Patent GB-264928-A (GB)
Title: Improvements in or relating to signal transmission systems and more particularly to picture transmission systems
IPC: H04N1/00
Filed: 1925-10-28
Abstract: 264,928. Western Electric Co., Ltd., (Bell Telephone Laboratories, Inc.). Oct. 28, 1925. Copying-telegraphs.-In order to obtain a constant overall transmission equivalent, particularly in a picture-transmission system, variations of alternation in the line due to temperature &c. changes are compensated by causing currents transmitted over the line during non-signalling periods to control the gain at a receiving amplifier so that if, for example, attenuation increases, the amplification factor is correspondingly increased and vice versa. In a modification, the compensation in a picture-transmission system is continuous and is effected by a carrier current of a frequency different from that employed for carrying the picture modulations. In the system shown, modulated carrier waves transmitted over the line L are amplified by thermionic devices 12, 14 and then control a light valve 15 to record the picture on a revolving drum 17. The drum is driven in synchronism with a corresponding drum at the transmitting end, both drums having corresponding non-sensitive portions 23. During the period termed the " underlap period," when the portions are opposite the light-sensitive devices the carrier wave is immodulated and is used for controlling the gain of the amplifier 14, this control being effected by a rectifying valve 32 the grid circuit of which is periodically connected by a cam-controlled switch 26 to the incoming line. At the commencement of operations the motors are started up, unmodulated carrier current is sent out, and switch 65 is momentarily closed, operating relay 66 which disconnects the recording device 15 and connects battery 72 to the grid of amplifier 14. This battery causes.the amplifier to act as a detector and the rectified current operates relay 69 which holds open the circuit of relay 46. Switch 76 is then closed and necessary adjustments are made. When ready to commence the transmission of a picture, the carrier wave is momentarily interrupted, causing relays 66, 69 to release. Relay 46 now operates and. at its armature 47 causes the driving gear 21 to be gripped between friction discs. 48, 50 and thus to drive the shaft 45: At the same time, armature 54 which has a fork co-operating with a groove in a loose collar 24 movesthis collar from its adjustment position in engagement with the disc 50 into engagement with a fixed collar 44, a spring pin 42 engaging with a hole 43 ensuring the correct relation between the cam 25 and the underlap strip 23. With relay 66 released, the tube 14 functions as an amplifier having a normal grid potential supplied by a battery 85 which however is opposed by potential from battery 36 through resistance 37. The point 35 is connected to the rectifier 32 by means of condensers 33, 34 and inductance 90 arranged as shown, the effect of which is that the potential of the point 35 remains substantially constant as long as the amplitude of the unmodulated current passing to the grid 31 during the underlap periods remains constant. If, however, the amplitude changes, the potential of point 35 and, consequently, the potential of grid 86 are correspondingly varied. As an alternative arrangement, the rectifier 32 might control the plate potential or filament current of the amplifier 14. In order to prevent condensers 33, 34 from assuming an abnormal charge if the carrier wave is interrupted, a manual switch may be provided to disconnect point 35 from resistance 37 and connect it to a separate battery and potentiometer, this switch being automatically restored when the carrier current is resumed. In the modified system in which the gain of the amplifier is continuously controlled, this is effected by current of a different carrier frequency the signalling and controlling currents being separated at the receiving end by means of filter circuits. The controlling current may serve also for synchronizing the sending and receiving motors. Specification 242,694 is referred to.