Omnidirectional Sound Field Reproducing System

Abstract

Systems for producing a sound field in a region surrounding a listener wherein three information channels are utilized for conveying a first signal representative of total sound pressure (A) at a point in space, and two additional signals, A cos .theta. and A sin .theta., for describing the directional characteristics of the sound field. An array of microphones having, for example, equal cardioidal response characteristics are arranged in the original sound field for detecting sound information. Linear, additive signal combining networks are coupled to the microphones for producing the three signals. A plurality of loudspeakers (e.g., four) are coupled to the three information channels by additional linear signal combining networks for reproduction of the sound field. Systems for synthesizing the three signals by electronic devices are also described.

Description

This invention relates to systems for producing a sound field in a region surrounding a listener and, in particular, to systems for transmitting and/or storing, in three information channels, signals representative of surround stereophonic sound field information.

In a concurrently filed U.S. Pat. application Ser. No. 251,836 of Roy Martin Christensen, three-channel systems for surround stereophonic sound reproduction are described in which the three channels carry, respectively, an audio sum (total sound pressure) signal, a first audio difference signal (e.g., front minus back) and a second orthogonal audio difference signal (e.g., left minus right). Such signals conveniently may be derived from four discrete sound sources (of the type, for example, used in recording surround stereophonic sound on a four track magnetic tape).

The present invention relates to systems for producing a surround stereophonic sound field which may include an arbitrary number of sound sources disposed at various azimuthal locations around a "listening point" in space. The invention to be described contemplates describing the sound field at that point in terms of three quantities, the total sound pressure "A" at that point and a pair of quantities "A sin .theta." and "A cos .theta." indicative of the gradients (i.e., direction) of the sound pressure at that point with respect to a reference direction.

In accordance with one aspect of the present invention, the three quantities described above are produced by means of an array of, for example, four microphones located at a virtual central point in a sound field, the microphones facing outwardly from the point and being displaced from each other by equal angular increments. Linear signal combining means are coupled to the microphones for combining the outputs thereof to produce first, second and third audio frequency signals proportional to A, A cos .theta. and A sin .theta. representative of the total sound pressure and sound pressure gradient at the point in space. Three signal transmission channels are provided for coupling the three signals to a further linear signal combining means wherein signals proportional to A, A cos .theta. and A sin .theta. are recombined with predetermined relative amplitudes to produce a plurality of audio frequency signals suitable for driving a like number of loudspeakers so as to recreate a surround stereophonic sound field at a listening position.

In accordance with a preferred embodiment of the invention, the three signals A, A cos .theta. and A sin .theta. are formed by linear, additive combinations of outputs provided by four closely spaced microphones having like cardioidal response characteristics, the microphones being directed along orthogonal axes.

In accordance with a further aspect of the invention, a surround stereophonic sound field is synthesized by linearly, additively combining a plurality of signals representative of individual sound sources to form a total sum signal A. Means are provided for multiplying the quantity A by factors proportional to cos .theta. and sin .theta. to form second and third directional signals A cos .theta. and A sin .theta..

For a more complete understanding of the present invention, reference should be made to the following detailed description, in conjunction with the accompanying drawing, in which:

FIG. 1 is a schematic, block diagram of a system constructed in accordance with the present invention which utilizes three signal transmission channels for recreating a sound field surrounding a listener;

FIG. 2 is a diagram of the cardioidal characteristics of two microphones shown in FIG. 1; and

FIG. 3 is a schematic, block diagram of a portion of a system constructed in accordance with the present invention for synthesizing three signals suitable for recreating a sound field surrounding a listener.

Referring to FIG. 1, four microphones 10, 12, 14 and 16 are mounted at a virtual point 18 in a sound field, the microphones 10 and 14 being disposed along a first axis and are directed towards the left front (L.sub.f) and right rear (R.sub.r), respectively. The microphones 12 and 16 are disposed along a second axis orthogonal to the first and are directed towards the right front (R.sub.f) and left rear (L.sub.r), respectively. The microphones 10, 12, 14, 16 have predetermined directional response characteristics which, in the illustrated embodiment, preferably are all equal and are of a cardioidal pattern. Two such response patterns are shown in FIG. 2 directed along the axis associated with microphones 10 and 14. The patterns associated with microphones 12 and 16, as noted above, preferably are similar in shape but are directed along their associated axis.

Sound waves which are intercepted by the microphone array 10, 12, 14, 16 produce electrical signal outputs from one or more of the microphones depending upon sound direction and amplitude. The direction of a sound source with respect to the microphone array may be specified in terms of an angle .theta. measured from a reference line passing through the point 18. In the discussion which follows, .theta. will be specified as 0.degree. for a frontward direction (i.e., midway between L.sub.f and R.sub.f) and increasing counterclockwise.

In that case, a sound source located at a point distant from the central point 18 and producing a sound pressure of amplitude A at point 18 will produce the following responses in microphones 10, 12, 14 and 16, respectively:

L.sub.f = A/2 [1 + sin (.theta. + .pi./4)] 1

R.sub.f = A/2 [1 + cos (.theta. + .pi./4)] 2

R.sub.r = A/2 [1 - sin (.theta. + .pi./4)] 3

L.sub.r = A/2 [1 - cos (.theta. + .pi./4)] 4.

Any additional sound sources produce relative responses in the several microphones according to the above expressions, the resultant sound signals being additive according to superposition principles.

The four electrical signal outputs from microphones 10, 12, 14 and 16 are combined in summing amplifiers 20, 22 and 24 to form three signals which define the pressure and directional characteristics of the measured sound field. Specifically, amplifier 20 is arranged to produce an output signal "A," proportional to total sound pressure sensed at point 18 and defined by the expression:

A = 1/2 (L.sub.f + R.sub.f + R.sub.r + L.sub.r) 5.

The quantity "A" will hereafter also be referred to as M (a "main channel" signal) and, as specified in equation (5) is proportional to the sum of the responses of the four microphones 10, 12, 14 and 16 as defined by equations (1) - (4) above.

Summing amplifier 22, arranged to provide a relative gain of .sqroot.2/2, produces an output signal "A cos .theta." which is defined by the expression:

A cos .theta. = .sqroot.2/2 (L.sub.f + R.sub.f - R.sub.r - L.sub.r) 6.

The expression A cos .theta. is seen to be proportional to the difference between the response of the front (L.sub.f, R.sub.f) microphones 10, 12 and the response of the rear (R.sub.r, L.sub.r) microphones 14 and 16. The signal A cos .theta. therefore is sometimes referred to hereinafter as "X," or a front minus back signal.

The third amplifier 24, like amplifier 22, is arranged to provide a relative gain of .sqroot.2/2 and produces an output signal "A sin .theta." defined by the expression:

A sin .theta. = .sqroot.2/2 (L.sub.f - R.sub.f - R.sub.r + L.sub.r) 7.

The expression A sin .theta. is thus seen to be proportional to the difference between the response of left side (L.sub.f, L.sub.r) microphones 10, 16 and the response of right side (R.sub.f, R.sub.r) microphones 12 and 14. The signal A sin .theta. is therefore sometimes referred to hereinafter as Y, or a left minus right signal.

The three signals A, A cos .theta. and A sin .theta. (or M, X and Y) are coupled via appropriate separate transmission channels 26, 28 and 30 to reproducing means. The transmission channels 26, 28 and 30 may, for example, include a storage medium such as magnetic tape on which the three signals are recorded in separate tracks for subsequent recovery in a well known manner. Alternatively, the transmission channels may be included in an FM surround stereophonic radio system of the type described in the above-referenced Christensen patent application.

The outputs of transmission channels 26, 28 and 30 are coupled to linear, additive signal combining means for application to an appropriate array of loudspeakers 32, 34, 36 and 38 in a listening location 40. The loudspeakers 32, 34, 36 and 38 are illustrated as arranged in a "square" pattern. That is, each loudspeaker is in a separate corner of a square room. Such an arrangement is suitable for producing left front (L.sub.f '), right front (R.sub.f '), right rear (R.sub.r ') and left rear (L.sub.r ') audio signals.

To this end, the additive signal combining means are illustrated as comprising first and second inverting amplifiers 42 and 44 supplied, respectively with A cos .theta. and A sin .theta. information, an amplifier 46 exhibiting a gain of 1/2 and coupled to the "A" signal transmission channel 26 and four linear matrixing amplifiers 48, 50, 52 and 54 coupled, respectively, to loudspeakers 32, 34, 36 and 38.

The output of amplifier 46 (in the "A" channel) is coupled to each of matrix amplifiers 48, 50, 52 and 54 so as to provide equal signals representative of total sound pressure to all such amplifiers. The outputs of inverters 42 and 44 and the remaining outputs of transmission channels 28 and 30 are coupled to matrix amplifiers 48, 50, 52 and 54 so as to satisfy the following relationships:

L.sub.f ' = (M/2) + .sqroot.2/4 X + .sqroot.2/4 Y 8

r.sub.f ' = (M/2) + .sqroot.2/4 X - .sqroot.2/4 Y 9

r.sub.r ' = (M/2) - .sqroot.2/4 X - .sqroot.2/4 Y 10

l.sub.r ' = (M/2) - .sqroot.2/4 X + .sqroot.2/4 Y 11.

substituting previous expressions for the quantities M, X and Y and applying trigonometric identities to the resulting expressions, it can be shown that the outputs of matrix amplifiers 48, 50, 52 and 54 can be expressed, respectively, as:

L.sub.f ' = A/2 [1 + sin (.theta. + .pi./4)] 12

R.sub.f ' = A/2 [1 + cos (.theta. + .pi./4)] 13

R.sub.r ' = A/2 [1 - sin (.theta. + .pi./4)] 14

L.sub.r ' = A/2 [1 - cos (.theta. + .pi./4)] 15.

Comparison of equations (12) -- (15) with previously stated equations (1) -- (4) demonstrates that the reproduced sound field components correspond to the measured components in the original sound field.

The illustrated arrangement for three channel transmission of surround stereophonic information also may be characterized as a uniform system. That is, regardless of the azimuthal orientation of a sound source in the original sound field, the reproduced sound source will appear to the centrally located listener in the listening room 40 as appearing from the same azimuthal orientation as the original. Furthermore, a constant amplitude sound source moving around the horizon of the original sound field will be reproduced without fluctuations in power in the listening room 40 where four like loudspeakers 48, 50, 52 and 54 are employed.

The sound field components produced by each of the speakers 32, 34, 36 and 38 for a given location (.theta.) of a sound source in the original sound field may be calculated by means of the equations set forth above. Thus, for example, where a sound source of detected amplitude A is located to the left in the original sound field (i.e., at .theta. = .pi./2), the loudspeakers 32, 34, 36 and 38 will provide relative outputs of 0.85A, 0.15A, 0.15A and 0.85A, respectively. The equal outputs of loudspeakers 32 and 38 will produce a virtual sound source midway between such loudspeakers. A standing wave (non-directional) sound component of 0.15A radiated by all four loudspeakers may be considered to be superimposed upon the traveling wave component (e.g., 0.7A) appearing to come from a phantom source midway between loudspeakers 32 and 38. It should be noted that standing wave information is carried by the M signal channel.

By similar substitution to that employed above, it can be shown that a sound source along one of the axes (e.g., L.sub.f) associated with microphones 10, 12, 14 and 16 produces response in the principal speaker (e.g., L.sub.f) equal to that of the original sound source and one-half that response in the flanking speakers (e.g., L.sub.r and R.sub.f). Directionality of the sound source is therefore maintained in the reproduced sound field.

Similar results will be obtained for location of sound source in others of the four spatial quadrants. That is, as stated above, the system provides a uniform response. The effect of multiple sound sources at different locations also may be evaluated by applying superposition principles.

It should also be noted that different speaker arrangements may be excited by proper matrixing of the M, X and Y signals. For example, a diamond arrangement (front, back, left and right) of speakers may be so excited.

Referring to FIG. 3 of the drawing, apparatus is shown whereby M, X and Y signals representative of a desired sound field may be formulated utilizing a different microphone technique than that shown in FIG. 1. Specifically, a series of separate magnetic recording tracks 100, 102, 104, etc., each representing the sound contribution of a single musical instrument are shown. Such multi-track recordings are conventionally used in commercial sound recording studios. The information on each track 100, 102, 104 of the tape is converted by means of a conventional magnetic pickup head 106, 108, 110 to electrical audio frequency signals for application to an amplifier 112, 114, 116. Variable attenuators (gain controls) 118, 120, 122 are provided in each signal path for control by the sound engineer. The outputs of each of attenuators 118, 120, 122 are coupled to a first summing amplifier 124 to produce an audio sum signal (M) as defined in connection with FIG. 1. Furthermore, apparatus is provided in each signal path for selectively "placing" each sound source in a particular spatial position. To this end, sine-cosine potentiometers 126, 128 and 130 of the type, for example, commonly employed as phase resolvers in the servomechanism art, are included in each signal path. Adjustment of the shaft position of the potentiometers 126, 128, 130 will determine the relative angular spatial orientations of the several sound sources in the resulting sound field. The "cosine" outputs of all potentiometers are added together in a second summing amplifier 132 while the "sine" outputs thereof are added together in a third summing amplifier 134. In this manner, signals proportional to X and Y components defined previously are provided.

Claims

What is claimed is:

1. Apparatus for producing signals representative of a sound field in a spatial region comprising:

a plurality of transducers responsive to audio information signals for converting said information signals into a corresponding plurality of electrical signals,

first means for additively combining said plurality of electrical signals to produce a signal A proportional to the sum thereof,

seconds means for additively combining said plurality of electrical signals to produce a signal proportional to A cos .theta., wherein .theta. is the angular disposition of a sound source with respect to a reference direction in said spatial region, and

third means for additively combining said plurality of electrical signals to produce a signal proportional to A sin .theta..

2. Apparatus according to claim 1 wherein:

said plurality of transducers comprise four like microphones disposed about a point in a sound field, said microphones being equally angularly spaced around said point and each exhibiting a cardioidal response characteristic.

3. Apparatus according to claim 2 wherein:

said first additive combining means comprises a summing circuit providing a relative gain of one-half for each of signals supplied by said four microphones,

said second and third additive combining means providing relative gains of .sqroot.2/2 for each of said signals from said four microphones,

said second combining means further providing inversion of signals from two adjacent ones of said microphones and said third combining means providing inversion of signals from one of said two adjacent ones of said microphones and of signals from a third one of said microphones adjacent to said one.

4. Apparatus according to claim 1 wherein:

said transducers comprise a plurality of magnetic pickup devices, each associated with a corresponding magnetic sound recording track, said apparatus further comprising a like plurality of sine-cosine potentiometers coupled in circuit between each said pickup device and said second and third combining means.

5. In a system for reproducing a sound field surrounding a listener wherein said sound field is represented by three signals A, A cosine .theta., and A sine .theta., the quantity A being representative of total sound pressure at a point in space and the parameter .theta. being representative of a spatial orientation of a sound source with respect to a reference direction, a sound reproducing system comprising:

first, second and third audio signal transmission channels for said three signals;

four sound reproducing means; and

first, second, third and fourth linear additive signal combining means for coupling said transmission channels to said sound reproducing means to produce, by means of said three signals, four output signals representative of a sound field surrounding a spatial region suitable for reproduction by synthesis of said four signals from said three signals in additive and subtractive combinations.

6. A sound reproducing system according to claim 5 wherein:

signal A, proportional to total sound pressure, is defined by the expression: A = K(L.sub.f + R.sub.f + R.sub.r + L.sub.r) wherein L.sub.f, R.sub.f, R.sub.r and L.sub.r correspond respectively to sound sources disposed at left front, right front, right rear and left rear locations in a sound field;

signal A cosine .theta. is porportional to the difference between the front and rear sound sources defined by the expression: A cosine .theta. = .sqroot.2 K(L.sub.f + R.sub.f - R.sub.r - L.sub.r); and

signal A sine .theta. is proportional to the difference between the left side and right side sound sources defined by the expression: A sine .theta. =.sqroot.2 K(L.sub.f - R.sub.f - R.sub.r + L.sub.r), the quantity K representing a numerical constant.