Patent GB-653043-A (GB)
Title: Improvements in or relating to electric pulse code modulation systems of communication
Inventor: EAGLESFIELD CHARLES CECIL
IPC: H03M7/08
Filed: 1948-10-22
Abstract: 653,043. Multiplex pulse code signalling ; pulse circuits. STANDARD TELEPHONES & CABLES, Ltd. Oct. 22, 1948, No. 27452. [Classes 40 (v) and 40 (vi)] In a pulse code modulation system the signal amplitude is compared with a specified amplitude scale having a finite number of steps and a pulse or pulse group is generated in response to a signal amplitude change from one scale step to another, the pulse or pulse group indicating whether the change is an increase or decrease of amplitude according to a specified code. When the principle is applied to a multichannel system, each channel produces pulses or pulse groups which indicate both the identity of the channel and the sign of the amplitude change. The transmitter of a single channel system may comprise a cathode-ray tube 1, Fig. 1, having a stepped target plate 7, Fig. 2. The signal applied to the vertical deflection plate 3 determines, at any instant, which step of the target the beam is swept across by the oscillator 8 feeding the horizontal deflection plate 5. By this means, the signal is converted into a stepped wave in loadresistance 13 which is differentiated by circuit 15, 16 to produce at the output terminal 17, short positive pulses when the amplitude rises from one step to the next and short negative pulses when the amplitude falls. A demodulator which reproduces the signal wave from these pulses comprises an integrating condenser 22, Fig. 3, to which the pulses are applied through biassed oppositely-directed diodes 23, 24 which pass the negative and positive pulses respectively but otherwise isolute the condenser. The condenser 22 is shunted by a high resistance 28 and the step wave voltage produced across them is converted to the original signal wave by a low-pass filter 29. If it is not desired to transmit pulses of both signs they may be converted to pulses of the same sign but of different amplitudes or durations, or pairs of pulses with different spacing &c. A logarithmic or other amplitude scale may be used, by suitable variation in the height of the steps in the target plate 7, Figs. 1 and 2, and the deflection plates 5, 6 omitted, the beam being in the form of a thin horizontal blade wide enough to cover plate 7 at its maximum width. Multiplex transmitter.-A four-channel system is described in which the equipment for each channel is identical, that for two of the channels A, B being shown in Fig. 4. The signal from input terminal 32 is fed to a stepwave generator and differentiator 33, as in Fig. 1, the positive and negative output pulses being separated in the discriminator 34 and passed on separately over conductors 35, 36 as positive pulses in each case. Each output is fed to a " ringer " circuit such as 37 which upon receipt of a pulse commences to generate a train of pulses, each of which is capable of operating an acceptor circuit 38 for transmitting to line a code group of pulses indicating that an increase in signal amplitude has occurred in channel A. The acceptor circuit 38 cannot be operated if any other acceptor circuit in the system is operated, all the acceptors being inter-connected over conductor 58. When, however, it responds, it sends a pulse over conductor 39 to stop the operation of ringer 37, and a pulse to the coder 40 which generates the appropriate pulse code group for transmission to the line 41. This code group consists of a fixed first pulse and the individual presence or absence of three others, and indicates the identity of the channel and the sign of the amplitude change. The first pulse is a " start " pulse, necessary as the groups are transmitted at irregular times. The number of pulses in each group necessary to transmit 2n channels is thus n+2. The pulse discriminator 34 comprises two cathode follower output valves 58A, 59, Fig. 5, biassed to cut-off and the grid of valve 59 is coupled directly to the input terminal 63 so that a positive input pulse is repeated at output terminal 35. A negative input pulse cannot operate valve 59 but operates valve 58A through reversing valve 65 to produce a positive output pulse at terminal 36. The ringer and acceptor circuits 37, 38, Fig. 4, are shown in Fig. 6. The ringer consists of a closed chain of three similar flipflop multivibrators which trigger each other to produce a succession of pulses when once started and until stopped by the acceptor circuit. The starting pulse from the discriminator is applied to the grid of the lefthand triode 68 of the first multivibrator which triggers the second 72, which in turn triggers the third 73 which then triggers the first again at the grid of the right-hand triode 69, if the coupling valve 77 is conducting. The acceptor circuit consists of a similar flip-flop multivibrator 82, 83. The cathode of valve 82 is connected, through terminal 85, to the similar point on all the other acceptor circuits in the system so that if any one of them has been triggered and not returned to normal its lefthand valve is conducting and all the others are therefore disabled by a positive potential applied to their corresponding cathodes. When however, valve 82 is triggered by a positive pulse from the anode of valve 69 of the ringer, valve 82 sends a negative pulse to cut-off the coupling valve 77 at its suppressor grid to shut down the ringer, and valve 83 sends a pulse to the output terminal 89, this pulse being a short positive pulse derived from a differentiating and clipping circuit 86, 87; 88. The multivibrators 72, 73 in the ringer circuit may be replaced by a passive delay network. The output pulse from the acceptor circuit is passed to a shaper 93, Fig. 7, and then converted into the desired code group by passage to the line 103 through isolating diodes 99 ... 102 and tappings 96 ... 98 on delay line 94. The first pulse of the group is undelayed and is fed through diode 99. The others are timed by the delays introduced by the line 94 and if any pulse is to be omitted the corresponding connection to the tapping point and the corresponding diode are omitted. In another embodiment, a single coder replaces the eight codes fed by the eight acceptor circuits, Fig. 8 (not shown). Multiplex receiver.-The incoming pulse code groups at terminal 117, Fig. 9, are applied to the pulse separator 118 which may be a multivibrator triggered by the first pulse of a group and insensitive thereafter for a period sufficient to prevent triggering by a subsequent pulse of the same group. The single pulse from circuit 118 produces a succession of three gating pulses from tappings 121 ... 123 of a delay network 119 occurring respectively when the other three code pulses occur, and a subsequent activating pulse from tapping 124. The gate circuits 125 ... 127 are fed with all the code pulses, and the individual gating pulses, to separate the three code pulses which are passed respectively to the shaper circuits 128 ... 130. These latter lengthen the code pulses so that each overlaps the activating pulse from tapping 124. The first code pulse, if present, operates the two-way electronic switch 131 from its lower to its upper position. The second pulse operates the two switches 132, 133 and the third pulse operates the four switches 134 ... 137 so that the subsequent activating pulse is routed through the switches to the correct one of the channel demodulators 138 ... 141 and to its amplitude rising or amplitude falling input circuit as the case may be. The activating pulse is negative and so the amplitude rising input circuit of each demodulator contains a pulse inverter 142...145, respectively. The demodulators are similar to that of Fig. 3, but each diode has a separate input circuit. The gating circuits are pentode valves with the gating pulse applied to the suppressor grid, Fig. 10 (not shown). The electronic switches each comprise a pentode 149, Fig. 11, to the suppressor grid of which is applied the positive switching pulse. In the absence of a switching pulse the suppressor grid bias cuts-off current to the anode and the negative input pulse at terminal 153 is inverted in triode 154, applied to the control grid of the pentode and a negative output pulse occurs at the screen grid which is coupled to terminal 151. When the switching (lengthened code) pulse is applied to the suppressor grid, the input pulse from terminal 153 produces an output pulse at terminal 152 coupled to the anode. The small residual pulse at the screen grid is cut-off by the biassedrectifier 155 which is normally conducting but is blocked by a large negative pulse. The code pulses may be controlled by a master periodic generator so that a series of equally spaced instants are fixed at which a code pulse might occur, the first pulse of a group being transmitted at the predetermined time next following the corresponding amplitude change. At the receiver a synchronized generator may then be arranged periodically to unblock the receiver when code pulses are due to arrive and thus minimize the effect of interfering pulses.