PCM (Pulse-Code Modulation) is the raw, uncompressed representation of digital audio: a stream of sample values, one per channel per tick of the sample clock. It is what an ADC produces and what a DAC consumes, the common denominator every codec encodes from and decodes back to. WAV and AIFF files are mostly just PCM with a header; professional IP links (AES67, SMPTE ST 2110-30) and SDI embed PCM directly. Its size is simple to predict — sample rate times bit depth times channels — so 48 kHz, 24-bit stereo is about 2.3 Mbit/s, which is why delivery almost always compresses it.
