GB1502792A - Analogue to digital converter - Google Patents

Abstract

1502792 Analogue-to-digital conversion COMMUNICATIONS SATELLITE CORP 16 May 1975 [17 May 1974] 20920/75 Heading H3H An input voltage signal having a predetermined range of variation is digitized by a quantizer 40 covering only a part of the range, an offset being added to or subtracted from the input voltage when it lies below or above the range of the quantizer, the offset being removed when the input voltage returns to a value well within the range of the quantizer. For a T.V. signal, which lies within 25-75% of its maximum value for 95% of the time, the range of the quantizer is 25-75% and the offset is plus or minus 25% as appropriate and is removed when the T.V. signal returns to at least 40% or drops back to no more than 60% of its maximum value. The quantizer 40 may comprise eight threshold circuits in parallel (Fig. 3, not shown). In Fig. 2, under- and over-range signals are fed to flip-flops 52, 36 respectively for adding or subtracting offset V x from the input voltage, sampled and held at 16. Encoder 42 produces a binary output code to which compensatory subtraction or addition of the binary equivalent of V x is performed at 48. Decoders 44, 46 detect return to values of 35% and 65% (# input voltages of 60% and 40%) for removing the offset. The offset is also automatically removed at the start of each scanned line of a T.V. picture by the T.V. synch pulses LS. In another arrangement, Fig. 4 (not shown) application and removal of the offset is controlled by analogue comparators (92, 94, 102, 104) responsive to 75%, 60%, 25% and 40% input voltages respectively. A delay (90) allows the offset switching induced transients at the summing point (30) to subside before the input signal is sampled.