PCM (Pulse Code Modulation)


PCM or Pulse Code Modulation has become the de facto standard for digitally encoding analogue signals in both computer software and home theater audio applications.

Linear pulse-code modulation is based on very old mathematical theorem developed by Alec Reeves in the 1930's. PCM's method divides an analogue voltage into regular increments and assigns a binary number (a "pulse code") to each cycle. Using PCM, any analogue waveform can be completely converted into binary code.

PCM rose to the prominence it enjoys today when it was chosen as the audio digitization method for the Compact Disc. It's still the standard in newer technologies like DVD-Audio and the principals of PCM are at work in compression standards like Mpeg and MP3.
CD uses two channels, each sampling 16-bit pulse codes at 44Khz. These numbers were picked because they were capable of a frequency response close to the limits of human hearing and they could still fit a whole album on the limited storage of the CD (about 650MB). PCM was again employed for the development of DVD-Audio. Since DVD can store over 8Gigs, they could push PCM's numbers well beyond the range of human hearing. The pulse code is wider, up to 24bits, and at a much higher frequency of 96Khz. And instead of just two channels DVD-Audio uses 5.1 channels.

Competition between the two high-resolution audio standards - DVD-Audio and SACD - is in a technical sense competition between PCM and Sony's own newer analogue to digital process DSD.