SE306351B - - Google Patents

Abstract

1,114,641. Automatic gain control. INTERNATIONAL STANDARD ELECTRIC CORPORATION. 13 Aug., 1965 [14 Aug., 1964], No. 34726/65. Heading H4R. A continuous control of the gain of go and return channel amplifiers of a two-way transmission system is made in dependence upon the signal level of both channels in a manner which discriminates against noise in a channel. Gain control signals are produced by rectifying a signal representative of a channel energy, then removing the D.C. level from the resulting signal, which renders an alternating signal for further rectification to remove the noise component and leave only speech components. The time constant characteristic of rectifiers making the second rectification is such as to retain the adjusted degree of gain between syllables of a spoken word. In the arrangement shown in Fig. 2 the total quiescent amplification of the amplifiers VF1 and VF2 exceeds the total acoustic attenuation D 1 and D 2 at the terminating stations M 1 -H 2 , M 2 -H 1 . A control voltage U 1 or U 11 comprising a noise and speech signals after rectification at L 1 passes through circuit DF to remove the direct voltage components and through fullwave rectifier L, where the noise component is removed to provide one input U 4 to the summing circuit S 1 or S 2 , which input acts to adjust the gain upwards. The other input U 7 which is dependent upon the signal strength in the other channel is always greater than U 4 and acts to adjust the gain downwards. The sum U 5 or U 6 of these voltages is then fed to the amplifier gain control. Any self-commencing oscillation in the loop reduces the gain of both amplifiers. In another arrangement (Fig. 1, not shown) where the total quiescent amplification is less than the total attenuation, the voltage U 7 is omitted and the voltages U 5 and U6 are the mean of voltages U 4 and U 41 but so oppositely poled as to provide a greater gain in the active channel than the passive one.