Patent GB-1046485-A (GB)
Title: Improvements in or relating to the detection of alternating current signals
Inventor: SONES RALPH ARCHIBALD
IPC: F01D1/30
Filed: 1962-05-31
Abstract: 1,046,485. Loud-speaking telephones. H. M. POSTMASTER GENERAL. May 31, 1963 [May 31, 1962], No. 21088/62. Heading H4R. In a circuit for detecting the presence of a first A.C. signal in the presence of a second A.C. signal a D.C. signal is generated corresponding to the envelope of the combined first and second A.C. signals and compared with a D.C. signal corresponding to the envelope of the second A.C. signal to produce a signal corresponding to the difference in instantaneous amplitudes of the D.C. signals. The invention is described in relation to a loud-speaking telephone in which the presence of a received signal on the line is detected in the presence of a high amplitude transmitted signal. Fig. 2 shows a loud-speaking telephone incorporating the detecting circuit SD which detects a signal received over the hybrid on terminal 3 of circuit SD in the presence of the signal generated by microphone 21 which is to be transmitted over the line 27 and is applied to terminal 1 of circuit SD. The detecting circuit SD comprises a unity gain amplifier 2 and two constant volume amplifiers 6 and 8, the input to amplifier 6 comprising only the locally generated transmitter signal while the input to amplifier 8 comprises the locally generated signal plus the signal received over the line. The amplifier output signals are envelope detected in rectifiers 7 and 9 and mixed, in opposition, in a resistive pad 10 so that the signal appearing at terminal 11 is the difference signal corresponding to the envelope of the signal received from the line. Constant volume amplifiers 6 and 8 cause the signal received from the line to be weighted in its effect on the output at 11 in dependence on the level at input 1, the larger the signal at input 1 the smaller the effect of the signal received from the line on the output at 11, and vice versa. In the loud-speaking telephone the signal at output 11 is compared with the signal generated by microphone 21 in the control circuit 31 to differentially operate the variable attenuator pads 24 and 28 in the transmit and receive lines of the set. In addition a gate 34 is provided in the transmit line which is closed by a signal in the receive line, the maximum attenuation of the gate being limited so that the speaker may regain control of the circuit by shouting. Circuits for the control device 31 comprising rectifiers, resistive adders, and transistor triggers are described with reference to Figs. 3 and 4 (not shown). The variable gain amplifiers 6 and 8 may be of the form shown in Fig. 5 in which speech from the input IP is applied via a transformer T1 to a variable shunt resistor comprising transistors VT1 and VT2. The incoming speech signals are tapped off across R1 in the primary circuit of transformer T1, amplified in linear amplifier 2, and rectified in the base emitter junction of transistor VT3, the resulting D.C. being used to control the base-emitter impedances of transistors VT1 and VT2 to tend-to maintain the speech level applied to transformer T2, and hence the output OP, constant. An additional signal volume compressor may be included to feed the terminals IP to increase the constant volume range of the amplifier.