Patent GB-1411994-A (GB)
Title: Signal matrixing for directional reproduction of sound
IPC: H04S3/00
Filed: 1972-10-03
Abstract: 1411994 Multi-channel audio systems D H COOPER 3 Oct 1972 [6 Oct 1971 13 Sept 1972] 45558/72 Heading H4R There is described a matrixing and dematrixing arrangement for reproducing n source signals Sk at bearing angles #k relative to a reference direction, via a number of transmission paths less than or equal to the number of source signals, at a number of devices, e.g. loud-speakers producing presentation signals Pi, the presentation signals being produced at bearing angles #i relative to the reference direction, the arrangement resulting in presentation signals of the form where F((#i - - #k) is a function only of (#i - #k). Where two transmission channels Talpha, TR are used the source signals Sk are matrixed according to the relationships where is the square root of - 1 and represents a 90 degree phase shift, and are dematrixed to give presentation signals Pi according to the relationship i.e. The Specification mainly relates to the case of such matrixing/dematrixing in quadraphonic systems of the 4-2-4 type, in which the transmission paths may include storage means such as disc grooves or tape tracks. Where radio transmission is used the sum and difference of the two transmission signals Ta and TR are transmitted in known manner, the sum signal being designated T# and the difference signal TA. By using signals Talpha and TR or T# and TA as defined above mono or two channel stereo reproduction can be provided with existing equipment without the need for any dematrixing. Figs. 6-10 (not shown) show in block diagrammatic form arrangements for producing the transmission signals Talpha and TR or TE and TA and for dematrixing them which follow from a consideration of the equations 1, 2 and 3. In one particular arrangement (Fig. 8, not shown) the dematrixing is performed to provide presentation signals Pi for loud-speakers spaced around a room at bearing angles # = 0, 15, 30 ... 345 degrees. The results of the above form of matrixing/ dematrixing are shown in Fig. 13 which is a polar diagram showing the amplitudes and phase angles of reproduction for the above twotransmission channel system as a function o f the angle between each speaker location and the source signal (Sk) location portrayed, the source signal being positioned at #k=0 0 degrees. There is also shown an improved directivity pattern that can be achieved by the use of two supplementary transmission signals to form a four transmission channel system. Such a system for use in radio broadcasting is diagrammatically shown in Fig. 11, and assumes four source signals S# and four presentation signals P#. Two main channels carry the signals TE and TA which are transmitted as the sum and difference signals of conventional mono-compatible two channel stereo broadcasting. The two supplementary signals TT, TQ given by respectively are bandwidth limited to the range 130 Hz - 3 kHz and are alternated at a sampling frequency of 9À5 kHz and multiplexed together as a composite modulation of the quadraturephased 38 kHz carrier known in the art for speaker-identified quadraplex FM transmission. Dematrixing is performed according to the equation Pi = T# + T#(e<-j(#i+(M)/(2))>) + TTe<j(#i+(M)>/(2)) + TQe<-2j(#i+(M)/(2))>) 4 to give a modified form of equation 3 such that Pi (equation 4) = where <2>/M is equal to a factor by which the signals TT' TQ may be attenuated relative to signals T# and TA, this factor being introduced to allow some variation in the directivity patterns to suit individual listener needs. The directivity curve shown for the 4 transmission channel system in Fig. 13 is for m = 1. An arrangement using four signals Talpha, TR, TT, TQ may be used for four track tape recordings without bandlimiting the signals TT TQ. These signals may also be substituted for speaker-identified channels in a known four channel disc reproducing system. Several other arrangements based on the same principles are described, mainly from the mathematical viewpoint.