Patent GB-664401-A (GB)
Title: Improvements in thermionic valve circuits
Inventor: OXFORD ALAN JOHN HENRY
IPC: H03M1/00
Filed: 1948-03-25
Abstract: 664,401. Pulse code modulation systems; pulse circuits. NATIONAL RESEARCH DEVELOPMENT CORPORATION. March 8, 1949 [March 25, 1948], No. 8842/48. Classes 40 (v) and 40 (vi). In apparatus for driving a code signal, e.g. a pulse code modulation system, a five-unit code signal representing a given potential, e.g. a sample potential of a speech or other complex wave, is obtained by comparing the sample with a reference potential, the comparison being such that if the sample potential differs from the reference potential, a first code pulse is, or is not, generated depending on whether the sample is greater or less respectively than the references and the sample is reduced or increased correspondingly in geometric progression with a ratio of one half, the reference potential being compared successively with the modified sample potential. Alternatively the sample potential may be fixed and the reference potential varied. Speech signals from a microphone 1 (Fig. 1), are applied through a logarithmic amplifier 2 to a clamp circuit 3 which applies to an integrator 4 a voltage representing the instantaneous amplitude of the speech wave. The voltage from the integrator 4 is applied to a slicer circuit 5 which effects the comparison with the reference potential, a derived pulse being applied through a delay circuit 6 to allow a pulse from a master oscillator 10 to be transmitted to the output terminal 7. A pulse from the delay circuit 6 is also applied to a switch circuit 8 which selects pulses of one form or another from a pulseforming circuit 9 and applies them to the integrator 4 to modify the sample voltage set up therein for further comparison. The master oscillator 10 supplies control pulses (Fig. 3), C, D to the clamp circuit 3 and code pulses (Fig. 3), B for transmission to the delay circuit 6, so that a sample potential is set up in the integrator for every fifth code pulse. The control or secondary pulses (Fig. 3), C, D, are also applied to the pulse forming circuit 9 which includes a ringing circuit having a logarithmic decrement of two and produces the waveform of Fig. 3, E. From this waveform are derived a series of positive and negative going pulses F, G (Fig. 3), which serve to modify the sample voltage set up on a condenser in the integrator circuit 4. Thus, assuming that the sample voltage has an amplitude level of 19.3 in a 32 amplitude level system, and the reference potential has an amplitude level of 16 on the first comparison a coding pulse will be allowed to pass to the output 7. This pulse is also fed into the switching circuit 8 causing the first pulse of the train G (Fig. 3), to reduce the sample voltage by 8 levels to 11.3 (Fig. 3) K. Since this is below the reference level 16 the second code pulse from the master oscillator is suppressed and no pulse is supplied to the switch circuit 8 so that the second pulse of the positive train F (Fig. 3), is fed to the integrator 4 increasing the sample voltage from 11.3 to 15.3. The output pulse will result from the third comparison so that the sample voltage is increased to 17.3. This is above the reference potential so that a pulse is transmitted and the sample voltage reduced to 16.3. The fifth comparison results in a further code pulse being transmitted, after which the next control or secondary pulse from the master oscillator 10 is fed to the clamp circuit and a new sample voltage is obtained. At the receiver, (Fig. 2, not shown), the received code pulses cause positive and negative trains F, G to modify the reference voltage level 16 on a condenser in the manner shown in N (Fig. 3). The circuit for selecting either the positive or negative going trains F, G, is shown at Fig. 5. The wave trains F, G are applied to two diodes 31, 32, and code pulses from the delay circuit 6 are fed to the grid of a pentode 30. The presence or absence of a pulse on the grid of valve 30 causes the anode voltage to be at a high or low voltage to render one or other of the two diodes 31, 32 conductive, then selecting one or other of the wave trains F, G for transmission to the integrating circuit through a valve 33. A resistor R is connected between the cathode of valve 33 and screen grid of valve 30 to compensate for variations in the output level of the wave trains due to variations of the screen voltage of the valve 30, when the valve anode voltage changes.