Digital silence

Overview

The term "digital silence" is used to describe a digital audio signal in which all bits containing voltage level information are "zeroes;" and therefore represents complete silence.

History

Digital silence is directly related to CD technology because it is part of the commercial format Red Book standard. In order to assure that there were no noises when the CD started or ended playback; there must be complete silence at the beginning and end of the CD audio file. This was to allow for the use of muting relays in the audio output circuitry of the CD player to prevent undesired noise from being output. If there is any signal present when the relay contacts open or close, a “pop” or “thump” could occur in the audio output. Even a relatively low level encoded DC offset could cause a pop; so the simple way to address this was to specify that digital silence be present when the relay contacts would open or close. DC offset was actually a common problem in early converter designs; so it was not unusual for early digital recordings to have at least some low level DC encoded with the audio.

Basics

Although analog audio circuits can be designed to be extremely low-noise and therefore have no perceivable noise present unless amplified to an usually high level; there is always some measurable noise present in analog circuitry. In digital audio; it is possible to have "complete silence" by setting every bit in the digital audio word representing a voltage level to 0. When reproduced by a DA converter; digital silence will cease to be absolute because of real-world limitations caused by noise in the analog circuitry at its output.

PCM digital audio is typically encoded using the Two's Complement system. There are a number of advantages to this approach, which is beyond the scope of this discussion. This system uses all but one of the bits in a digital audio word to represent the voltage level of the encoded audio of either the positive or negative portion of the signal waveform. The other bit represents the “sign” which determines if the voltage was positive or negative at the point in time the audio was sampled.

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