Patent GB-1044979-A (GB)
Title: Two-way speech amplifier
IPC: H04M9/00
Filed: 1963-07-23
Abstract: 1,044,979. Self conditioning two way speech amplifiers. GYLLING & CO. A.B. July 23, 1963 [July 24, 1962], No. 29121/63. Heading H4R. In a two-way speech amplifier having separate amplifiers for each direction of transmission and in which each amplifier channel has a moderate amplification during silence and a control device is provided responsive to signals in the two channels to increase the amplification of one channel upon detection of a signal in that channel and to reduce the amplification of the other channel, at least two variable gain stages are provided in each channel, one. of the stages being located before, and the other after, the point at which signals are taken out to the control device. Fig. 3 shows the preferred embodiment in which two speech amplifier channels A and B each comprise a microphone, M1 or M2, feeding an amplifier #F whose output is connected to a variable gain stage Fp the output from which feeds an attenuator, comprising series resistors. RS1 and RS2 and a variable shunt resistor DR, and a second variable gain stage Fs which feeds a loud-speaker, H2 or H1, via a loud-speaker amplifier #S. The speech signals in each channel are monitored at points PA and PB and the signals amplified in circuits STA and STB, rectified, and compared to provide the control voltages VSA and VSB, VSA controlling the gain of amplifiers Fp and Fs in channel A and the loss of attenuator DR in channel B while VSB controls amplifiers Fp and Fs in channel B and the attenuator DR in channel A. Optimum values of gain and gain and attenuation variation are derived mathematically in the Specification. Variable gain amplifiers and attenuator, Fig. 4. The output from the microphone amplifier #F, Fig. 3, feeds over conductor 100, Fig. 4, to the base of a transistor amplifier TP, this amplifier is provided with negative feedback over the unbypassed emitter load RE1, the gain of this stage being controlled by varying the base bias and therefore the emitter collector impedance of transistor TPE1 which is connected to shunt the emitter load of transistor TP. The output from the collector of transistor TP is applied via a potentiometer comprising the internal impedance of the transistor TP, resistor RS2 and resistor RP shunted by the collector emitter impedance of transistor TD to the base of a second transistor amplifier TS, whose gain is controlled in a similar manner to that described in respect of transistor TP. The potentiometer which includes the shunt resistance RP forms an attenuator the loss of which is controlled by a voltage applied over line 103 to control the emitter collector impedance of transistor TD. The point PA at which the signal level in the channel is monitored is between the two variable gain amplifiers TP and TS and in addition the signal level is influenced by the loss in the attenuator which includes the transistor TD since some attenuation will occur at point PA due to the internal impedance of the transistor TP. The way in which the control voltages for the variable gain amplifiers and attenuators is derived by rectification and comparison of the voltages at the monitoring points PA and PB is described with reference to Fig. 5 (not shown).