BBC R&D

Abstract

In a pulse-code modulation (PCM) sound system, the decoding process at the receiving terminal to reconstitute the original signal should ideally be carried out at precisely the same regular rate as the coding process at the sending terminal. However, in a practical system, the timing of the decoding process may be subject to fluctuation - that is, timing errors or timing 'jitter' may occur - and the quality of the reproduced sound may consequently be impaired.

Apparatus has been built to simulate the effect of timing errors in a sound PCM system. Subjective tests have been carried out to enable tolerance limits to be estimated for both sinusoidal and random jitter on high-quality monophonic programmes. Tolerances are proposed for jitter on a practical sound system, based on the results of the present work and other previous investigations. The tolerance varies from a timing jitter amplitude of 3.5 milliseconds (RMS), for sinusoidal variation over a period of 50 seconds or more, to 35 nanoseconds for jitter frequencies of 2 kHz or higher. Comparable figures apply to timing jitter with a random-noise variation.