Patent GB-652212-A (GB)
Title: Volume control for pulse code modulation communication systems
IPC: H04B14/04
Filed: 1949-04-29
Abstract: 652,212. Pulse code modulation systems. STANDARD TELEPHONES & CABLES, Ltd. April 29, 1949, No. 11463. Convention date, June 19, 1948. [Class 40 (v)] In a pulse code modulation transmitter using groups of pulses representing quantized signal amplitudes on a binary code, means are provided for inspecting the distribution of pulses in each transmitted group and for adjusting the signal level according to the results of the inspections so that the quantized amplitude varies over substantially the whole of the amplitude range for which provision is made. The inverse process is used at the receiver to compensate the level of the recovered signal wave for the signal level adjustments at the transmitter. In a transmitter using the simple addition binary code a source 1, Fig. 1, of pulses modulated in amplitude by the signal, feeds a binary pulse coder 3 through an amplifier 2. The code pulses for the coder 3 are provided by a base pulse generator 5 which also generates other control pulses. In addition to being fed to the transmitter the code groups a, Fig. 3, are fed to a gating circuit 4 which selects the pulses in the first two code positions under control of the gating pulses b from generator 5. The selected pulses c are fed to a scale-of-two flipflop circuit 6, which is switched to the operated condition by one pulse and back to normal by a second pulse. Restoring pulses f from generator 5 ensure that circuit 6 is always in the unoperated condition just prior to the occurrence of each gating pulse b. Sampling pulses e from generator 5 enable the gating circuit 8 to supply pulses g to an integrating circuit 7 which depend for their polarity on the condition of circuit 6 after the pulses in the first two code positions of each code group have been applied to it. In the examples shown, the code groups representing amplitudes 21 and 15, lying in the amplitude range 8 to 24, each have one pulse only present in the first two positions, so that the flip-flop circuit 6 is in the operated condition when its output is sampled. Negative pulses g are therefore supplied to circuit 7. The code groups 26 and 7, lying outside the amplitude range 8 to 24, have both and neither respectively of the possible pulses in the first two positions. In these cases, circuit 6 is in the unoperated condition when its output is sampled so that positive pulses are supplied to circuit 7. The integrated pulses g produce a control voltage for amplifier 2 of such a nature that its gain is reduced if the signal amplitude lies outside the range 8 to 24 and increased if it is within this range. At the receiver, Fig. 5, the pulse code groups from the receiver 11 are passed to the decoder 12, the signal output of which is fed through amplifier 13 to a suitable translator. A base pulse generator 18, gating circuit 14, scale-of-two circuit 15, gating circuit 16 and integrator circuit 17 produce a control voltage from the pulse code groups, as at the opposite sense and is applied to the amplifier 13 to restore the output signal to its original form. If the cyclic permutation code described in Specification 649,376 is used a simple transmitting and receiving arrangement in which the presence or absence of a pulse in the second code position is used as the controlling factor, Figs. 2 and 4 (not shown).