Patent GB-682233-A (GB)
Title: Improvements in or relating to electrical circuits for the production of subjective auditory effects
IPC: H03G5/20
Filed: 1947-03-21
Abstract: 682,233. Transmission systems. STANDARD TELEPHONES & CABLES, Ltd. March 21, 1947 [Oct. 8, 1941], No. 7819/47. Class 40 (iv). In a circuit of the kind described in Specification 649,709 in which a non-linear device is used to distort the frequencies transmitted within a narrow band so as to give the listener the impression of a wideband transmission, the non-linear device is automatically controlled so as to reduce distortion during transients. In the arrangement shown in Fig. 8, the valve T1 is normally biased to act as a linear device. On receipt of a signal through a filter F, a class " C " amplifier T2 applies a positive bias to T1 which then operates on a non-linear basis. A delay circuit R4, R5, C4 defers the change-over from linear to non-linear operation for the initial transient portion of the signal and permits a rapid return to linear operation at the commencement of a decay transient. In another arrangement, Fig. 9, the valve T1 is biased so as to operate on a non-linear basis only for signals above a critical amplitude. A voltage derived from the output of a pentode T3, amplified at T4 and rectified by a double diode T5 is applied to the second control grid of T3 to limit the output of T3 to an amplitude very little greater than the amplitude required to operate T1 on a non-linear basis. In the arrangement shown in Fig. 10, frequencies outside the range to be transmitted are selected by a tuned circuit TC amplified at A and applied to the second control grid of pentode T13. The valve T1 is biased so as to operate as a class " A " amplifier for signals below a critical amplitude. Non-linear distortion is introduced only when the frequencies selected by TC raise the output of T13 above the critical amplitude for T1. In another arrangement, Fig. 11 (not shown), the amplitude of the input to T1 is raised above the critical amplitude only when the output of T13 is raised by regeneration brought about by feedback from the plate of T13 to the second control grid over a tuned phase shifting network. Two distorting valves T11, T12 (Fig. 12) may be arranged in push-pull. The bias of each valve is automatically controlled by a circuit comprising an amplifier T9, a double-diode T10 and a delay circuit R14, R15, C10. Valve T11 is biased by a battery EC4 and normally operates as a linear amplifier. When a signal arrives, T10 operates and builds up a voltage across R15 until, after a delay, the bias on T11 is destroyed and the valve operates on a non-linear bases. In a similar manner, T12 which is normally operated on a non-linear basis changes over to linear operation on the arrival of a signal. The circuits are so adjusted that both valves T11, T12 operate together on a linear basis for a short interval after the onset of the signal. An amplifier PA, a filter FX0 and a loud-speaker LS are used to simulate the narrow band transmission characteristics of the apparatus to be used in practice so that the circuit can be adjusted by experiment to give the most satisfactory results.