Patent GB-693134-A (GB)
Title: Improvements in or relating to electric pulse signalling systems
IPC: H04J3/06
Filed: 1951-02-27
Abstract: 693,134. Multiplex pulse code signalling; pulse generating circuits. GENERAL ELECTRIC CO., Ltd., ARBON, R. S., ELSON, D. W., and HUNT, G. J. Feb. 27, 1952 [Feb. 27, 1951], No. 4749/51. Classes 40 (v) and 40 (vi). In a pulse code modulation signalling system, the synchronizing signal consists of a characteristic pattern of digit pulses transmitted periodically in place of the intelligence carrying digit pulses. In the twelve-channel five-unit code system described, the channel recurrence period is divided into sixty pulse time-positions, the pulses being of such duration that adjacent pulses join up. The time positions occur at a frequency of 420 Kc/s so that the channel recurrence frequency is 7 Kc/s. The synchronizing signal occurs once every 1.44 seconds, the intelligence signals being blanked out for a period KN, Fig. 1, of sixty-five pulse time-positions, to allow the synchronizing signal to be inserted. It occupies a period LM of fifty-five pulse timepositions and consists of an elevenfold repetition of two adjacent pulse time-positions having pulses, followed by three positions without pulses. One of the channels carries the intelligence on twenty sub-channels, so that each pulse time-position in four successive groups allocated to this channel corresponds to one of the sub-channels. In connection with this feature a four-level staircase waveform, Fig. 3 (a), having a recurrence frequency of 1750 c/s, is available, and is applied to the terminal 10, point (A), Fig. 2, of the circuit which inserts the synchronizing signal in the channel pulse train. Pulses 27, Fig. 3 (b), and 45, Fig. 3 (c), corresponding respectively to the lower and upper levels of the staircase waveform are derived by diodes 12, 13, and appear at points (B), (C), respectively. Each of these pulses has a duration of sixty pulse time-positions. The pulses 27 at (B) are twice inverted, amplified and applied as negative-going pulses to conductor 17, being limited by diode 15. These pulses are applied through cathode-follower stages 19, 18 to two pairs of frequency dividers 21b, 21c and 21a, 24 respectively, followed by clipping circuits 32, 38 respectively. The four frequency dividers divide by nine, eight, seven and five respectively, the circuit of divider 24 being shown. It comprises a Miller integrator transitron-connected valve 23, which produces the staircase waveform, Fig. 3 (d), at the point (D), having nine steps. The diodes 25, 26 cause the steps to commence at the trailing edge of the input pulses and diodes 28, 29 limit the positive and negative excursions of the anode of valve 23. The staircase waveform at (D) is applied to diode 34, which passes the lowest level to produce pulses 44, Fig. 3 (e), at the point (E) at the divided frequency. These pulses are mixed, in the anode resistance 37 of valve 35, with those similarly produced by the dividers 21a, 21b, 21c and occur at the junction 43 to which are also applied pulses 45, Fig. 3 (c), from the point (C) through stages 39, 41. When pulses from all five sources occur simultaneously, once every 1.44 seconds, the diode 42 passes the pulse 46, Fig. 3 (f), to the point (F), this pulse being of duration sixty pulse time-positions. The normal pulse code signals are applied to terminal 59 and thence through valve 58, normally-conducting diode 79 and stage 83 to the output terminals 84. When the synchronizing signal is to be transmitted the diode 79 is rendered non-conducting to blank out the pulse code signals for the period KN of sixty-five pulse time-positions. For this purpose the pulse from point (F) is fed through switch 47, stages 48, 51, with D.C. restoration by diode 49, and diode 57 to diode 79, to render the latter non-conducting for a period KM of sixty pulse time-positions. A pulse for blocking diode 79 for the last period MN of five pulse time-positions is derived by differentiating the output of stage 51 in circuit 54, 55 and cutting-off valve 53 by the resulting trailing-edge pulse, the anode of valve 53 passing the resulting pulse through diode 56 to the diode 79. The synchronizing signals are supplied to terminal 63 and fed through stage 64 and delay line 65 to the suppressor grid of a gate valve 67 feeding the output stage 83. This gate valve is opened at its control grid for a period LM of fifty-five pulse time-positions, by feeding thereto the pulse from stage 48 through valve 76. The valve 67 however is cut off, for the first five pulse time positions, by a pulse from valve 74 responsive to the leading edge differential pulse from circuit 72, 73 fed from stage 48 through stage 68. The synchronizing signals may alternatively be transmitted at a frequency of 1750 c/s by operating switch 47 to connect stage 48 to the diode 13. At the receiving terminal of the system, a five-level staircase wave controls the decoder for the intelligence signals, the timing of which is determined by the transmitted synchronizing signal. The incoming pulses, Fig. 1, are applied to the terminal 101, Fig. 4, and passed through amplifiers 102, 103, giving positive-going and negative-going pulse signals at points (A), (B) respectively. A supply line 108, voltage stabilized by the neon tube 109, feeds anode voltage to stage 103 and supplies a saw-tooth generator comprising the condenser 113 and resistance 116. Pulses at point (B) are applied to valve 112 to render it non-conducting during which time the condenser 113 charges. The condenser discharges at the end of a pulse when valve 112 conducts again and the voltage developed across the condenser is thus proportional to the number of consecutive received pulses. This voltage is arranged to have a maximum value equal to that due to three such pulses. It is fed through stage 115 to stage 117 arranged to produce a pulse, initiated by the trailing edge of the triangular input pulse, and of duration proportional to its amplitude and 50 per cent longer than the received pulse. Due to the limitation of the value of the triangular pulsel the output pulses of stage 117 at point (C) wil, therefore be limited to a maximum duration of 4.5 pulse time positions. Stage 118 produces an inverted version of these pulses at point (D). Waveforms (a), (b), Fig. 5, are of a typical portion of pulse signals developed at points (A), (B) and the corresponding outputs of stages 117, 118 at points (C), (D) are given as waveforms (c), (d), Fig. 5. Coincidence valves 119, 123 are supplied from points (A), (C), and (B), (D), respectively, and produce the mixture of pulses 121, 122, 125, graph (e), Fig. 5, at the point (E), pulse 125 deriving from valve 123 and pulses 121, 122 from valve 119. When the synchronizing pulses are received, waveforms (a), (b), (c), (d), Fig. 6, are present at points (A), (B), (C), (D), respectively and no pulses, graph (e), are produced by valves 119, 123, at point (E). The circuit is adjusted to this condition by appropriately setting condenser 120 in stage 117. Pulses occurring at point (E) after clipping by diodes 128, 129 are fed through valve 125 to a saw-tooth generator 126, 127, 130 similar to 112, 113, 116 to produce at point (F), the waveform (f), Fig. 5, for a normal signal, waveform (a), and the waveform (f), Fig. 6, for a synchronizing signal. The voltage across resistance 130 is applied to diode 132 which does not conduct until condenser 127 has charged for a period of approximately forty pulse time - positions. Thereafter, the resulting clipped triangular waveform is passed through stages 134, 135 to the terminal 136. The sharp trailing edge of this output is used to synchronize the fivelevel staircase decoding waveform generator, being applied to terminal 166, Fig. 7. The generator comprises a condenser 153, which is charged in steps during the positive half waves of a sine-wave 420 Kc/s oscillation applied to valve 152 from terminals 151. The blocking oscillator 155 triggers to discharge condenser 153 when the condenser voltage exceeds its cathode voltage and diode 159 prevents the output terminal 161 from going negative. The negative-going triangular synchronizing pulse from terminal 166 is applied through stage 162 and diode 163 to reduce the voltage at point 161 below the lowest staircase level until the sharp trailing edge occurs. The staircase generator thereafter commences-to operate normally again, diode 163 becoming non-conducting.