Application of p.c.m. to broadcast video signals. Part 2: Subjective study of digit errors and timing jitter

This part of the paper concerning p.c.m. video signals describes two series of subjective tests performed to examine picture impairments caused by unwanted components which may be introduced into system I {PAL, 625-line} video signals while they are encoded in p.c.m. form. The first series of testsexamined the subjective effects of digital transmission errors. Both random singal-digital errors and burst errors were examined. The tests included an investigation of a system for concealing errors based on the use of parity bits. For random errors it was found that error concealment reduced the maximum tolerable error rate from 1 in 107 to 1 in 104. The second series of tests examined the subjective effects of timing jitter in the subjective effects of timing jitter in the analog samples of video signals obtained from a p.c.m. digital-to-analog convertor. Certain alignment errors in the PAL decoder used in conjuction with the display monitor can considerably increase the imparement caused by timing jitter. One of the most citical adjustments in the PAL decoder is the alignment of the static phase of the reference colour subcarrier. The results of the tests indicate that the amplitude of random jitter should not exceed about 0.3 ns r.m.s. assuming that PAL decoders are aligned to within specified tolerances which allow for long term drifts.