Sound reproduction systems

A basic decoder designed to feed signals to at least three loudspeakers disposed at respective azimuths around a reference point at equal distances therefrom is modified to make it suitable for feeding loudspeakers at the same azimuths but at non-uniform distances from the reference point. Each output of the basic decoder is subject to a time delay proportional to the difference of time of travel of sound from the first pair of loudspeakers to the reference point and the time of travel of sound from the second pair of loudspeakers to the reference point, divided by the sum of said times of travel to an adder connected between the first velocity signal input and the corresponding high-pass filter.

This invention relates to directional sound reproduction systems 
reproducing sound via three or more loudspeakers spaced round a listening 
position. 
In designing decoders for sound reproduction systems of this type, it is 
customary to assume that all loudspeakers are at the same distance from a 
central listening point. The signals fed to the various loudspeakers are 
so adjusted as to produce a sound field in the immediate neighbourhood of 
this reference point which provides information to the ears of a listener 
at the point sufficient to create an impression of directionality. 
Provided that this information satisfies a sufficient number of the 
various mechanisms used by the human ear to localise sounds for listeners 
at the reference point, it is found in practice that good directional 
reproduction is obtained for listeners at other positions in the 
surrounding listening area. 
The various relevant psychoacoustic criteria for a listener at the 
reference point are discussed in M. A. Gerzon, "Surround Sound 
Psychoacoustics", Wireless World, December, 1974, pages 483 to 486, U.K. 
Patent Specfication No. 1,494,751 and co-pending Application No. 46822/75 
and corresponding U.S. Pat. Nos. 4,081,606 and 4,086,433. The latter two 
references disclose matrix circuitry for use in decoders for such systems. 
It has been found that, even when all loudspeakers in a layout are 
equidistant from a reference point, there are some shapes of layout for 
which it is not possible to design decoders using matrix circuitry such 
that sufficiently many psychoacoustic criteria are satisfied for a 
listener at the reference point. In the following description, a 
loudspeaker layout for which it is possible to design a decoder using 
matrix circuitry will be referred to as a "solvable equidistant layout" 
and the decoder for such a layout will be described as a "solvable 
equidistant decoder". 
According to the invention, there is provided a decoder for producing 
output signals which, if fed to at least three loudspeakers disposed at 
respective azimuths around a reference point at non-uniform distances 
therefrom, would produce a desired directional effect, comprising a basic 
decoder which, if fed to loudspeakers disposed at said azimuths at a 
uniform distance from the reference point would produce said desired 
directional effect, means for subjecting each output of the basic decoder 
to a relative time delay proportional to the difference between the 
distance from the reference point of the loudspeaker at the azimuth to 
which the output relates and the distance from the reference point of the 
most distant loudspeaker and to an amplitude gain proportional to the 
distance of the loudspeaker at said azimuth at the reference point. 
The applied time delay is preferably so chosen as to be exactly equal to 
the difference in the time of travel of sound signals from each of the 
loudspeakers to the reference point. The constant of proportionality which 
determines the applied time delay is therefore at least approximately 
equal to the reciprocal of the speed of sound in air. The applied gain is 
chosen to compensate for the attenuation of sound intensity with 
increasing distance from a sound source. 
It is already known to use delay lines to feed loudspeakers placed around a 
listener but these have been used to cause sounds other than those 
intended to come from loudspeakers in front of the listener to be heard by 
the listener with a delay relative to such sounds from loudspeakers in 
front of the listener of between 5 and 50 milliseconds. In accordance with 
the present invention, the sound from all loudspeakers arrive at the 
reference position at the same time from all loudspeakers. In addition, 
the invention requires the signals for the various loudspeakers to be 
produced by a solvable equidistant decoder for the same azimuth angles.

FIG. 1 shows a layout of four loudspeakers 10, 11, 12 and 13 equidistant 
from and surrounding a point 14. The loudspeakers are disposed at corners 
of a trapezium. There is no known way of designing a solvable equidistant 
decoder for this layout using, matrix circuitry with the point 14 as a 
reference. 
The diagonal lines joining the loudspeakers 10 and 12 and loudspeakers 11 
and 13 respectively intersect at a point 15. The loudspeakers 10 and 11 
are closer to this point than are the loudspeakers 12 and 13 but, 
referring to FIG. 2, the loudspeakers 16, 17, 18 and 19 which are located 
on these diagonal lines at equal distances from the reference point 15, 
form a solvable equidistant layout. A decoder for this layout can be as 
described in either the above-mentioned patent specification or the 
above-mentioned co-pending application. 
FIG. 3 shows a decoder in accordance with the invention. Two or more input 
signals are fed to a solvable equidistant decoder 20 which produces output 
signals LB', LF', RF' and RB' suitable for the loudspeakers 16, 17, 18 and 
19 respectively of the layout shown in FIG. 2. The two output signals LF' 
and RF' for the front loudspeakers are fed to respective delay devices 21 
and 22 which produce output signals LF and RF respectively for feeding to 
the loudspeakers 10 and 11 of the layout shown in FIG. 1. Similarly, the 
signals LB' and RB' are fed to respective amplifiers 23 and 24 which 
produce output signals LB and RB respectively for the loudspeakers 12 and 
13. The delay applied by the delay device 21 is equal to the difference 
between the distance of the furthest loudspeaker, i.e. the loudspeaker 12 
or the loudspeaker 13, from the reference point 15 and the distance of the 
loudspeaker 10 from the reference point 15 divided by the speed of sound 
in air. A similar delay is applied by the delay device 22. The amplifiers 
23 and 24 apply amplitude gains which are proportional to the distance of 
each of the loudspeakers 12 and 13 from the reference point 15, the 
constant of proportionality being such that the equivalent gain for the 
distance of the loudspeakers 10 and 11 from the reference points 15 would 
be unity. More generally, for an array of loudspeakers in which the 
distance of the i'th loudspeaker from the reference point is r.sub.i and 
the maximum loudspeaker distance from the reference point is r.sub.max 
then i'th loudspeaker is fed from its associated solvable equidistant 
decoder output via an amplitude gain proportional to r.sub.i and a time 
delay given by: 
EQU (r.sub.max -r.sub.i)/c 
where c is the speed of sound in air. Thus, the signal for the most remote 
loudspeaker will be fed via an amplifier only, the signal for the closest 
loudspeaker to the reference point would be fed via a delay device only 
while the signals for intermediate loudspeakers will be fed via both 
respective amplifiers and respective delay devices. 
If desired, the various gains and time delays may be applied to the input 
signals to solvable equidistant decoders rather than to the output 
signals. FIG. 4 illustrates a decoder of this type which is equivalent to 
the decoder of FIG. 3 when the latter is adapted to receive two input 
signals only. One of the input signals is applied to the amplifier 23 and 
the delay device 21 and the other input signal is applied both to the 
amplifier 24 and the delay device 22. Two solvable equidistant decoders 25 
and 26, both of which as identical with the decoder 20, are provided. The 
outputs from the two amplifiers 23 and 24 are applied to the inputs of the 
decoder 25, two of the outputs of which comprise the signals LB and RB 
respectively. The other outputs are not used. Similarly, the outputs of 
the delay devices 21 and 22 are applied to the inputs of the decoder 26, 
two of the outputs of which produce the signals LF and RF, the other two 
outputs not being used. In general, the delay devices and amplifiers may 
be incorporated in any part of the circuitry provided that the required 
output signals are produced. 
Certain of the decoders described in the above-mentioned patent 
specification and co-pending application include so called "distance 
compensation" which compensates for the effect of the curvature of the 
sound field at the reference point due to the distance of the loudspeaker 
from the reference point being finite. Such compensation consists of an RC 
high-pass filter in all signals paths representative of reproduced 
velocity at the reference point with a -3dB point 54/r Hz, where r is the 
loudspeaker distance in meters. Decoders in accordance with the present 
invention are for use with loudspeaker layouts which do not have a single 
value for r. An economical, although not strictly correct, method of 
providing compensation of sound field curvature in decoders in accordance 
with the invention is to apply compensation for an average value of the 
loudspeaker distances involved. 
It is in fact possible to compute the circuitry required to compensate for 
sound field curvature for layouts not equidistant from the reference point 
and this compensation can always be realised by a matrix using 
non-cascaded low-pass and high-pass RC networks acting on the signals 
representative of pressure and velocity to produce modified output signals 
representative of velocity. 
FIG. 5 illustrates the later stages of a decoder, similar to the decoder of 
FIG. 3, with compensation for sound field curvature. Three input signals 
W', X' and Y' are representative respectively of the desired pressure, 
forward components of velocity and lateral components of velocity at the 
reference position. The distance compensated signals, W, X and Y are 
applied to the output matrix 30 of a solvable equidistant decoder for the 
layout of FIG. 2 which produces output signals as follows: 
LB'=1/2(W-X+Y) 
lf'=1/2(w+x+y) 
rf'=1/2(w+x-y) 
rb'=1/2(w-x-y) 
the output signals for the matrix 30 are applied to the delay devices 21 
and 22 and the amplifier 23 and 24 to produce the signals LB, LF, RF and 
RB as described with reference to FIG. 2. 
To provide the required distance compensation, the two inputs signals X' 
and Y' representative of velocity are applied to the matrix 30 via 
respective high-pass filters 31 and 32 with time constant given by: 
EQU 2(t.sub.1.sup.-1 +t.sub.2.sup.-1) 
where t.sub.1 and t.sub.2 are the time sound takes to travel from the two 
front loudspeakers 10 and 11 to the reference point 15 and from the rear 
loudspeakers 12 and 13 to the reference point 15 respectively, so that the 
-3dB frequency is equal to the average of that associated with the front 
loudspeaker distance and that associated with the rear loudspeaker 
distance. In addition, the pressure signal W' is RC low-pass filtered by a 
filter 33 having the same time constant as the filters 31 and 32, passed 
through an attenuator 34 having amplitude gain given by: 
EQU (t.sub.2 -t.sub.1)/(t.sub.2 +t.sub.1) 
and added to the output of the filter 31 by an adder 35. 
When the loudspeaker layout departs only slightly from being equidistant 
from the reference point, the required delays are small and may be 
provided by cascaded RC all-pass networks. For example if the required 
delay corresponds to a 20 centimeter difference between r.sub.i and 
r.sub.max an approximation to the required delay may be provided by a 
cascaded pair of RC all-pass networks, each of time constant equal to a 
quarter of the required delay, i.e. a time constant of 0.147 msec. The 
pair of all-pass networks acts as a delay of the required time for 
frequencies up to about 1 kHz. At higher frequencies, it does not alter 
the polarity of signals. It is considered to be desirable for good high 
frequency localisation that the relative polarities of signals from all 
loudspeakers should be undisturbed by the delay circuitry.