Patent GB-649998-A (GB)
Title: Electric pulse communication systems employing supersonic cells
IPC: H04N5/66
Filed: 1948-04-23
Abstract: 649,998. Light valves. STANDARD TELEPHONES & CABLES, Ltd. (Federal Telecommunication Laboratories, Inc.). April 23, 1948, No. 11180. [Class 40 (iii)] [Also in Group XL (c)] A receiver for a pulse multiplex system comprises a supersonic-wave light-modulating cell, means for propagating mechanical supersonic waves derived from the received pulse-modulated signals through the cell, through which is also passed light in a direction transverse to that of the supersonic waves, and a stop which blocks the light passing through the cell in the absence of a supersonic wave, the light passing the stop when supersonic waves pass through the cell and impinging upon one or more photo-electric cells which deliver outputs corresponding to the modulating signals. In the arrangement of Fig. 1, pulse-modulated signals received at 2 are used to modulate the output of a supersonic-wave generator 3. The output of generator 3 is applied to a supersonic cell 4 comprising quartz-crystal transducer 5, liquid 6 and an absorber 7 terminating the mechanical line formed by the cell. Unit 18 selects synchronizing pulses from the output of receiver 2 and applies them to a light-control unit 17 which causes light source 13 to flash briefly once for every synchronizing pulse received. In the absence of any mechanical signals in the cell, the light from the source 13 is brought to a focus upon the stop 8 ; however, the light passing through a mechanical pulse is deflected past the stop and brought to focus upon the appropriate one of the photoelectric cells 11 which are arranged behind respective apertures in the plate 12. The output from each cell comprises the modulation conveyed by a respective channel ; the arrangement described acts as a demodulator which may be used for either amplitudemodulated or time-modulated pulse trains. In Fig. 3 a continuous light source 13 is employed, the light from which is passed through a cell 21 through which supersonic waves corresponding to the synchronizing pulses are passed. The light passing through the cell, apart from that deflected by the mechanical pulse, is focused on to a stop 27, the light passing the stop being limited by an aperture (not shown) situated close to lens 28 to produce a narrow parallel beam through cell 4 to which the received pulse trains are applied. The light from the cell 21 accordingly scans the cell 4, passing each of the pulses therein in turn causing the corresponding one of the photo-electric cells 32 to be illuminated. In a further embodiment, Fig. 4 (not shown), an arrangement similar to that of Fig. 3 is used except that the spot of light from the cell 22 scans the cell 4 in the same direction as the supersonic waves travel through it and that a single photo-cell only is used. A single mechanical pulse is consequently illuminated during the whole of its passage through the cell 4 and the output of the photo-cell corresponds to one channel only. Different channels may be selected by a delay network inserted into the circuit after the synchronizing-pulse selector 18 or by adjusting the length of delay line 21.