Patent GB-555169-A (GB)
Title: Band width compression in electric wave transmission systems
IPC: H03G5/20
Filed: 1942-10-02
Abstract: 555,169. Band-width compression : television. STANDARD TELEPHONES & CABLES, Ltd. Oct. 2, 1942, No. 13847. Convention date, Jan. 17, 1941. [Class 40 (iv)] [Also in Group XL] In a transmission system for a complex signal the band-width of which varies, means are provided whereby when the band-width is large it is compressed and when it is small it is expanded so that the whole signal can be sent over a channel the width of which corresponds to the mean band-width instead of the maximum band-width of the original signal. This may be effected by recording the signal on a portion of a telegraphone tape that runs at a uniform speed and reproducing it for transmission from a portion of the tape of which the speed varies in accordance with the rate of change and higher order derivatives of the signal voltage. Alternatively the signal may be recorded on a variable-speed portion and reproduced for transmission from a constantspeed portion of the tape. In the case of television, the scanning speed may be varied in accordance with the rate of change and higher order derivatives of the signal voltage obtained by scanning. Systems of general application.-In the arrangement shown in Fig. 1, an incoming signal produces a voltage across a resistance 23 which follows the signal wave without distortion and a voltage across a resistance-condenser combination 20, 21 which departs from the signal wave to an extent that depends on the rate of change and higher order derivatives of the signal voltage. The difference between these voltages appears in the output circuit of tubes 28, 29 and, after rectification by tubes 31, 32, produces a biasing voltage across a resistance 36 in the grid circuit of a tube 37 the anode circuit of which is included in the circuit of a motor 12 driving the recording portion of a telegraphone tape. The speed of this portion thus depends on the rate of change and higher derivatives of the signal voltage. The distorted signal is picked up at 44 from a portion of the tape that is driven at a uniform speed by a motor 13, integrated by a circuit 48, 48<SP>1</SP> in the anode circuit of an amplifying tube 47 and sent over the transmission circuit. A voltage developed across a resistance 38 in the anode circuit of the speed-controlling tube produces a signal for transmission over a pilot channel for the control of the speed of a motor forming part of the converse arrangement at the receiving end, Fig. 3 (not shown). If the variable-speed motor lags too far behind the motor 13, relay 2a is operated by the engagement of a contact 5 on a disc driven by 13 with a contact 2 on a disc driven by 12, locks itself and modifies the grid-circuit of tube 37 so as to bring the motor 12 to its maximum speed. When the relative movement of the discs has been reversed for a period, contact 5 engages contact 1 and relay 1a releases 2a to restore control to the signal. An analagous operation takes place if motor 12 gets too great a lead. In a modified arrangement, Fig. 5 (not shown), the speed-controlling voltage is derived from the signal after the latter has been recorded at a uniform and reproduced at a variable speed. Receiving arrangements are described, Figs. 4 and 6 (not shown), in which the driving motor is controlled as at the transmitter by the signals instead of by a pilot current. Television system, Figs. 7 and 8.-A photosensitive cathode on which the scene is projected is scanned by an electron beam controlled by deflecting coils 206, 207 supplied by sawtooth generators in which a condenser 215 (225), is charged slowly through a resistance constituted by a tube 216 (226) and discharged abruptly through a gas-filled tube 221 (231), the voltage of the condenser controlling the plate current of an amplifier. A portion of the output signal is made, with the aid of circuits similar to those shown in Fig. 1, to produce a voltage across a resistance 36 which depends on the rate of change and higher order derivatives of the signal voltage and is used to modify the scanning speed by altering the bias on tubes 216, 226 and hence the charging rate of condensers 215, 225. It is arranged nevertheless that the total time per frame is constant since the tube 231 can only discharge to cause the vertical fly-back under control of a constant frequency generator 240 which also discharges tube 221 of the horizontal scanning generator. The impulse from 240 is also sent over a separate channel to the receiving point, Fig. 8, at which the incoming signal voltage, as in previously-described arrangements, produces a voltage in accordance with its rate of change and higher order derivatives. This voltage, which appears across resistance 136, governs the scanning speeds of a cathode beam reproducer. The scanning of each frame however begins at a time determined by the synchronizing impulse from the transmitter. The beam intensity is modulated in accordance with the scanning-speed by means of an additional grid 262 connected to resistance 136 to ensure that the picture brightness shall not be affected by changes in scanning speed.