For many organizations involved in the production of DAISY books, the starting point of the digital audio recording process is an analog audio signal coming through a microphone, a tape reproducer or any other analog source. These numbers, zeros and ones, are the actual digital audio file.
Virtually all digital audio systems, including CD, DAT, etc., use PCM.
The PCM encoding process is based on a series of measures - or "samples" - of the amplitude of the analog signal. The rate at which these measurements are taken, and the number of bits used to describe each measurement, are two central concepts in digital audio.