GB1251967A - - Google Patents

Abstract

1,251,967. A/D and D/A converters. BRITISH BROADCASTING CORP. 31 Jan., 1969 [31 Jan., 1968], No. 5046/68. Heading G4H. To convert an analogue signal to digital form a sawtooth signal is compared with the analogue signal, clock pulses from a main clock 14 (Fig. 1) being accumulated in a multiples counter 15 until coincidence is detected when a vernier clock 16 is enabled delivering pulses of a slightly different frequency, these pulses being accumulated in a units counter 18 until coincidence is detected between pulses of both clocks. The vernier clock may comprise an oscillator delivering a signal sin #t which is phase shifted by 90 degrees in a quadrature network (31, Fig. 3, not shown) the signal from which is fed to a modulating function generator (32), the original signal being fed to a similar generator 30. The trigger signal from the comparator is also fed to the generators 30, 32 to hold their outputs at their value at the time t 0 of the triggering signal so that from modulators (29, 33), signals sin #t cos #t are fed to a subtractor until time t 0 , the signals thereafter being sin #t cos #t 0 and cos #t sin #t 0 respectively to give an output sin #(t - t 0 ). To convert a two digit decimal number to an analogue signal one digit is entered in units store and counter 37 (Fig. 4) the other digit being entered in a multiples store. The multiples counter is set to 9 and counted down at a first frequency, a sawtooth generator being started at equality of the counter and store. When the multiples counter and units store are at the same value the units counter is counted down to zero but at a second frequency slightly different from the first, the sawtooth generator being stopped when the units counter reaches zero and its value sampled at that time to give the analogue output signal.