Patent GB-974721-A (GB)
Title: Pulse code modulation coder
IPC: H03M1/12
Filed: 1963-06-07
Abstract: 974,721. Pulse code modulation systems. STANDARD TELEPHONES & CABLES Ltd. June 7, 1963 [June 8, 1962], No. 22758/63. Heading H4L. In a p.c.m. coder means are provided for producing a given group of pulses during the absence of information signals, for activating the coder during predetermined repetitive periods to check the production of the given code group, and to detect and compensate for any drift in the output of the coder relative to the given code group. As shown in Fig 1, time division multiplex amplitude samples from sources 4 are supplied via a multiplexer 3 to the coder 1 which is arranged to supply a code group of pulses representing the particular amplitude coded together with a residual voltage representing the difference between the quantized amplitude level represented by the code group and the actual amplitude of the sample. The p.c.m. signal is supplied via INHIBIT gate 15 and OR gate 16 for transmission and is also converted to parallel form at 8 and supplied to a matrix 7 which produces output signals indicating the pattern of the code group of pulses as these are generated by the coder when it is activated by a reference bias pulse from source 13. The code pattern information from matrix 7 is coupled to AND gates 9, 10, 11 which are actuated by timing pulse source 12 during the period of the reference pulse. If the code pattern is correct a signal is supplied via gate 9 to bias source 13 which operates to produce the given code group of pulses when no information is supplied from any of the sources 4. If, due to drift, the coder 1 produces a code group representing an amplitude greater than the reference amplitude, this is detected by matrix 7 and a signal is supplied via gate 10 to reduce the bias from 13. Similarly, if the reference pulse causes coder 1 to produce a code group representing an amplitude less than the reference value a signal is supplied via gate 11 to adjust the bias from source 13 accordingly' In this way the matrix 7 causes a coarse correction of the reference bias voltage, the residual voltage from the coder providing a fine control via gate 14. The INHIBIT gate 15 removes the code group representing the reference pulse from the outgoing signal and permits the substitution via OR gate 16 of synchronizing or supervisory signals from source 17. In an alternative arrangement, Fig. 2 (not shown), a 6-digit coder which can code sixty four different amplitude levels is stabilized at level thirty one. Levels thirty two to sixty three have a " 1 " as the most significant bit so that when a " 1 " appears in the pulse group generated during the reference pulse period the coder has drifted upwards and the reference bias is reduced by reducing a charge on a capacitor so that the next code group generated during the next reference pulse duration is brought down by one level and all other outputs are down one level. Similarly, logic circuits detect when code pulse groups representing amplitudes one to thirty are produced during the reference pulse period and increase the charge on the capacitor enough to produce a one level increase in all outputs of the coder. The residual output of the coder is compared with a reference voltage and used to obtain a fine bias control of the coder.