A sliding-scale direct-feedback PCM coder for television

A sliding-scale coder for television signals was built which extends the range of the quantizing scale by processing the input signal twice when the input signal exceeds a prescribed threshold. On the second pass the quantizing range is effectively moved outward to reduce the errors in coding large signals. Double processing nearly triples the number of quantizing levels of a basic three-bit coder. Measurements of the number of extra bits required, that is, those in excess of three-bits per sample show that they may be accomodated on a three-bit per sample transmission channel by reducing the sampling rate five percent. The experimental coder generates 19 quantizing levels. Its performance approaches that of a seven-bit pulse code modulation coder. Busyness or streaking, common to most three-bit differential type coders, is eliminated. Acceptable pictures are reproduced with ±5 dB changes in the input signal's range. Over this range the signal-to-noise ratio of the reproduced pictures varies from 47 dB to 54 dB and the rise-time of a regenerated step-signal varies from 1 microsecond to 1.45 microsecond when the input signals rise-time is limited to 1 microsecond.