SE316796B - - Google Patents

Abstract

1,132,274. Data transmission systems. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 17 March, 1966 [20 March, 1965], No. 11697/66. Heading H4P. In a single sideband digital data transmission system the transmitter comprises a network having a transmission characteristic equivalent to that of a subtraction circuit to which a signal is fed both direct and via a delay. In a transmitter, Fig. 1 (not shown), binary pulses are converted to differential form (3, 4) and pass to the network (5) referred to above, the delay in this instance being one bit. After filtering (6), the resultant 3-level signals are amplitude modulated (7) in a suppressed carrier modulator, and one of the sidebands is passed to the transmission line. A pilot carrier is added to the signal from the modulator. At the receiver, Fig. 3 (not shown), the pilot is used in the demodulation (14), the 3-level signal being full-wave rectified (20) to recover binary data identical to that at the transmitter input. D.C. level variations are countered by subtracting (21) from the binary signal a D.C. signal, derived from the demodulator by filtering (22), and passing the difference signal through a threshold circuit (24). The resulting signal is sampled (25) for regeneration of the binary data. Level control of the received signals may be achieved by selecting from the demodulator output a component at twice the carrier frequency which is then rectified and used to control an attenuator preceding the modulator. In a modification, Fig. 8 (not shown), the demodulated three-level signal is sampled at bit rate, the samples passing to positive and negative threshold circuits (50, 51) which feed a bi-stable circuit (52). The signal from this circuit, which is in differential form, passes to a modulo 2 adder (55) both direct and via a 1-bit delay (54), the adder output being the required binary data. In a further embodiment, Figs. 10 and 11 (neither shown), generally similar to that described with reference to Figs. 1 and 3, a square wave at half bit-rate is added to the output of the subtractor (58), and at the receiver this signal is extracted by a filter (73), full wave rectified (75) and passed to a tuned filter (75) which develops a sine wave at bit frequency. This is clipped (76) and differentiated (77) to provide bit-rate sampling pulses.