Noise which is present only in conjunction with a signal is called modulation noise. In analog tape recorders, the recording process has a certain granularity due to the fact that the magnetic characteristics of the tape are not completely uniform as the magnetic domains are of finite size. A recorded signal has an irregularity which sounds like the addition of noise. In digital audio systems, there is also an uncertainty in the level of the signal because of quantization error in the A/D converter. This uncertainty also sounds like added noise and is not present if the signal is not present. Compare with distortion.
See Barkhausen effect, granularity.
