Patent GB-715130-A (GB)
Title: Electric pulse code modulation communication system
IPC: H03M1/00
Filed: 1951-09-24
Abstract: 715,130. Pulse code modulation systems: valve circuits. COMPAGNIE FRANCAISE THOMSONHOUSTON. Sept. 24, 1951 [Sept. 22, 1950; Sept. 22, 1950; Jan. 26, 1951 ; July 31, 1951 Aug. 3, 1951], No. 22288/51. Classes 40 (5) and 40 (6) In a pulse code modulation system the coder is preceded by an instantaneous amplitude compression device and the decoder is followed by a complementary decompression device, the supply voltage to the latter being controlled in accordance with that of the compression device. In the embodiment described, channel signals from the multiplex distributer 1, Fig. 4, are compressed in circuit 3, sampled in circuit 4, which feeds the coder 6 the output from which is applied to the radio transmitter 8. The synchronizing signal generator 2 controls the distributer 1, the sampling circuit 4 and a reference signal generator 5 for the coder 6 and the instantaneous decoder 7 associated therewith. At the receiver pulses from the amplifier 9 are fed to the synchronizing signal separator 10 which controls the decoder 11 and multiplex distributer 15. The decoded channel signals are fed through the decompression circuit 13 to the distributer. The value of the supply voltage of the compression circuit at the transmitter is signalled to the receiver by amplitude or position modulated pulses controlling the charge or discharge of a condenser providing the supply voltage for the decompression circuit. Compression and decompression circuits.- A symmetrical compression circuit comprises two parallel amplifier valves T1, T2, Fig. 11, with a control valve T3 in their common cathode circuit to maintain constant the sum of their anode currents. The input signal is applied to the grid E of valve T1 and the output taken from either of the anodes VS, VS1. The anode resistances R4, R5 are associated with a potential divider Pl ... P5 connected between H.T. positive and earth and tappings of which are connected through diodes to the anodes of valves T1, T2. Thus as the input signal to either valve increases in amplitude, the diodes conduct successively to decrease the anode load impedance and reduce the gain. For achieving exact symmetry, potential dividers R<SP>1</SP>1, R'2 and R<SP>1</SP>3 are provided to adjust the control grid biases of valves T1, T2, T3 respectively. A similar decompression circuit is described in which the successive change of impedance takes place in the cathode circuits of the amplifier valves, Fig. 13 (not shown). Coding circuits.-The binary coders described are of the type in which the exponentiallydiminishing steps of a reference waveform are compared with the signal amplitude diminished at each comparison by the value of the signal already coded and provided by an instantaneous decoder. The preferred form of reference signal generator comprises a source 2, Fig. 6b, shown twice for convenience, producing positive-pulses at digit frequency and at code group recurrence frequency. The latter render valve V4 conducting through transformer T to earth the upper electrode of condenser C2 at the commencement of each coding operation. The digit frequency pulses are applied to condenser Cl to produce a standard charge which is shared with condenser C2 when the trailing edge of the pulse causes diode V3 to conduct. The voltage across condenser C2 there or constitutes the tapped reference voltage which is derived from terminal E. Due to the self-capacity C3 of diode V3 the digit frequency pulses produce distortion of the step waveform by direct charging of condenser C2 and valve V6 is provided to neutralize this effect by feeding equal and opposite charges to condenser C2 through a variable condenser C5. The stray capacity C4 from the grid of valve V4 to earth would prevent complete discharge of condenser C2 by the valve but this effect is neutralized by coupling a third winding of transformer T to earth through a variable condenser C6. A comparison circuit for producing code pulses comprises a valve VI, Fig. 9a, to which the reference waveform at terminal E is applied, a valve V2 to which the signal to be coded less the output of the instantaneous decoder at terminal A is applied and a transformer T to which highfrequency digit pulses from source 2 are applied The cathodes of valve V1, V2 are connected respectively to the cathode and anode of a diode V3 the former connection including the secondary of transformer T. Only when the signal at A is less negative than the reference signal E does the diode V3 conduct to let a digit pulse pass to the output conductor S connected to the diode anode. The pulses thus passed form the binary code group. In a modified comparison circuit the triode valves to which the signals to be compared are applied have a common cathode impedance formed by a constant-current pentode which is rendered conducting by d.c. digit pulses, the code pulses being taken from the anode of the second triode, Fig. 9b (not shown). Decoding circuits.-In one of the instantaneous decoding circuits described, the decoded signal appears on conductor A, Fig. 8a, being the voltage to which condenser Cl is charged. Just before a decoding operation, condenser C1 is charged to a predetermined voltage by rendering the valve V1 conducting by a pulse applied to its grid. The code pulses are applied to valve V3 so that the charge on condenser C1 is shared with that on condenser C3 at these instants. The latter is discharged before decoding commences by rendering valve V4 conducting by a pulse on its grid. Thereafter it is charged to the values of the steps of the reference signal at T derived from conductor E through the cathode-follower V5. Other decoding circuits operating in a similar manner are described, Figs. 8b and 10 (not shown).