Patent GB-1494751-A (GB)
Title: Sound reproduction systems
IPC: H04H20/88
Filed: 1974-03-26
Abstract: 1494751 Sound reproduction systems NATIONAL RESEARCH DEVELOPMENT CORP 10 March 1975 [26 March 1974] 13292/74 Heading H4R In a sound reproduction system in which four loud-speakers are located along the diagonals of a non-square rectangle, a decoder receives at least two input signals representative respectively of the total sound pressure (called the omnidirectional component W) and the direction from which the apparent second source arrives (called the phasor component P) and produces outputs including a first difference signal component X indicating the difference in signal strength between the sum of the signals for a first adjacent pair of loud-speakers and that for the opposite pair, and a second difference signal component Y indicating the difference in signal strength between the sum of the signals for a third pair of loud-speakers and that for its opposite pair, and a "layout control" applies to the difference signals respective gains having a ratio equal to the sine of half the angle subtended at the centre of the rectangle by the first pair of loudspeakers divided by the sine of half the angle subtended by the third pair of loud-speakers. This compensates for the apparent distortion of the sound field occasioned by the loud-speakers not being arranged in a square. The X difference signal is the front-back difference, Y is the Left-Right difference and where there are eight loud-speakers at the covers of a cuboid there is a third difference signal Z representing the Up-Down difference. These signals are subsequently matrixed to form loud-speaker signals LF, RF, LB, RB. In Fig. 6 the gains in the X and Y channels are adjusted by a centre tapped potentiometer U calibrated according to the half angle # subtended by the front pair of loud-speakers. The control may be limited to 25 <#< 65 by a fixed resistor at each end of the potentiometer. The gains may simply be #2 sin# and #2cos# but it is preferred to use #2Ksin# and #2 Kcos# where for frequencies below 500 Hz K=1/sin2# and for higher frequencies K = 1. For an eight loud-speaker system the gain ratios are where # is a gain factor and # the subtended halfangle. Different ratios may be used for low frequencies and the signals fed to the layout control may be linear combinations of the XYZ components such as Fig. 8, not shown. The transition frequency between the low and high ranges is theoretically centred at 700 Hz but it is better in practice to use 320 Hz, especially where the listener is off-centre. The transition concept is related to the effect of the distance between the human ears in relation to the wavelengths of received soundwaves. Directional accuracy in the sound field is ensured by satisfying four criteria of which two concern voltage gain at low frequency and two energy gain at high frequency. The criteria are satisfied in practice by the use of shelf-filters 48, 52, Fig. 10, in the omnidirectional and in the difference signal paths, the potentiometer network 28 providing the X, Y outputs and ensuring that their energies are the same as at the input and the high pass filter 82 compensating for the bass boost and phase shift encountered in the low frequency components of the sound field at a non-central listener as a result of different distances between the listener and the loud-speakers. The filter 82 is calibrated in terms of such distance but effectively its time constant is adjusted to equal the time taken by sound to reach the listener from the loud-speaker. The gains of the shelf filters relative to W are 1 at high frequency and 2/sin2# at low frequencies (i.e. very approximately 2 for 30 <#< 60) and the shelf filter gains for an eight loud-speaker system are 1 at low frequencies and #2/3 or #2 respectively for the phasor and omnidirectional components at high frequencies. In Fig. 11 (not shown) four-channel inputs are matrixed to produce W, X, Y components and additionally an F component representing the diagonal difference. These are mixed, with a 90 degree shift applied to F, to form further W, X, Y components which are subjected to shelf-filtering and the X and Y components are respectively highpass and low pass filtered then remixed with W to reconstitute LB, LF, RF, RB outputs. The filter gains are both 1 at low frequencies but rise to #3/4 for W and #3/2 for X and Y. Signals L. R (Figs. 12, 13, not shown) representing linear combinations of W, X, Y signals may be combined to produce a W output and ajP (phasor) output which is added to a third signal T T , the complex conjugate of P, to produce X and subtracted from T T  to produce Y. It is also stated that in an eight loud-speaker system, the gains associated with the differences between the sum of the four loud-speaker signals on one face and the sum of the four signals on the opposite face in the three orthogonal directions F-B, L-R and U-D are inversely related to the distances between the opposed pairs of faces. This is tantamount to the same relationship as the ratio of the sines of the subtended half angles.