Sound is captured as a sequence of numbers taken at a steady rate. Each number uses a fixed count of bits, and nothing in that sequence is removed by a perceptual codec.
The linear form of Pulse Code Modulation (PCM) means the steps between amplitude levels are equal. A step near silence represents the same change in level as a step near the loudest peak.
How Equal Steps Work
- Sample clock: Values are taken at even intervals, so the time from one sample to the next does not change with loudness.
- Fixed width: Each sample is a whole number with a set count of bits, and a quiet sample occupies the same space as a loud one.
- Equal levels: The step from one stored number to the next stands for the same change in amplitude from the bottom of the range to the top.
- No perceptual codec: A hearing model is not used to drop detail. The numbers that were sampled are the numbers that are stored.
Compact Disc and Video Discs
On a compact disc the stream is 16-bit stereo at 44.1 kilohertz. That is two channels, with 44,100 samples each second and 16 bits in each sample. Video discs commonly keep the same kind of uncompressed audio beside the picture, so the samples stay intact instead of passing through a perceptual codec.
What the Compact Disc Numbers Mean
| Figure | Value |
| Sample rate | 44.1 kilohertz (44,100 samples per second on each channel) |
| Bit depth | 16 bits for every sample |
| Channels | Stereo, with a left channel and a right channel |
| Amplitude steps | 65,536 equal steps, because 16 bits can count that many levels |
| Raw bit rate | 1,411,200 bits per second, from 16 bits times 44,100 samples times 2 channels |