GB451676A - Improvements in or relating to sound transmitting recording and reproducing systems - Google Patents

Abstract

451,676. Reproducing music &c. MARCONI'S WIRELESS TELEGRAPH CO., Ltd., Electra House, Victoria Embankment, London. -(Assignees of Beers, G. L. ; 119, Summerfield Avenue, Collingswood, New Jersey, U.S.A.) Feb. 11, 1925, No. 4401. Convention date, Feb. 9, 1934. [Class 40 (iv)] [See also Group XL] In a binaural system, the outputs of two or more microphones A, B are combined in a transformer 12 and transmitted over a single channel (which may have a recording and reproducing link), and in addition an auxiliary frequency, outside the sound range and supplied by a generator 22, is modulated in accordance with the difference in output of the microphones, transmitted over the same channel, and at the receiving end is separated out by a filter 25 to control the relative gains of amplifiers 17, 19 feeding spaced loud-speakers 18, 20. Fig. 2 shows means for alternatively modulating the auxiliary frequency in accordance with amplitude or frequency. The output circuits of additional amplifiers 101, 131 associated with the microphones are fed to a double - diode rectifier 23-24 the circuits of which contain resistances 26, 27 connected in series to the grid of an amplifier 29 whereby the outputs of the amplifiers 10<1>, 13<1> shift the grid bias in opposite directions. With switches 31, 34 in positions 37, 36, changes in grid bias of amplifier 29 cause changes in amplitude of the oscillations generated by tube 22, and with the switches in their alternate positions, the anode-cathode circuit of amplifier 29 is connected in series with condenser 33 across the upper part of the inductance of the tuned circuit 30 so that changes in the impedance of the anode-cathode circuit cause changes in the frequency generated. Fig. 2 also includes the provision of automatic volume control for maintaining the changes in the control energy independent of the absolute sound values. Thus valves 38, 38<a>, are fed from the outputs of valves 10<1>, 131 and resistances 40, 40<a> in series supply potential to the grids of these amplifying valves to keep constant their feed to the rectifier 23-24. In receiving an amplitude-modulated auxiliary frequency, Fig. 3, the audiofrequency output from receiver 16, Fig. 1, is fed to amplifiers 17, 19 through transformers 41, 42, and the auxiliary frequency is fed through transformer T to two rectifiers 25, 25<1> having resistances connected in their circuits whereby the grids of the amplifiers are oppositely biassed in accordance with the amplitude of the auxiliary frequency. In a frequency-modulated system, Fig. 4, two filters 43, 44 passing bands above and below the mean frequency of the auxiliary energy (with or without an overlap) feed two oppositely-arranged double-diode rectifiers similar to that in Fig. 2, and similarly and oppositely control the grids of amplifiers 17, 19.