Patent GB-1359509-A (GB)
Title: Decoder apparatus for use in a multidirectional sound system
IPC: H04H20/88
Filed: 1971-06-14
Abstract: 1359509 Stereophonic sound systems; automatic gain control P SCHEIBER 14 June 1971 [15 June 1970] 27689/71 Heading H4R In a stereophonic system in which sounds are picked up at four or more positional locations to provide directional information and are distributed to four or more spaced sound reproducers to impart direction to the reproduced sound, the sound signals from each pickup transducer are 'resolved into components' corresponding to two different directions, applied one to each of two channels common to all pick-ups and distributed to the reproducers in accordance with the relative signal levels on the two channels. As described true resolution into components in the normal vector sense is not effected. Instead the angle subtended between each directional information input and a fixed direction is divided by two and the input is treated as though it subtends the half angle to the fixed direction for resolution purposes. In the preferred embodiments four pick-ups (microphones) and four reproducers (loud-speakers or loud-speaker units) are employed. A two dimensional and a three dimensional arrangement are described. In the three dimensional arrangement, for example, the microphones and loud-speakers are disposed at the vertices of respective tetrahedrons. The signals in the two channels which may for example be the two channels of a stereo radio transmission or the two tracks of a stereo record convey the directional information in the form of two angular co-ordinates measured relative to origins within the tetrahedrons. One angular co-ordinate for each pick-up is transmitted by the relative magnitudes in which the signals from each microphone are distributed between the channels by resolving a unit vector in the direction of each microphone along two axes at right angles and distributing the signal from each microphone to the two channels at amplitudes proportional to the two components of the associated vector. The other angular co-ordinate, i.e. the angle through which the plane of the two axes must be turned to bring it into a fixed X, Y plane is transmitted by delaying the signal applied to one channel relative to that applied to the other in proportion to the said other angle. In the two dimensional embodiment the phase displacement is necessary only to the extent of 180 degrees phase or polarity change. At the reproducer station the two channels are applied in different proportions to each loud-speaker. If the proportions correspond to the components from a particular microphone then that loudspeaker reproduces the output of that microphone loudest and should be placed in a position corresponding to the microphone. The signals may be applied to and separated from the channels by analog calculating circuits, e.g. operational amplifiers with resistive inputs the resistive values of which are selected in accordance with the signal proportions to be applied to or separated from the channel. Automatic gain control.-In the two dimensional embodiment with four microphones and four loud-speakers a sound from one microphone is reproduced loudest in the correspondingly positioned loud-speaker and at a lower level in the two nearest loud-speakers. To increase the directional effect an automatic gain circuit may be incorporated to attenuate the lower level sounds and to increase the level of the loudest loud-speakers in a manner which maintains the total gain constant. Several embodiments with logarithmic gain control are described. In two embodiments (Figs. 9 and 10, not shown) located wholly at the reproducer station the levels of two loud-speakers positioned at opposite corners of a square are compared and if equal or nearly equal are attenuated while the levels of the other loudspeakers are increased and if unequal are increased while the levels in the other channels are attenuated. In another embodiment (Figs. 11 and 12, not shown) the signal levels of oppositely positioned microphones at the pick-up station are combined and a 20 Hz tone distributed between the channels in accordance with the ratio of the two combinations. At the reproducing station the tone levels are filtered out and used to control the gain at the loud-speaker inputs.