Synthesized Four Channel Sound Using Phase Modulation Techniques

Abstract

A sound reproduction system comprising a plurality of loud-speakers producing sounds corresponding to audio signals having a desired stereophonic relationship, at least one of which is phase modulated according to signals of ultralow frequency generated by a signal generator, sounds from all said loud-speakers being so designed as to converge into the same listening area.

Description

This invention relates to a sound reproduction system in which sounds from a plurality of loud-speakers are directed into the same listening area.

The conventional two-channel type stereophonic reproduction system is primarily intended to conduct sounds corresponding to stereophonic right and left audio signals convergently to the same listening area from two loud-speakers disposed slantwise on the right and left sides ahead of a listener. This system makes the listener feel that sounds are produced from a certain point in an intermediate region between the two loud-speakers placed in the aforesaid positions. With said system, however, the point from which the listener feels sounds to be sent forth is limited to an area lying between the loud-speakers. Since he can not listen to any sound from either aside or behind said point, the conventional sound reproduction system fails fully to allow the listener to have the so-called feeling of the presence of sounds.

As used herein, the term "feeling of the presence of sounds" means the acoustic sense of audience gathered, for example, in a concert hall to feel the presence of sounds actually played therein. In a concert hall each listener hears not only sounds conducted to him straight from a source of sounds such as a musical instrument disposed ahead of him (direct sounds), but also those reflected by the walls and ceiling of the concert hall (indirect sounds) and reaching him from all sides. These indirect sounds are somewhat delayed in phase from the direct sounds and moreover displaced from each other in phase, because they pass through different routes. In short, the essence of the aforementioned "feeling of the presence of sounds" may be boiled down to the listener's sense to feel not only direct but also indirect sounds which are brought to him in every direction substantially at the same time in all varieties of phase, wave form and level. Therefore, the reason is obvious why the prior art 2-channel type stereophonic sound reproduction system can not fully allow the listener to have the feeling of the presence of sounds. Namely, said system simply sends forth aggregates of direct and indirect sounds picked up by two microphones, for example, in a concert hall, separately from two loud-speakers set ahead of the listener, thus making it impossible to reproduce sounds exactly in the same manner as might otherwise reach the listener from every possible side and in a wide variety of forms.

The so-called 4-channel type stereophonic reproduction system often used in recent years cannot yet be deemed fully to eliminate aforesaid defects encountered in the 2-channel sound reproduction system. The reason is that though such system indeed enables the four loud-speakers to furnish the listener in a listening room with sounds ariving in four different directions, yet said system fails to reproduce actual sounds exactly in the complicated relationship which they bear to each other. Even of a sound reproduction system consisted of many more transmission channels, it would be still impossible to allow the listener fully to sense the presence of sounds, because the number of loud-speakers to be used is practically limited.

It is accordingly the object of this invention to provide a sound reproduction system enabling a listener to have a stronger feeling of the presence of sounds than before, though there are used a limited number of electroacoustic transducers.

According to this invention, there is provided a sound reproduction system including means for generating a plurality of signals having a desired stereophonic relationship and a plurality of electroacoustic transducers so arranged as to define a listening area and fed with said audio signals respectively, which is characterized in that it further comprises means for generating signal of ultralow frequency; and phase modulating means for modulating the phase of at least one of said plurality of audio signals according to said signal of ultralow frequency, thereby enabling a listener to feel the presence of sounds in a manner more approaching than before the form of reproduction which would be possible in a concert hall, though there are used a limited number of electro-acoustic transducers. Namely, according to the sound reproduction system of this invention, at least one of a limited number of loud-speakers gives forth sound corresponding to a phase-modulated audio signal, so that sounds generated by all these loud speakers momentarily vary in phase. This has an effect of allowing the listener to have a full feeling of the presence of sounds, though it may be impossible to reproduce sounds exactly in the same complicated phase relationship as would actually occur in a concert hall. The sound reproduction system of this invention permits a listener to have a full feeling of the presence of sounds without using many electroacoustic transducers, realizes prominent reduction of cost and is also particularly adapted for household application.

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

FIG. 1 is a block circuit diagram of a sound reproduction system according to an embodiment of this invention;

FIG. 2 is a block circuit diagram of a sound reproduction system according to another embodiment of the invention; and

FIG. 3 is a circuit diagram of a sound reproduction system according to still another embodiment of the invention.

FIG. 1 is a block circuit diagram of a sound reproduction system according to an embodiment of this invention. There are used four composite signals consisting of a combination of stereophonic right and left signals. Of said composite signals, those which are conducted to the two loud-speakers disposed slantwise behind the listener are phase modulated.

Right and left signals picked up from a source 11 of stereophonic signals such as a magnetic tape, or disk record are respectively amplified by ordinary right- and left-channel amplifiers 12 and 13 and supplied to phase splitters 14 and 15. Said phase splitters 14 and 15 may each consist of, for example, a known collector-emitter split circuit utilizing outputs of reverse phase generated at the collector and emitter of a transistor when its base is supplied with an input signal. With an input signal to the phase splitter 14 designated as R, its output signals may be expressed by +R and -R. Similarly with an input signal to the phase splitter 15 indicated by L, its output signals may be expressed by +L and -L. Four signals represented by said notations of +R, -R, +L and -L are supplied to a matrix circuit 16 formed of resistors, where they are formed into first, second, third and fourth composite signals which may be indicated by the notations of R-.DELTA..sub.1 L, R+.DELTA..sub.2 L, L+.DELTA..sub.3 R and L-.DELTA..sub.4 R (where the notations .DELTA..sub.1, .DELTA..sub.2, .DELTA..sub.3 and .DELTA..sub.4 are proportion constants each having a prescribed value ranging from zero to 1). The first composite signal is phase modulated by a phase modulator 18 according to an output signal of ultralow frequency from a signal generator 17 and then conducted through a power amplifier 21 to a loud-speaker 25 disposed slantwise on the right side behind a listener 29. The second composite signal is directly supplied to a power amplifier 22 for amplification and sent to a loud-speaker 26 placed slantwise on the right side ahead of the listener 29. The third composite signal is supplied through a power amplifier 23 to a loud-speaker 27 located slantwise on the left side ahead of the listener 29. The fourth composite signal is phase modulated, like the first composite signal, by a phase modulator 19 according to an output signal of ultralow frequency produced by a signal generator 20, and then supplied to a loud-speaker 28 positioned slantwise on the left side behind the listener 29.

There will now be described the individual right and left signals constituting the first, second, third and fourth composite signals indicated by the aforesaid notations of R-.DELTA..sub.1 L, R+.DELTA..sub.2 L, L+.DELTA..sub.3 R and L-.DELTA..sub.4 R. Right and left stereophonic signals R and L are electrical signals subjected to acoustoelectrical conversion by two microphones juxtaposed at an equal space ahead of a source of sounds. Accordingly, said signals R and L contain signals representing direct sounds from the source of sounds and reflected indirect sounds. The signals associated with the direct sounds have the same wave form, level and phase, whereas the signals of indirect sounds reflected from the walls and ceiling to the microphones through different routes are very much varied in wave from, level and phase. Accordingly, the first and fourth composite signals which are formed of a balance between the right and left signals contain larger proportions of indirect sound signals, because direct sound signals are extinguished therein. On the other hand, the second and third composite signals which represent a sum of the right and left component signals contain more predominant amounts of direct sound signals than indirect sound signals. According to the embodiment of FIG. 1, therefore, there are produced by the loud-speakers 26 and 27 ahead of the listener those sounds corresponding to the second and third composite signals containing larger proportions of direct sound signals and by the loud-speakers 25 and 28 behind the listener these sounds corresponding to the first and fourth composite signals bearing larger amounts of indirect sound signals. Moreover, the first composite signal (R-.DELTA..sub.1 L) and fourth composite signal (L-.DELTA..sub.4 R) are individually phase modulated by output signals of ultralow frequency produced by the signal generators 17 and 20 respectively. Therefore, sounds given forth by the loud-speakers 25 and 28 behind the listener according to said first and fourth composite signals enable a listener in a listening area to have the same full feeling of the presence of sounds as would be made possible in a concert hall. The reason is that indirect sounds from the loud-speakers 25 and 28 momentarily vary in phase not only among themselves, but also with respect to direct sounds from the loud-speakers 26 and 27, so that installation of electroacoustic transducers only at four spots can produce sounds substantially in the same complicated phase relationship as actually occurs in a concert hall.

The phase modulators 18 and 19 may be of the same type as is generally used with, for example, an electronic musical instrument, and description thereof is omitted. Output signals from the signal generators 17 and 20 are preferred to be independent of each other and have irregular ultraflow frequencies of several Hertz, because sounds actually generated, for example, in a concert hall from all sides have very much complicated and quite random relationships in wave form, level and phase. Accordingly, the signal generators 17 and 20 are only required to be an oscillator generating a signal of irregular ultraflow frequencies, a type capable of envelope detecting audio signals from the stereophonic source 11 to obtain an output signal of irregular ultraflow frequencies, a combination thereof, or a type capable of drawing out ultraflow frequency components from a random noise signal to produce similar output signals. All these signal generators are already known and further description is omitted. The power amplifiers 21 to 24 may also be of an ordinary type.

According to the embodiment of FIG. 1, four composite signals supplied to the four electroacoustic transducers consisted of those derived from right and left stereophonic signals. It will be apparent that said composite signals may, of course, be substituted for discrete 4-channel audio signals (not shown). It will also be noted that this invention is not limited to a four-channel type sound reproduction system wherein there are supplied stereophonic audio signals to four electroacoustic transducers, but may be further applicable to a multi-channel type sound reproduction system using a plurality of electroacoustic transducers, if permitted by economic and other considerations. Also arrangement of electroacoustic transducers may be effected in various modifications according to the listener's desire without departing from the technical concept of the invention.

There will now be described by reference to FIG. 2 another type of sound reproduction system which is more simplified and less expensive than the embodiment of FIG. 1. Right and left signals are supplied to right and left channel amplifiers 12 and 13, which are of the same type and denoted by the same numerals as those in FIG. 1. An output signal from the right channel amplifier 12 is conducted through a power amplifier 43 to a loud-speaker 46 disposed slantwise on the right side ahead of a listener 49, and an output signal from the left channel amplifier 13 is supplied through a power amplifier 44 to a loud-speaker 47 positioned slantwise on the left side ahead of the listener 49. This arrangement is the same as in the prior art stereophonic sound reproduction system. Therefore, the listener 49 can enjoy an ordinary type of stereophonic reproduction when sitting in a listening area defined by the loud-speakers 46 and 47.

If this invention is applied in such arrangement, it will easily enable the listener to have a full feeling of the presence of sounds. Namely, according to this invention, an output signal from the right channel amplifier 12 (designated as R) and an output signal from the left channel amplifier 13 (designated as L) are supplied to a matrix circuit 41 consisting of resistors. There are drawn out therefrom composite signals (indicated by R-L) representing a balance between the right and left signals R and L. Said composite differential signal is conducted to a phase modulator 42, where it is phase modulated according to output signal of ultralow frequency from a signal generator 50 to be supplied through a power amplifier 45 to a loud-speaker 48 located right behind the listener 49. Said phase modulator 42 and signal generator 50 are only required to be of the same type as those of FIG. 1. When composite signal (R-L) representing a balance between the right and left signals and containing large proportions of indirect sound signals is produced after phase modulation from the loudspeaker 48 to a listening area, then it will be easily understood from the description of FIG. 1 that there will be obtained the same reproduction effect as if the listener was in a concert hall.

The foregoing description relates to the case where this invention was employed in a sound reproduction system using right and left stereophonic audio signals, but the invention may also be applicable to a monophonic sound reproduction system. There will now be described this embodiment by reference to FIG. 3. Signal picked up from a source 51 of monophonic audio signal such as, for example, a magnetic tape or disk record is supplied to an amplifier 52 and then to a loud-speaker 57 through a power amplifier 53. This arrangement is exactly the same as in an ordinary monophonic sound reproduction system.

According to this invention, however, output signal from the amplifier 52 is further supplied to a phase modulator 54 to be phase modulated according to output from a signal generator 55 and then through a power amplifier 56 to a loud-speaker 58 disposed right behind a listener 59. Said loud-speaker 58 may, of course, be disposed in any other place. The point is that both loud-speakers 57 and 58 can define such a listening area as will enable the listener 59 to sense the presence of sounds. The phase modulator 54 and signal generator 55 may be of the same type as those of FIG. 1. If the sound reproduction system is arranged as described above, the listener will be able to hear both monophonic sound and phase modulated sound from the loud-speakers located at different spots, enjoying the same stereophonic reproduction as in the preceding embodiments of FIGS. 1 and 2.

Claims

What we claim is:

1. A sound reproduction system for operation with a source of audio frequency signals, including in combination:

first means having said source as an input for producing audio signals at four outputs,

said first means including means for producing stereophonically related right and left signals R and L, and means for combining said right and left signals R and L and producing four composite signals represented by R-.DELTA.1L, R+.DELTA.2L, L+.DELTA.3R and L-.DELTA.4R, respectively at the respective outputs of said first means, where .DELTA.1, .DELTA.2, .DELTA.3, and .DELTA.4 are predetermined proportionality factors having values ranging from 0 to 1;

electroacoustic transducer means corresponding in number to said first means outputs and defining a listening area, each of said outputs being coupled to a corresponding transducer means, said electroacoustic transducer means defining the listening area including four loudspeakers in correspondence to said four outputs of said first means;

second means for producing signals having subsonic frequencies; and

third means connected between the outputs of said first means at which the signals R-.DELTA. 1L and L - .DELTA. 4R are produced and the corresponding electroacoustic transducer means, for phase modulating such signals by said subsonic frequency signals produced by said second means,

said third means including two phase modulators, one of which being adapted to phase modulate the composite signal R-.DELTA.1L, the other being adapted to phase modulate the composite signal L-.DELTA.4R.

2. A sound reproduction system according to claim 1 wherein said signals produced by said second means have irregular frequency of several hertz.

3. A sound reproduction system according to claim 1 wherein said four loud-speakers are located at front-left and right sides and rear-left and right sides in the listening area, respectively, and wherein two loudspeakers at front-left and right sides are adapted to receive signals L+.DELTA.3R and R+.DELTA.2L, respectively and two loudspeakers at rear-left and right sides are adapted to receive phase modulated signals L-.DELTA.4R and R-.DELTA.1L, respectively.