GB1519607A - Digital control of a loudspeaking telephone system - Google Patents

Abstract

1519607 Transmission systems NORTHERN TELECOM Ltd 5 Dec 1975 [12 Dec 1974] 50016/75 Heading H4R A loudspeaking telephone system has a transmit and a receive channel, means for generating a control voltage dependent on comparative signal levels in the channels and two identical, digitally controlled attenuators, one in each channel, one attenuator responding to the digital representation of the control voltage and the other to its complement so that the total attenuation and hence the total gain in the loop is constant. In the embodiment, Fig. 1, A/D converter 30 converts the analogue control voltage VC obtained from the generator G by comparison of the signal levels of the microphone, transmit and receive signals S M , S T and S R respectively, into a binary signal which directly controls the attenuator 10 in the transmit channel and through a complement generator 50 the attenuator 20 in the receive channel. The A/D converter 30 is a tracking type converter i.e. it monitors its output and it adjusts the output in accordance with the input. In the embodiment, Fig. 3, the control voltage VC added with the complement of the output, converted by D/A converter 38 is monitored by comparators 32, 33 which through a logic circuit 40 control an up/down binary counter 37 whose output is the digital representation of the input Vc. Comparator 32 steps up the counter 37 when the converted output is less than the input Vc and comparator 33 steps down the counter when the converted output is more than Vc. The output of counter 37 controls, directly, the attenuator 10 (R11-R20) through FETs Tl to T5. The complement generator 50 (inverters 51-55) controls the attenuator 20, which is identical to attenuator 10, through FETs not shown. The timing of the A/D converter is controlled by a clock signal generated by a clock generator (Fig. 4, not shown). An end limit decoder 36 prevents the counter 37 from exceeding its limits (all '0's or all '1's) to prevent a "jump" in the attenuation of attenuators 10 and 20.