PCM Digital Audio

Canonical Definition

Pulse-code modulation represents audio as a sequence of numerical samples for one or more channels at a stated sample rate and word length. PCM describes the sample values and channel ordering; it does not by itself define the cable, connector, clock topology, file container, compression format, or analog performance.

Keep These Layers Separate

Layer	Examples	Question it answers
Audio representation	PCM samples, encoded AC-3 or DTS payload	What do the bits mean?
Channel format	mono, stereo, 5.1, 7.1, TDM slot map	Which sample belongs to which channel?
Transport	S/PDIF, USB Audio, HDMI, I2S, TDM	How are data and timing moved?
Physical layer	coax, optical, differential pair, PCB traces	What electrical or optical connection carries it?
Control	I2C, SPI, USB HID, GPIO	How is the hardware configured?

A Complete PCM Description

At minimum, record:

sample rate
nominal word length and serial slot width
signed numeric format and justification
channel count and channel order
interleaving, data-lane, or TDM-slot map
full-scale convention and mute or silence code

A label such as “24-bit digital audio” is incomplete without those details and does not guarantee 24 useful bits of analog dynamic range.

PCM Versus Encoded Surround

Two-channel PCM can usually be handed from a digital receiver to a PCM DSP directly after format and clock matching. A compressed multichannel payload is different: a stream detector and compatible decoder must first produce separate PCM channels.

S/PDIF encoded 5.1
→ digital interface receiver
→ non-PCM detector
→ Dolby / DTS-compatible decoder
→ six decoded PCM channels
→ I2S / TDM / multiple lanes / DACs
→ DSP processing

Verification

Mute downstream hardware.
Send one channel at a time.
Confirm sample rate, word format, and channel identity.
Test silence and low-level signals before full scale.
Verify whether the source is PCM, downmixed PCM, or an encoded payload.