GB1511487A - Loudspeaking telephone instruments - Google Patents

Abstract

1511487 Loudspeaking telephones GENERAL ELECTRIC CO Ltd 23 May 1975 [27 Sept 1974] 42099/74 Heading H4R In a loudspeaking telephone in which signals on the send or receive path are selectively attenuated in accordance with the relative amplitudes of the signals in those paths, as deteremined by a comparator fed with the rectified signals of the respective paths, at least one of the signals fed to the comparator is reduced by an amount proportional to the mean level of the signal in the respective path taken over a period of the order of a second to reduce the effect of ambient noise on the operation of the comparator. As described the send path 1 and receive path 2 each contain a variable attenuator comprising the source drain path of a FET, the impedances of the paths being differentially varied by the output of comparator 16 which compares the signal level in the two paths as determined by rectifier circuits 14, 15 connected to monitor the signals on either side of the attenuator circuits in each path. The rectifier circuits, Fig. 2, comprise operational amplifiers 19 having diodes 21 in their feedback circuits, to give them rectifier characteristics, together with additional diodes 25 which form signal limiters by increasing feedback on high signal levels. A D.C. voltage proportional to signal level appears on capacitor 26 as the signal to feed the comparator. This signal is also fed to the input of a second differential amplifier 29 and, via resistor 34 and capacitor 32 having a time constant of about 5 seconds, the output voltage of amplifier 29 is fed to the second input of differential amplifier 19, to be subtracted from the D.C. voltage resulting from the A.C. signal inputs at 17, 18. The diode 33 is arranged to be reverse biased by a fall in the voltage on capacitor 26, as when a speech signal ends, so as to remove the negative feedback from the amplifier 29, the resulting rapid decrease in voltage at the output of amplifier 29 causing diode 35 to conduct to rapidly discharge capacitor 32 so that the voltage fed to the second input of amplifier 19 rapidly follows decreases in the signal down to the ambient noise level, i.e. is primarily responsive to the ambient noise level. Similar circuits can be used for both rectifiers 14 and 15, Fig. 1, if circuit conditions require.