphony orchestra, he hears sounds arriving from many different directions and from a variety of instruments, as well as reflections from the walls and ceiling, which gives him an accustomed illusion of space perspective. When reproduction is accomplished by utilizing only two channels, however, it is difficult if not impossible, to produce true reality with respect to spatial perspective. Early experiments have demonstrated that a minimum of three independent channels are needed to convey a satisfactory illusion of reality in the reproduction of orchestral music.
Recently, some investigators have developed a multi-channel sound system, so called a four channel stereo system, providing third and fourth playback channels to an otherwise two-channel system by feeding third and fourth loudspeakers with signals.
If suitable sound media are prepared for the above mentioned multichannel sound system having the third and fourth channels, listeners may hear the sound from the sound system with a full illusion of reality. At present, however, sound media which can be used for such multichannel sound systems are insufficient and stereophonic sound media for two channels command a prominent majority.
In an attempt to provide a four channel system for use with stereophonic sound media one proposed prior system separates a stereophonic sound information signal into left and right sound signals defined as L and R, respectively, by a channel separator. The L signal is supplied to a loudspeaker placed on the left-hand side of a listener and in front of him while the R signal is supplied to a loudspeaker positioned on the right-hand side of the listener and in front of him as in a conventional manner. The proposed system further inverts the phase of the L signal to make it a signal L and supplies it to a loudspeaker positioned on the left-hand side of the listener at his back. Similarly the R signal is inverted in phase to be R and is supplied to a loudspeaker positioned on the right-hand side of the listener at his back.
In other prior systems the signal R is supplied to the left rear loudspeaker while the signal L is supplied to the right rear loudspeaker. In still other prior systems a matrix circuit is employed for applying signals L-R and R-L to the loudspeakers at the left-hand and right-hand rear sides of the listener, respectively.
With such conventional systems, since the signals having inverted phase components are supplied to the loudspeakers at the back of the listener, he generally hears uncomfortable sounds peculiar to the inverted phase components when they are reproduced. Although there are provided the two rear loudspeakers in addition to the two front loudspeakers so that the so-called sound field is widened there still occurs a drawback in that the separation ability for the rear channels is greatly deteriorated.
SUMMARY OF THE INVENTION
The above and other disadvantages are overcome by the present invention of a multisound reproducing system comprising a first and a second input circuits for receiving a first and a second signal, respectively, means connected to the first and the second input circuits for separately shifting the phase of the first and the second sound signals by a predetermined amount, means responsive to the phase shifted first and second sound signals for separately reproducing them, means for filtering the first sound signal and for combining it with the phase shifted second sound signal to produce a third sound signal, means for filtering the second sound signal and for combining it with the phase shifted first sound signal to produce a fourth sound signal, and means responsive to the third and the fourth sound signals for separately reproducing them.
In one embodiment when the first and the second sound signals initially have the same phase and are at a predetermined frequency, the phase shifting means is designed to shift their phases by one hundred and eighty degrees with respect to their phases after being filtered by each of the combining means.
In another embodiment the system is provided with switches and an additional set of phase shifting circuits connected to the first and the second input circuits for providing 90° of additional phase shift over the primary phase shifting means. The switches allow the listener to selectively and separately reproduce the third and the fourth sound signals or the output signals from the additional phase shifting circuits, thereby making the system compatible with either stereophonic or quadraphonic sound media, respectively.
Accordingly, one object of the invention is to provide a multisound reproducing system in which four independent signals are produced from stereophonic sound signals supplied by two channels and the four independent signals are supplied to at least four loudspeakers.
Another object of the invention is to provide a multisound reproducing system for reproducing a sound signal about the circumference of the listener without discomfort to the listener.
A further object of the invention is to provide a multisound reproducing system in which a monaural signal is reproduced as a monaural sound positioned in front of a listener.
A still further object of the invention is to provide a multisound reproducing system which allows the sound to be positioned in accordance with the frequency which is most sensitive for the listener.
Yet another object of the invention is to provide a multisound reproducing system which is compatible with both stereophonic and four-channel reproducing systems.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram showing one embodiment of the invention;
FIGS. 2A, 2B and 2C are graphs for explaining the operation of the embodiment depicted in FIG. 1;
FIG. 3 is a plane view of the loudspeaker arrangement of the embodiment of FIG. 1;
FIGS. 4A, 4B, 5A and 5B are plane views of the reproduced sound fields for different input signals to the embodiment of FIG. 1, diagrammatically illustrating the directions of the sounds reproduced by the embodiment of FIG. 1;
FIG. 6 is a block diagram of another embodiment of the invention; and
FIG. 7 is a circuit diagram of a part of the embodiment depicted in FIG. 6.
DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS
Referring now more particularly to FIG. 1, reference numeral 10 denotes a stereophonic signal source having two channels such as a tape recorder, record player, FM multiplex receiver or the like. One of the channels of the stereophonic signal source supplies a signal L which is fed through an amplifier 51 to a phase shifter circuit 61 which produces as its output a signal Lf. The output of the other channel of the stereophonic signal source 10 is a signal R which is fed through an amplifier 52 to a phase shifter circuit 62 which produces as its output a signal Rf. The output signals Lf and Rf from the phase shifter circuits 61 and 62 are respectively supplied through power amplifiers 71 and 72 to loudspeakers 1 and 2, respectively. The loudspeaker 1 is placed on the left-hand side of the listener 5 in front of him and the loudspeaker 2 is placed on the right-hand side and in front of the listener as illustrated in FIG. 3.
The phase versus frequency characteristics of the phase shifter circuits 61 and 62 are illustrated in FIG. 2A and are designated φ6. Within a predetermined frequency range centered about a predetermined frequency f6 the phase shift for an incoming signal varies approximately in direct proportion to the logarithm of its frequency from 0° of phase shift to 360° of phase shift.
In particular, at the predetermined frequency f6, (which is, for example about 400 Hz) the phase of an incoming signal is shifted 180°. At the outer limits of the frequency range of the phase shifter circuits there is no phase shifting (0° and 360° phase shifts). Thus the signals Lf and Rf are inverted in phase when the signals L and R are at the frequency f6 and have the same phase as the signals L and R at the fringes of the band range.
The amplitude versus frequency characteristic of the phase shifter circuits 61 and 62 are illustrated by the curve A6 in the FIG. 2B. As illustrated, the response of the phase shifter circuits is substantially flat over a broad range of audio frequencies.
The signal L from the amplifier 51 is also supplied to the input of a filter 84 which produces a signal La at its output. The signal R from the amplifier 52 is supplied to the input of a filter 83 which produces a signal Ra at its output. The filters 83 and 84 have amplitude versus frequency characteristics designated A8 in the FIG. 2B. As illustrated in the Figure, the filters are both low pass filters having a 3db cutoff at a predetermined frequency f8, which is, for example a frequency slightly higher than 400 Hz, and they attenuate at -6db/octive.
The phase versus frequency characteristic of each filter is depicted by the curve φ8 in FIG. 2A. As illustrated in the Figure the output signals La and Ra are phase shifted from 0° to 90° in direct proportion to the logarithm of the frequency of the input signals L and R within a band range centered about a frequency f8. At the predetermined frequency f8 the signal La or Ra is shifted by 45° with respect to the signal L or R. At the lower end of the frequency range it has the same phase and at the upper end of the frequency range it is phase shifted by 90° with respect to the signal L or R.
The signals La and Rf are combined in an adder circuit 94 to produce a combined output signal Rb which is fed through an amplifier 74 to a loudspeaker 4. The signals Lf and Ra are combined in an adder circuit 93 which produces as its output a signal Lb which is fed through a power amplifier 73 to a loudspeaker 3. As illustrated in FIG. 3 the loudspeaker 3 is at the left rear side of the listener 5 and the loudspeaker 4 is at the right rear side of the listener 5.
In operation the signals Lf and Rf are both shifted in phase by equal amounts by the phase shifters 61 and 62, respectively, and their amplitudes are constant over the frequency range of the system. Therefore the signals L, R, Lf and Rf are equal with each other in level and the phase difference between the signals Lf and Rf is equal to the phase difference between the signals L and R. It also follows that the sounds S1 and S2 are reproduced with the same phase and the same level from the loudspeakers 1 and 2, respectively, as shown in FIG. 4A.
In the adder circuit 93 the signals Lf and Ra are added together to produce the signal Lb. The phase versus frequency characteristics of the phase shifters 61 and 62 and the filters 83 and 84 are such that at a predetermined frequency f7 (which is, for example between 650 to 700 Hz) there is 180° difference in phase between signals emerging from either of the filters 83 and 84 and signals emerging from either of the phase shifters 61 and 62. Thus at the predetermined frequency f7 the signals Lf and Ra are phase inverted with respect to each other and cancel.
Thus the signal Lb has no signal components near the frequency f7 as illustrated in FIG. 2C by dotted lines. The signal Ra delivered from the filter 83 has an amplitude characteristic as illustrated by the curve A8 in FIG. 2B and as a result the frequency spectrum of the signal Lb has no signal components near the frequency f7 as illustrated by the solid line curve in the FIG. 2C and is attenuated in its intermediate and high frequency signal components. The main part of the signal Lb is a signal component lower than the frequency f7. Since the signal L is equal in level and phase to the signal R, it follows that the above considerations apply equally to the signal Rb with the result that the signal Rb is equal to the signal Lb.
The signals Lb and Rb are respectively fed through the power amplifiers 73 and 74 to be reproduced as sounds S3 and S4, respectively, from the loudspeakers 3 and 4 as illustrated in FIG. 4A. Since the main components of the signals Lb and Rb are low frequency, the reproduced sounds S3 and S4 have almost no contribution to the position of the sound image as perceived by the listener 5. Therefore if the signals L and R are equal the position of the sound image as perceived by the listener is midway between the loudspeakers 1 and 2.
In the situation where only the signal L exists (there is no signal R), then the sound S1 is reproduced from the loudspeaker 1 from the signal Lf but there is no sound signal reproduced from the loudspeaker 2 as illustrated in FIG. 4B. The signal Lb derived from the adder circuit 93 contains only the signal Lf which is then reproduced through the loudspeaker 3 as the sound S3 having the same level as the sound S1. The signal Rb from the adder circuit 94 contains only the signal La obtained from the filter 84. The amplitude characteristic of the signal Rb is shown by the curve A8 in FIG. 2B, so that the signal Rb consists primarily of the low frequency signal component of the signal L and the attenuated intermediate frequency signal component thereof. The phase of the signal Rb is also substantially the same as the phase of the signal Lf. The signal Rb is reproduced from the loudspeaker 4 as the sound S4.
Thus where only the signal L exists and the signal R does not exist, the reproduced sound S1 from the loudspeaker 1 and S3 from the loudspeaker 3 are the same in phase and level and the reproduced sound S4 from the loudspeaker 4 contains only the low frequency signal component and the attenuated intermediate frequency signal component of the signal L. The sound image as perceived by the listener 5 is positioned near the loudspeaker 3. The separation for left and right side sounds is effectively kept because the sound S4 contains no high frequency signal component of the signal L and its intermediate frequency signal component is substantially attenuated.
Similarly in the case where only the signal R exists, and no signal L exists, the sound image is positioned near the loudspeaker 4 as perceived by the listener 5.
The above examples of sound positioning by the reproducing system of the invention indicate that although the signals L and R are normally reproduced by a two channel stereophonic reproducing system as a sound image S evenly distributed in front of the listener (FIG. 5A) the same signals when reproduced by the embodiment of FIG. 1 provide a sound image S which is expanded to surround the listener from his left rear side, around in front of him, to his right rear side (FIG. 5B). This increases the illusion of reality for the sound effect. For example the system of the invention is capable of giving the illusion of a singer standing directly in front of the listener 5 and with musical instruments being played around the listener.
It is presently considered that a man can hear a sound having a frequency of approximately 700 Hz with best sensitivity. This is the frequency f7 as discussed above. Therefore in the situation where the signals L and R have the same phase the loudspeakers 3 and 4 behind the listener 5 produce no sounds having a frequency in the vicinity of 700 Hz, so that the position of the sound image of the reproduced signal is positioned in front of the listener 5.
The system of the invention is also suitable for use with a quadraphonic stereo system. Referring now more particularly to FIG. 6 a quadraphonic stereo reproducing system 100 according to the invention has a first input terminal 101 a second input terminal 102 and a first, second, third and fourth output terminals 103, 104, 105 and 106, respectively. A first and a second phase shifter 107 and 108 are connected to the first input terminal 101 and a third and a fourth phase shifter 109 and 110 are connected to the second input terminal 102. The phase shifter 108 provides a phase difference of 90° with respect to the phase shifter 107 and a phase shifter 109 provides a phase difference of 90° with respect to the phase shifter 110.
Quadraphonic signals supplied to the first and second input terminals 101 and 102 are encoded to comprise signals LT and RT from, for example, four original sound informational signals LF, RF, LB and RB. The composite signal LT consists of three components LF, 0.7LB and 0.7RB. The signals LF and 0.7RB have the same phase whereas the signal 0.7LB is 270° different in phase from the signal LF. The other composite signal RT similarly consists of three components RF, 0.7RB and 0.7LB. The signal 0.7LB is 180° out of phase with the signal RF and the signal 0.7RB is 90° out of phase with the signal RF.
The composite signal LT applied to the input terminal 101 is supplied through a first phase shifter 107 and a power amplifier 114 to the output terminal 103 from which a signal LF' is derived. A loudspeaker (not shown) positioned at the left front side of the listener is connected to the output terminal 103. The loudspeaker reproduces a sound having the signal LT as its main component. The composite signal RT applied to the second input terminal 102 is passed through phase shifter 110 and the power amplifier 116 to emerge as the signal RF' from the fourth output terminal 106. A loudspeaker (not shown) is connected to the output terminal 106 and is placed at the right front side of the listener to reproduce a sound having the signal RF as its main component.
The composite signal LT supplied to the first input terminal 101 is also fed to the second phase shifter 108 where is is shifted in phase 90° with respect to the output of the first phase shifter 107. The phase-shifted composite signal from the phase shifter 108 is supplied through a resistor 120 to an adder circuit 121 where it is combined with the output from the phase shifter 110 supplied to the adder 121 through a resistor 122.
The output from the adder circuit 121 is fed through a phase inverter 123 to a fixed contact b of a single pole, double throw switch 124. The moving contact c of the switch 124 is connected to the input of a power amplifier 126 whose output is supplied to the output terminal 104. The signal LB' derived at the output terminal 104 has the components LB + 0.7LF + 0.7RF. It has the same phase as the signal LB but is 90° out of phase with the signal 0.7LF and is 180° out of phase with the signal 0.7RF. The output terminal 104 is connected to a loudspeaker (not shown) placed at the rear right hand side of the listener.
The composite signal RT is also supplied to the phase shifter 109 where it is phase shifted by 90° with respect to the output signal from the phase shifter 110. The phase-shifted composite signal from the phase shifter 109 is supplied through a resistor 130 to an adder circuit 131 where it is combined with the output signal from the phase shifter 107 which is supplied to the adder 131 through a resistor 132. The output signal from the adder circuit 131 is supplied directly to a fixed terminal b of a single pole, double throw switch 134. The contact arm c of the switch 134 is connected to the input of a power amplifier 136 which has its output connected to the output terminal 105. The signal RB' derived at the output terminal 105 contains the signal components RB + 0.7LF + 0.7RF. It has the same phase as the signals RB and 0.7LF but is 270° out of phase with the signal 0.7RF. The terminal 105 is connected to a loudspeaker (not shown) which is positioned at the rear right hand side of the listener.
In order to make the above described system compatible with both conventional stereophonic signals and quadraphonic signals the switches 124 and 134 each have two fixed contacts a and b and a movable contact c. The fixed contact a of each switch serves to transmit a conventional stereophonic signal to the respective output terminals 104 and 105 and the other fixed contact b serves to transmit quadraphonic signals to the respective output terminals 104 and 105.
The input side of a first low pass filter 151 is connected to the input terminal 101 and its output side is connected to an adder circuit 154 through a resistor 153. The input side of a second low pass filter 152 is connected to the input terminal 102 and its output side is connected to an adder circuit 156 through a resistor 155.
The output side of the phase shifter circuit 110 is connected through a resistor 157 to the adder circuit 154. The signals from the low pass filter 151 and the phase shifter circuit 110 are combined by the adder and its output signal is connected to the terminal a of the switch 134. The output side of the phase shifter 107 is connected to the adder circuit 156 through a resistor 158. The signals from the low pass filter 152 and from the phase shifter circuit 107 are combined in the adder circuit 157 and its output is connected to the fixed terminal a of the switch 124.
If the movable contacts c, which are ganged together, of the two switches 124 and 134 are connected to their respective fixed contacts a then the stereophonic signals L and R supplied to the input terminals 101 and 102 are reproduced as the signals LF and RF at the output terminals 103 and 106 while the respective signals LB and RB are derived at the output terminals 104 and 105 to achieve an effect similar to that described with respect to the embodiment of FIG. 1. It should be noted that in this mode of operation the phase shifters 107 and 110 are employed.
As described above when the movable contacts c of the switches 124 and 134 are connected to the fixed contacts b and the quadraphonic signals LT and RT are supplied to the input terminals 101 and 102, respectively, then the signal LF' is derived at the output terminal 103, the signal LB' is derived at the output terminal 104, the signal RB' is derived at the output terminal 105 and the signal RF' is derived at the output terminal 106.
Referring now more particularly to FIG. 7 some of the features of the embodiment of FIG. 6 are described in more detail. The first phase shifter 107 comprises two transistors 107A and 107B. A series circuit of a capacitor 107c and a resistor 107d is connected between the emitter and collector electrodes of the transistor 107A. The connection point between the capacitor 107c and the resistor 107d is connected to the base of the transistor 107B. A series circuit of a capacitor 107e and a resistor 107f is connected between the collector and emitter electrodes of the transistor 107B. The connection point between the capacitor 107e and the resistor 107f is connected to the output terminal 103 through a resistor and a power amplifier 114.
The other phase shifting circuits 108, 109 and 110 are similarly constructed and an explanation of their construction is omitted for the sake of simplicity. Since the phase shifters 108 and 109 are used for phase shifting signals by 90° with respect to the signals passed through the phase shifters 107 and 110, the value of the capacitors and resistors connected between the leads of the transistors of the phase shifters 108 and 109 are selected to be different from those of the capacitors and the resistors connected to the transistors of the phase shifters 107 and 110.
The low pass filter 151 comprises a resistor 151R connected between the input terminal 101 and one lead of a capacitor 151C. The other lead of the capacitor 151C is connected to the circuit ground. The output from the low pass filter 151 is taken from the connection point between the resistor 151R and the capacitor 151C. Similarly the low pass filter 152 comprises a resistor 152R connected between the input terminal 102 and one lead of a capacitor 152C. The other lead of a capacitor 152C is connected to the circuit ground. The output from the low pass filter 152 is derived from the connection point between the resistor 152R and the capacitor 152C.
The terms and expressions which have been employed here are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions, of excluding equivalents of the features shown and described, or portions thereof, it being recognized that various modifications are possible within the scope of the invention claimed.
This invention relates to a multisound decoding apparatus and more particularly to apparatus for reproducing a monaural audio information signal or stereophonic audio information signal with four separate loudspeakers without accompanying unnaturalness.
There is increasing interest in multiple channel recording and reproducing systems because of the variety of sounds and music forms which such systems reproduce. It is a well known phenomenon that the quality of music reproduction is enhanced when the number of reproduction channels is increased. In the early days of the phonograph, only a single channel or monophonic recording was used. Investigators soon realized the value, however, of recording and transmitting two separate channels of information, which in modern parlance is known as binaural or stereophonic sound.
However, even two channels of information are not considered sufficient for a full illusion of reality. For example, when a listener is placed in front of a symphony orchestra, he hears sounds arriving from many different directions and from a variety of instruments, as well as reflections from the walls and ceiling, which gives him an accustomed illusion of space perspective. When reproduction is accomplished by utilizing only two channels, however, it is difficult if not impossible, to produce true reality with respect to spatial perspective. Early experiments have demonstrated that a minimum of three independent channels are needed to convey a satisfactory illusion of reality in the reproduction of orchestral music.
Recently, some investigators have developed a multi-channel sound system, so called a four channel stereo system, providing third and fourth playback channels to an otherwise two-channel system by feeding third and fourth loudspeakers with signals.
If suitable sound media are prepared for the above mentioned multichannel sound system having the third and fourth channels, listeners may hear the sound from the sound system with a full illusion of reality. At present, however, sound media which can be used for such multichannel sound systems are insufficient and stereophonic sound media for two channels command a prominent majority.
In an attempt to provide a four channel system for use with stereophonic sound media one proposed prior system separates a stereophonic sound information signal into left and right sound signals defined as L and R, respectively, by a channel separator. The L signal is supplied to a loudspeaker placed on the left-hand side of a listener and in front of him while the R signal is supplied to a loudspeaker positioned on the right-hand side of the listener and in front of him as in a conventional manner. The proposed system further inverts the phase of the L signal to make it a signal L and supplies it to a loudspeaker positioned on the left-hand side of the listener at his back. Similarly the R signal is inverted in phase to be R and is supplied to a loudspeaker positioned on the right-hand side of the listener at his back.
In other prior systems the signal R is supplied to the left rear loudspeaker while the signal L is supplied to the right rear loudspeaker. In still other prior systems a matrix circuit is employed for applying signals L-R and R-L to the loudspeakers at the left-hand and right-hand rear sides of the listener, respectively.
With such conventional systems, since the signals having inverted phase components are supplied to the loudspeakers at the back of the listener, he generally hears uncomfortable sounds peculiar to the inverted phase components when they are reproduced. Although there are provided the two rear loudspeakers in addition to the two front loudspeakers so that the so-called sound field is widened there still occurs a drawback in that the separation ability for the rear channels is greatly deteriorated.
SUMMARY OF THE INVENTION
The above and other disadvantages are overcome by the present invention of a multisound reproducing system comprising a first and a second input circuits for receiving a first and a second signal, respectively, means connected to the first and the second input circuits for separately shifting the phase of the first and the second sound signals by a predetermined amount, means responsive to the phase shifted first and second sound signals for separately reproducing them, means for filtering the first sound signal and for combining it with the phase shifted second sound signal to produce a third sound signal, means for filtering the second sound signal and for combining it with the phase shifted first sound signal to produce a fourth sound signal, and means responsive to the third and the fourth sound signals for separately reproducing them.
In one embodiment when the first and the second sound signals initially have the same phase and are at a predetermined frequency, the phase shifting means is designed to shift their phases by one hundred and eighty degrees with respect to their phases after being filtered by each of the combining means.
In another embodiment the system is provided with switches and an additional set of phase shifting circuits connected to the first and the second input circuits for providing 90° of additional phase shift over the primary phase shifting means. The switches allow the listener to selectively and separately reproduce the third and the fourth sound signals or the output signals from the additional phase shifting circuits, thereby making the system compatible with either stereophonic or quadraphonic sound media, respectively.
Accordingly, one object of the invention is to provide a multisound reproducing system in which four independent signals are produced from stereophonic sound signals supplied by two channels and the four independent signals are supplied to at least four loudspeakers.
Another object of the invention is to provide a multisound reproducing system for reproducing a sound signal about the circumference of the listener without discomfort to the listener.
A further object of the invention is to provide a multisound reproducing system in which a monaural signal is reproduced as a monaural sound positioned in front of a listener.
A still further object of the invention is to provide a multisound reproducing system which allows the sound to be positioned in accordance with the frequency which is most sensitive for the listener.
Yet another object of the invention is to provide a multisound reproducing system which is compatible with both stereophonic and four-channel reproducing systems.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram showing one embodiment of the invention;
FIGS. 2A, 2B and 2C are graphs for explaining the operation of the embodiment depicted in FIG. 1;
FIG. 3 is a plane view of the loudspeaker arrangement of the embodiment of FIG. 1;
FIGS. 4A, 4B, 5A and 5B are plane views of the reproduced sound fields for different input signals to the embodiment of FIG. 1, diagrammatically illustrating the directions of the sounds reproduced by the embodiment of FIG. 1;
FIG. 6 is a block diagram of another embodiment of the invention; and
FIG. 7 is a circuit diagram of a part of the embodiment depicted in FIG. 6.
DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS
Referring now more particularly to FIG. 1, reference numeral 10 denotes a stereophonic signal source having two channels such as a tape recorder, record player, FM multiplex receiver or the like. One of the channels of the stereophonic signal source supplies a signal L which is fed through an amplifier 51 to a phase shifter circuit 61 which produces as its output a signal Lf. The output of the other channel of the stereophonic signal source 10 is a signal R which is fed through an amplifier 52 to a phase shifter circuit 62 which produces as its output a signal Rf. The output signals Lf and Rf from the phase shifter circuits 61 and 62 are respectively supplied through power amplifiers 71 and 72 to loudspeakers 1 and 2, respectively. The loudspeaker 1 is placed on the left-hand side of the listener 5 in front of him and the loudspeaker 2 is placed on the right-hand side and in front of the listener as illustrated in FIG. 3.
The phase versus frequency characteristics of the phase shifter circuits 61 and 62 are illustrated in FIG. 2A and are designated φ6. Within a predetermined frequency range centered about a predetermined frequency f6 the phase shift for an incoming signal varies approximately in direct proportion to the logarithm of its frequency from 0° of phase shift to 360° of phase shift.
In particular, at the predetermined frequency f6, (which is, for example about 400 Hz) the phase of an incoming signal is shifted 180°. At the outer limits of the frequency range of the phase shifter circuits there is no phase shifting (0° and 360° phase shifts). Thus the signals Lf and Rf are inverted in phase when the signals L and R are at the frequency f6 and have the same phase as the signals L and R at the fringes of the band range.
The amplitude versus frequency characteristic of the phase shifter circuits 61 and 62 are illustrated by the curve A6 in the FIG. 2B. As illustrated, the response of the phase shifter circuits is substantially flat over a broad range of audio frequencies.
The signal L from the amplifier 51 is also supplied to the input of a filter 84 which produces a signal La at its output. The signal R from the amplifier 52 is supplied to the input of a filter 83 which produces a signal Ra at its output. The filters 83 and 84 have amplitude versus frequency characteristics designated A8 in the FIG. 2B. As illustrated in the Figure, the filters are both low pass filters having a 3db cutoff at a predetermined frequency f8, which is, for example a frequency slightly higher than 400 Hz, and they attenuate at -6db/octive.
The phase versus frequency characteristic of each filter is depicted by the curve φ8 in FIG. 2A. As illustrated in the Figure the output signals La and Ra are phase shifted from 0° to 90° in direct proportion to the logarithm of the frequency of the input signals L and R within a band range centered about a frequency f8. At the predetermined frequency f8 the signal La or Ra is shifted by 45° with respect to the signal L or R. At the lower end of the frequency range it has the same phase and at the upper end of the frequency range it is phase shifted by 90° with respect to the signal L or R.
The signals La and Rf are combined in an adder circuit 94 to produce a combined output signal Rb which is fed through an amplifier 74 to a loudspeaker 4. The signals Lf and Ra are combined in an adder circuit 93 which produces as its output a signal Lb which is fed through a power amplifier 73 to a loudspeaker 3. As illustrated in FIG. 3 the loudspeaker 3 is at the left rear side of the listener 5 and the loudspeaker 4 is at the right rear side of the listener 5.
In operation the signals Lf and Rf are both shifted in phase by equal amounts by the phase shifters 61 and 62, respectively, and their amplitudes are constant over the frequency range of the system. Therefore the signals L, R, Lf and Rf are equal with each other in level and the phase difference between the signals Lf and Rf is equal to the phase difference between the signals L and R. It also follows that the sounds S1 and S2 are reproduced with the same phase and the same level from the loudspeakers 1 and 2, respectively, as shown in FIG. 4A.
In the adder circuit 93 the signals Lf and Ra are added together to produce the signal Lb. The phase versus frequency characteristics of the phase shifters 61 and 62 and the filters 83 and 84 are such that at a predetermined frequency f7 (which is, for example between 650 to 700 Hz) there is 180° difference in phase between signals emerging from either of the filters 83 and 84 and signals emerging from either of the phase shifters 61 and 62. Thus at the predetermined frequency f7 the signals Lf and Ra are phase inverted with respect to each other and cancel.
Thus the signal Lb has no signal components near the frequency f7 as illustrated in FIG. 2C by dotted lines. The signal Ra delivered from the filter 83 has an amplitude characteristic as illustrated by the curve A8 in FIG. 2B and as a result the frequency spectrum of the signal Lb has no signal components near the frequency f7 as illustrated by the solid line curve in the FIG. 2C and is attenuated in its intermediate and high frequency signal components. The main part of the signal Lb is a signal component lower than the frequency f7. Since the signal L is equal in level and phase to the signal R, it follows that the above considerations apply equally to the signal Rb with the result that the signal Rb is equal to the signal Lb.
The signals Lb and Rb are respectively fed through the power amplifiers 73 and 74 to be reproduced as sounds S3 and S4, respectively, from the loudspeakers 3 and 4 as illustrated in FIG. 4A. Since the main components of the signals Lb and Rb are low frequency, the reproduced sounds S3 and S4 have almost no contribution to the position of the sound image as perceived by the listener 5. Therefore if the signals L and R are equal the position of the sound image as perceived by the listener is midway between the loudspeakers 1 and 2.
In the situation where only the signal L exists (there is no signal R), then the sound S1 is reproduced from the loudspeaker 1 from the signal Lf but there is no sound signal reproduced from the loudspeaker 2 as illustrated in FIG. 4B. The signal Lb derived from the adder circuit 93 contains only the signal Lf which is then reproduced through the loudspeaker 3 as the sound S3 having the same level as the sound S1. The signal Rb from the adder circuit 94 contains only the signal La obtained from the filter 84. The amplitude characteristic of the signal Rb is shown by the curve A8 in FIG. 2B, so that the signal Rb consists primarily of the low frequency signal component of the signal L and the attenuated intermediate frequency signal component thereof. The phase of the signal Rb is also substantially the same as the phase of the signal Lf. The signal Rb is reproduced from the loudspeaker 4 as the sound S4.
Thus where only the signal L exists and the signal R does not exist, the reproduced sound S1 from the loudspeaker 1 and S3 from the loudspeaker 3 are the same in phase and level and the reproduced sound S4 from the loudspeaker 4 contains only the low frequency signal component and the attenuated intermediate frequency signal component of the signal L. The sound image as perceived by the listener 5 is positioned near the loudspeaker 3. The separation for left and right side sounds is effectively kept because the sound S4 contains no high frequency signal component of the signal L and its intermediate frequency signal component is substantially attenuated.
Similarly in the case where only the signal R exists, and no signal L exists, the sound image is positioned near the loudspeaker 4 as perceived by the listener 5.
The above examples of sound positioning by the reproducing system of the invention indicate that although the signals L and R are normally reproduced by a two channel stereophonic reproducing system as a sound image S evenly distributed in front of the listener (FIG. 5A) the same signals when reproduced by the embodiment of FIG. 1 provide a sound image S which is expanded to surround the listener from his left rear side, around in front of him, to his right rear side (FIG. 5B). This increases the illusion of reality for the sound effect. For example the system of the invention is capable of giving the illusion of a singer standing directly in front of the listener 5 and with musical instruments being played around the listener.
It is presently considered that a man can hear a sound having a frequency of approximately 700 Hz with best sensitivity. This is the frequency f7 as discussed above. Therefore in the situation where the signals L and R have the same phase the loudspeakers 3 and 4 behind the listener 5 produce no sounds having a frequency in the vicinity of 700 Hz, so that the position of the sound image of the reproduced signal is positioned in front of the listener 5.
The system of the invention is also suitable for use with a quadraphonic stereo system. Referring now more particularly to FIG. 6 a quadraphonic stereo reproducing system 100 according to the invention has a first input terminal 101 a second input terminal 102 and a first, second, third and fourth output terminals 103, 104, 105 and 106, respectively. A first and a second phase shifter 107 and 108 are connected to the first input terminal 101 and a third and a fourth phase shifter 109 and 110 are connected to the second input terminal 102. The phase shifter 108 provides a phase difference of 90° with respect to the phase shifter 107 and a phase shifter 109 provides a phase difference of 90° with respect to the phase shifter 110.
Quadraphonic signals supplied to the first and second input terminals 101 and 102 are encoded to comprise signals LT and RT from, for example, four original sound informational signals LF, RF, LB and RB. The composite signal LT consists of three components LF, 0.7LB and 0.7RB. The signals LF and 0.7RB have the same phase whereas the signal 0.7LB is 270° different in phase from the signal LF. The other composite signal RT similarly consists of three components RF, 0.7RB and 0.7LB. The signal 0.7LB is 180° out of phase with the signal RF and the signal 0.7RB is 90° out of phase with the signal RF.
The composite signal LT applied to the input terminal 101 is supplied through a first phase shifter 107 and a power amplifier 114 to the output terminal 103 from which a signal LF' is derived. A loudspeaker (not shown) positioned at the left front side of the listener is connected to the output terminal 103. The loudspeaker reproduces a sound having the signal LT as its main component. The composite signal RT applied to the second input terminal 102 is passed through phase shifter 110 and the power amplifier 116 to emerge as the signal RF' from the fourth output terminal 106. A loudspeaker (not shown) is connected to the output terminal 106 and is placed at the right front side of the listener to reproduce a sound having the signal RF as its main component.
The composite signal LT supplied to the first input terminal 101 is also fed to the second phase shifter 108 where is is shifted in phase 90° with respect to the output of the first phase shifter 107. The phase-shifted composite signal from the phase shifter 108 is supplied through a resistor 120 to an adder circuit 121 where it is combined with the output from the phase shifter 110 supplied to the adder 121 through a resistor 122.
The output from the adder circuit 121 is fed through a phase inverter 123 to a fixed contact b of a single pole, double throw switch 124. The moving contact c of the switch 124 is connected to the input of a power amplifier 126 whose output is supplied to the output terminal 104. The signal LB' derived at the output terminal 104 has the components LB + 0.7LF + 0.7RF. It has the same phase as the signal LB but is 90° out of phase with the signal 0.7LF and is 180° out of phase with the signal 0.7RF. The output terminal 104 is connected to a loudspeaker (not shown) placed at the rear right hand side of the listener.
The composite signal RT is also supplied to the phase shifter 109 where it is phase shifted by 90° with respect to the output signal from the phase shifter 110. The phase-shifted composite signal from the phase shifter 109 is supplied through a resistor 130 to an adder circuit 131 where it is combined with the output signal from the phase shifter 107 which is supplied to the adder 131 through a resistor 132. The output signal from the adder circuit 131 is supplied directly to a fixed terminal b of a single pole, double throw switch 134. The contact arm c of the switch 134 is connected to the input of a power amplifier 136 which has its output connected to the output terminal 105. The signal RB' derived at the output terminal 105 contains the signal components RB + 0.7LF + 0.7RF. It has the same phase as the signals RB and 0.7LF but is 270° out of phase with the signal 0.7RF. The terminal 105 is connected to a loudspeaker (not shown) which is positioned at the rear right hand side of the listener.
In order to make the above described system compatible with both conventional stereophonic signals and quadraphonic signals the switches 124 and 134 each have two fixed contacts a and b and a movable contact c. The fixed contact a of each switch serves to transmit a conventional stereophonic signal to the respective output terminals 104 and 105 and the other fixed contact b serves to transmit quadraphonic signals to the respective output terminals 104 and 105.
The input side of a first low pass filter 151 is connected to the input terminal 101 and its output side is connected to an adder circuit 154 through a resistor 153. The input side of a second low pass filter 152 is connected to the input terminal 102 and its output side is connected to an adder circuit 156 through a resistor 155.
The output side of the phase shifter circuit 110 is connected through a resistor 157 to the adder circuit 154. The signals from the low pass filter 151 and the phase shifter circuit 110 are combined by the adder and its output signal is connected to the terminal a of the switch 134. The output side of the phase shifter 107 is connected to the adder circuit 156 through a resistor 158. The signals from the low pass filter 152 and from the phase shifter circuit 107 are combined in the adder circuit 157 and its output is connected to the fixed terminal a of the switch 124.
If the movable contacts c, which are ganged together, of the two switches 124 and 134 are connected to their respective fixed contacts a then the stereophonic signals L and R supplied to the input terminals 101 and 102 are reproduced as the signals LF and RF at the output terminals 103 and 106 while the respective signals LB and RB are derived at the output terminals 104 and 105 to achieve an effect similar to that described with respect to the embodiment of FIG. 1. It should be noted that in this mode of operation the phase shifters 107 and 110 are employed.
As described above when the movable contacts c of the switches 124 and 134 are connected to the fixed contacts b and the quadraphonic signals LT and RT are supplied to the input terminals 101 and 102, respectively, then the signal LF' is derived at the output terminal 103, the signal LB' is derived at the output terminal 104, the signal RB' is derived at the output terminal 105 and the signal RF' is derived at the output terminal 106.
Referring now more particularly to FIG. 7 some of the features of the embodiment of FIG. 6 are described in more detail. The first phase shifter 107 comprises two transistors 107A and 107B. A series circuit of a capacitor 107c and a resistor 107d is connected between the emitter and collector electrodes of the transistor 107A. The connection point between the capacitor 107c and the resistor 107d is connected to the base of the transistor 107B. A series circuit of a capacitor 107e and a resistor 107f is connected between the collector and emitter electrodes of the transistor 107B. The connection point between the capacitor 107e and the resistor 107f is connected to the output terminal 103 through a resistor and a power amplifier 114.
The other phase shifting circuits 108, 109 and 110 are similarly constructed and an explanation of their construction is omitted for the sake of simplicity. Since the phase shifters 108 and 109 are used for phase shifting signals by 90° with respect to the signals passed through the phase shifters 107 and 110, the value of the capacitors and resistors connected between the leads of the transistors of the phase shifters 108 and 109 are selected to be different from those of the capacitors and the resistors connected to the transistors of the phase shifters 107 and 110.
The low pass filter 151 comprises a resistor 151R connected between the input terminal 101 and one lead of a capacitor 151C. The other lead of the capacitor 151C is connected to the circuit ground. The output from the low pass filter 151 is taken from the connection point between the resistor 151R and the capacitor 151C. Similarly the low pass filter 152 comprises a resistor 152R connected between the input terminal 102 and one lead of a capacitor 152C. The other lead of a capacitor 152C is connected to the circuit ground. The output from the low pass filter 152 is derived from the connection point between the resistor 152R and the capacitor 152C.
The terms and expressions which have been employed here are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions, of excluding equivalents of the features shown and described, or portions thereof, it being recognized that various modifications are possible within the scope of the invention claimed.