Improvements in or relating to electrical signal compression networks

1,145,048. Pulse modulation circuits. STANDARD TELEPHONES & CABLES Ltd. 5 July, 1966, No. 30069/66. Heading H4L. A gating circuit comprises at least two transistors connected in series opposition with common emitter and base connections, the input to the gate being applied to the collector of one of the transistors and the output being taken from the collector of the other transistor via a capacitance and to earth potential via a resistance, and means for applying a non- linear gating signal to the transistors, the arrangement of the gate being such that the input or output currents thereof cannot exceed the sum of the gating signal currents in the gating circuits of the transistors. Basic sampling circuit. Fig. 1.-The input signal is supplied to the collector of transistor VT1, whose emitter is connected to the emitter of transistor VT2 and the output signal is taken across resistor R1, sampling pulses being applied via transformer T1 to the bases of the transistors. An input signal positive with respect to the voltage stored on capacitor C1 will cause a current Ic 1 to flow from the input via the path 1 and providing the transistor base currents Ib 1 , Ib 2 are maintained for a sufficient time the charge on capacitor C1 will increase until it is equal to the input voltage. At the same time a voltage is developed across R1 having an amplitude dependent on the rate at which C1 is being charged. The circuit is symmetrical and with a negative input signal a current Ic 2 will flow from C1 to the input path 2. The limiting factor is that 1c 1 or Ic 2 must not exceed Ib 1 + Ib 2 . Amplitude compression.-If the sampling pulses are non-linear so that the currents Ib 1 , Ib 2 are controlled according to, for example an exponential law, then the output voltage across R1 will represent the difference in amplitude between adjacent signal samples compressed by this law, thus providing " slope compression." Modifications.-In Fig. 4 (not shown) a balancing resistor is connected between the two transistors. An RC circuit may be included in the primary or the secondary circuit of transformer T1 to provide the non-linear sampling pulses. The leakage inductance of the transformer also may be arranged to produce the desired law. An arrangement is described, Fig. 8 (not shown), using photo-transistors to replace transistors VT1, VT2 of Fig. 1, the sampling pulses being replaced by a suitable light source.