GB1340327A - Coding arrangements - Google Patents

Abstract

1340327 Secret communication systems PLESSEY CO Ltd 19 April 1971 [14 Feb 1970] 7205/70 Heading H4R A radio or line communication system includes an encoder 2 and decoder 4 each including a storage arrangement comprising in the encoder a magnetic drum 12 which has the message written on to it by a single writing head 10 and read out by a plurality of reading heads 14a-14g. The output of the reading heads is sampled by switches 20a-20g which are closed by converter 36 in an order determined by code generator 34. A pulse generator 26 operates at a frequency of 2500 Hz which frequency is divided by 125 to give pulses every 50 ms. for closing switches 20a-20g to sample 50 ms. portions of the input signal. A further frequency division provides pulses every 400 ms. used to charge the code in code generator 34, control the drum speed in control 32 and modulate the 2500 Hz frequency at 38 so that the modulated waveform can be transmitted. The output of code generator 34 is modulated by a 1 KHz signal to provide a code synchronizing signal which is transmitted when switch 44 is closed. The decoder 4 operates in a complementary manner with the pulse generator 128 being kept in step by comparison with the 400 ms. pulses detected on the 2500 Hz carrier and the use of the correct code is ensured by operation of switches 44, 144 before transmission commences. The magnetic drum may be replaced by a magnetic disc or tape loop.