Idle-channel noise in p.c.m. sound-signal systems

This paper describes a theoretical investigation of idle-channel noise in p.c.m. sound-signal systems and presents the results of measurements made on a high-quality (13 bits per sample linear) p.c.m. soundsignal distribution system. The amplitude probability distribution and power-density spectrum of idle-channel noise are theoretically derived, including the complication introduced when dither is used. It is found that the statistics of idlechannel noise, with or without dither, are not Gaussian and show a strong dependence on the statistics of the noise present at the coder input. The spectrum of idle-channel noise is expected from theory to be essentially flat for equipment both with and without dither, but in practice quantised hum components weight the spectrum. The results of the measurements show satisfactory agreement with those predicted from theory. It is concluded that, of the various types of measurement made, the results given by a quasi-peak-reading meter preceded by a noise-weighting network give the best assessment of the subjective annoyance caused by idlechannel noise.