DEVICE FOR THE TRASMISSION OF RECTANGULAR SYNCHRONOUS INFORMATION PULSES

1278693 Digital transmission systems PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 4 July 1969 [9 July 1968] 33770/69 Heading H4P In order to transmit digital data within a certain frequency band the data modulates a rectangular carrier wave having a frequency nb/2, where b is the bit frequency and n is small, and the modulated signal is applied to a circuit, comprising a digital filter, which adjusts the sprectrum of the signal in the frequency band to avoid the effects of unwanted modulation products. In various embodiments, Figs. 6-13 (not shown), the data is read from a source under the control of bit-rate clock pulses and passes to a gating circuit fed also by a carrier wave of frequency 3b/2. The output of the gating circuit feeds the register of the digital filter, the register being stepped by shift pulses derived from a source common to the source clock pulses and to the carrier. In one embodiment, Fig. 6 (not shown), the gating circuit is fed by antiphase carriers and delivers a phase shift keyed square wave. In Fig. 7 (not shown), the circuit consists of a modulo-2 adder. In this embodiment the output of the digital filter is combined with carrier frequency pulses, the transmitted signal then being an on-off keyed signal. In Fig. 9 (not shown) the circuit consists only of an AND gate. This gate feeds the filter and its output is also combined with the filter output to provide an on-off keyed signal. In Fig. 12 (not shown), the circuit is fed with an additional carrier of frequency b/2 and provides two on-off keyed different frequency signals to respective digital filters whose outputs, when combined, give a frequency shift signal. In Fig. 13 (not shown) a frequency shift signal is obtained using only one digital filter. An arrangement developing a 4-phase differential phase shift signal, Fig. 15 (not shown), comprises a diode matrix fed direct and via a 1-bit delay with the digital data from the source. The matrix has two outputs feeding bi-stables via AND gates. The bi-stables are connected to modulo-2 adders fed by 90 degree carriers and also to the input of the matrix. The adders feed respective digital filters whose outputs are combined to give the required phase shift signal. Each phase represents a pair of bits.