Patent GB-983337-A (GB)
Title: Improvements in or relating to transmission systems for the transmission of pulse signals
IPC: H04L5/12
Filed: 1963-02-15
Abstract: 983,337. Telegraphy; multiplex transmission systems. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. Feb. 15, 1963 [Feb. 19, 1962], No. 6247/63. Headings H4P and H4R. In a pulse transmission system the transmitter comprises two channels each including a modulator, the modulators being fed with carrier oscillations phase displaced by 90 degrees. At least one of the channels includes a D.C. suppression network, and the outputs of the two channels, together with a pilot carrier, are sent to the receiver. The receiver also comprises two channels, the channel corresponding to the (or each) suppressed D.C. channel including a pulse generator having a low-pass feedback loop. A first embodiment, Fig. 1, shows a transmitter having separate input pulse sources 2, 3 each supplying data at 2250 Baud. The two transmitter channels comprise a low-pass filter 15, 16 having cut-off at 1350 c/s., a D.C. suppressor 17, 18 having a cut-off of 50 c/s. and an amplitude modulator 7, 8. The modulators are supplied with a carrier at 1850 c/s. via phase shifters 9, 10 giving + 45 degrees and - 45 degrees phase shift, respectively. The waveforms at the outputs of units 2, 15, 17 are, for example, as Fig. 3, a, b, d, respectively. The resultant signals on the two channels are sent, together with a pilot carrier, to the receiver, and occupy 2.7 kc/s. bandwidth. Due to the D.C. suppression the pilot carrier is distinct from the sidebands by  50 c/s. and is not influenced in amplitude or phase by the modulating pulses. At the receiver, Figs. 2 and 4, the signals pass to two channels and are demodulated at 27, 28, the demodulators being fed with a carrier from oscillator 31 by way of phase shifters 29, 30. A unit 37 for correcting the frequency of the oscillator is controlled by a voltage derived from the received waveforms: low-pass filters 34, 35 have a cut-off 0.1 c/s. and so present a voltage varying with the phase difference of oscillator 31 and the pilot frequency. The outputs of 34, 35 are differenced at 36 and control unit 37. To control the level of the signals sent to the two channels the D.C. voltage at the output of filter 34-which varies with the amplitude of the pilot carrieris fed to level control unit 25. The demodulated pulses trigger a bi-stable pulse generator 41, 42 whose output feeds a low-pass filter 43, 44 in a feedback loop. The filter provides an output D.C. varying with the D.C. component of the received pulses, the output being added at 47, 48 to the demodulated signals. The output of unit 47, 48 corresponds to that of unit 15, 16 at the transmitter, and unit 41, 42 squares the pulses for utilization at 45, 46. A circuit 49, 50 ensures that the pulse generator 41 is operated correctly even after a prolonged line interruption. A second embodiment, Fig. 8, shows a transmitter fed by only one pulse source 78. Since each channel in the transmitter can accommodate 2250 Baud pulse source 78 may work at 4500 Baud. The pulses are fed direct to a differentiator 81 and via an inverter to differentiator 85, negative pulses being suppressed at 82, 86, and the resultant pulse train triggering bi-stable unit 83, 87. The outputs of 83, 87 are at 2250 Baud, and the remainder of the transmitter corresponds to that of Fig. 1. At the receiver one channel includes an arrangement for deriving pulses corresponding to the leading edges of the data pulses, and the other channel one for deriving the trailing edge pulses. These pulses trigger a bi-stable unit to reproduce the original data pulses at 4500 Baud. Other arrangements are described for use with pulse code modulated signals or synchronous telegraph signals, Figs. 14, 15 (not shown), or Figs. 18, 19 (not shown), and for the combined transmission from a data pulse and a known channel (in which case an extra pilot signal is transmitted), Figs. 12, 13 (not shown).