Patent GB-915315-A (GB)
Title: Improvements in or relating to pulse code modulation systems
Inventor: HARLING DENIS WILLIAM
IPC: H03M1/00
Filed: 1960-05-18
Abstract: 915,315. Pulse code-modulation systems. GENERAL ELECTRIC CO. Ltd. May 15, 1961 [May 18, 1960], No. 17558/60. Class 40 (5). Apparatus for decoding a pulse code-modulation signal of the kind in which groups of pulses define the amplitudes of successive signal samples, comprises means to produce from each group an indication of a time position within a given time interval, means to produce a reference signal whose amplitude varies in a predetermined manner during the time interval, and means to derive an output signal whose amplitude is determined by the amplitude of the reference signal at said time position. The pulse code signal is produced by a modulator, Fig. 1 (not shown), of the type in which signal samples are repeatedly compared with a staircase wave until the amplitude of the staircase wave is less than that of the sample and a chain of bi-stable trigger circuits is controlled to produce a corresponding pulse code output. The remaining difference signal after multiplication is coded in the same way using the same trigger circuits and the two pulse code signals are transmitted in succession, constituting a train of eight pulses. At the receiver, Fig. 2, the incoming pulses are supplied via terminal 24 to gates 25 to 32 which are successively opened by pulses from a generator 33 in synchronism with the eight pulse positions, thus allowing pulses to pass where present at each position. Bi-stable circuits 34 to 40 which initially are reset to normal by a reset pulse via terminal 37, are connected to each gate and supply an output signal on a lead 35 or 36 according to whether a pulse is absent or present at that position. The gates 25 to 28 are associated with the first coarse approximation to the signal and the remaining gates are associated with the second fine approximation. A pulse generator 47 controls a binary chain 43 43 to 46 so that the binary 43 produces eight complete cycles of ON and OFF pulses in the time allotted to the first four pulse positions, the binary 44 four cycles, 45 two cycles and 46 one cycle. The output leads 50, 51 of binary 43 are connected to respective AND gates 48, 49 controlled by the output signal from bistable circuit 34 and the outputs of the AND gates 48, 49 are supplied to an OR gate 52 so that one or other of the outputs from the binary 43 will be passed according to whether or not there is a pulse in the first position. The remaining binaries 44 to 46 are connected and controlled in the same way during the first half of the group of pulses and when this has been done a generator 62 starts a staircase wave of sixteen steps the first step being of maximum amplitude and decreasing by an equal increment at each succeeding step. At one of the sixteen steps the OR gates 52 to 61 will all be supplying signals to an AND gate 63 which supplies a signal to stop the generator 62. The amplitude of the signal produced by the generator 62 at this instant is stored on a capacitor 64 to provide a first approximation of the audiofrequency signal sample. The gates 29 to 32 are associated with bi-stable circuits and a binary chain in a similar manner to gates 25 to 28 and this part of the apparatus operates in the second half of the incoming pulse group to control a further staircase generator 65 to provide an output signal having an amplitude approximating to that of the multiplied difference signal at the transmitter. This signal is then divided by sixteen at 66 before being combined with the first approximation signal at 64. The charge on the capacitor 64 is supplied to the appropriate channel gate, e.g. 67, controlled by gating pulses via 68, and the resulting series of amplitude modulated pulses are supplied via a low-pass filter to the output terminal 70. Specification 915,314 is referred to.