Patent GB-662611-A (GB)
Title: Improvements in or relating to pulse-code modulation communication systems
IPC: H04B14/06
Filed: 1949-09-30
Abstract: 662,611. Pulse code mediation systems. PHILIPS ELECTRICAL, Ltd. Sept. 30, 1949 [Oct. 4, 1948], No. 25170/49. Class 40 (v). For the transmission of signals by means of pulse code modulation, the signal controls a coder producing pulse groups and across the coder is connected a feed-back circuit including a code demodulator and a signal-frequencies integrating network which produces an approximation signal, the coder being fed with a difference voltage derived from the original signal and the approximation signal. At the receiver a signal-frequencies integrating network is included between the code-demodulator and the reproducer. The signal-frequencies integrating networks comprise circuits of which the output voltage is proportional to the timeintegral of the input voltage for a substantial part or the entire frequency range of the signals to be transmitted. Transmitter.-In one embodiment of radiotelephony transmitter, the output of a microphone 24, Fig. 4, is fed to a sampling circuit 26 through amplifier 25. The sampling circuit comprises two normally non - conducting, oppositely-directed triodes 27, 28 connected in parallel and which are rendered conducting periodically by pulses from transformer 33, 34, 35. These pulses occur at code group recurrence frequency and are supplied by a generator 38 controlled by a master pulse generator 39 operating at pulse recurrence frequency. The positive or negative signal samples are passed by triodes 27, 28 to charge a holding condenser 36, which thus produces a step-wave approximation to the signal wave. As described, the generator 38 operates at one-fifth of the frequency of generator 39, to allow a four-unit pulse code group and a synchronizing pulse to be transmitted for each signal sample. The step wave from condenser 36 is applied to a difference producer 40 to which is also fed through conductor 41 an approximation signal from the demodulator and integrator of the feed-back circuit. The output from the difference producer, taken across the resistance 42, is fed to a known cathode-ray tube coder 45, which produces four-unit code groups, the pulses of which are shaped by the coincidence mixer valve 61, fed with pulses from generator 39, and differentiating network 67, 68, and passed over conductor 69 to the radio transmitter. Synchronizing pulses from the generator 98, operating at half the group recurrence frequency and controlled by generator 38, are also fed to the transmitter over conductor 99 and so phased that each alternate code pulse group is preceded by an extra pulse. The coded pulses from valve 61 are also fed to a decoder 71 which comprises a normally non- conducting valve 72, the anode circuit of which includes a network consisting of resistance 75 shunted by condenser 76. The time constant of this network is such that when valve 72 is cut off, the voltage across condenser 76 decays to half its original value in a time equal to one period of the pulse recurrence frequency. The condenser 76 is discharged after the decoding of each group by triode 77, to the grid of which are applied pulses occurring at group recurrence frequency to render it conducting. The decoded signal output is fed, through cathodefollower 81, to a sampling circuit 85, similar to circuit 26, the output from which is fed to a holding condenser 86. The latter is shunted by a resistance 87, the centre point of which is earthed and which forms the balanced input of the signal frequencies integrating network 88. The integrating condenser 89 is shunted by the primary winding of a transformer 90, this circuit being tuned to a frequency preferably lower than the lowest signal frequency and connected between the anodes of a pair of pushpull connected hexodes 91, 92, biassed to cut-off at their first grids. These grids are supplied with pulses at group recurrence frequency and the second control grids are connected to the ends of resistance 87 so that on the occurrence of said pulses one or other of the hexodes conducts, depending on the polarities of the ends of resistance 87. Condenser 89 thus receives charging and discharging pulses of magnitude dependent upon the voltage across the holding condenser 86, and a step-wave approximation signal is set up thereacross which is fed by transformer secondary 97 to the difference producer 40. In another embodiment, the output of a microphone 100, Fig. 6, is fed to a sampling circuit 102 which provides one of the inputs of a difference producer 103, the other being an approximation signal from the demodulator and signal-frequencies integrating network 110 of the feed-back circuit. The output of the difference producer 103 is fed to the coder 105 which modulates the radio transmitter 106 and provides the input to the demodulator 110 through a pulse widener 109. The coder comprises a cathode-ray switching tube 131, the beam of which is directed to the one or other of the dynodes 139, 140, depending upon the polarity of the difference voltage from circuit 103 applied to the deflecting plates 136. The grid-like anode 138 is supplied with pulses from generator 111 which are repeated at that dynode upon which the beam is directed. The pulses from dynode 140 only, are fed to the radio transmitter 106, the two inputs of the flip-flop valve 150 in the pulse widener 109 being connected to the two dynodes 139, 140. The effect of the applied pulses is to switch over the flip-flop circuit whenever the beam in tube 131 changes from one dynode to the other, and the two outputs are fed to the suppressor grids of the demodulator valves 161, 162 respectively. The control grids of these valves are connected to a time constant circuit 170, 171 which is charged by a pulse from the code group recurrence frequency pulse generator 112, and the voltage across which decays exponentially to half its initial value during each pulse recurrence period. The amplitudes of the charging and discharging pulses fed to the anode condenser 166 are thus such as to demodulate the coded pulse groups and the signal frequencies integrating network 166, 167 feeds its output to the difference producer 103 over conductor 104. Receiver.-At the receiver, a local oscillator 176, Fig. 11, operating at pulse recurrence frequency is synchronized by a thermionic reactance 179 controlled by a discriminator circuit 203 to which are fed the incoming synchronizing pulses over conductor 174 and the oscillator output through condenser 205. The sine wave output of oscillator 176 is clipped in valve 188, differentiated in circuit 192, 193, and the resulting positive pulses are fed to the conductor 180 and the frequency divider 178 which produces pulses at code group recurrence frequency. The signal pulses from radio receiver 173 are widened in the flip-flop circuit 224 and gate the valve 235, to which the locally-generated pulses are applied over conductor 180. The resultant regenerated signal pulses are fed to the decoder 237, similar to decoder 71, Fig. 4, the output from which is fed through sampler 243 to the signal frequencies integrating network 245 followed by the low-pass filter 250 and reproducer 252. The operation of the decoder and sampler is controlled by the code group recurrence frequency pulses from circuit 178.