S/PDIF And Multichannel Audio

Canonical Definition

S/PDIF is the consumer form of the IEC 60958 digital-audio interface and carries a framed biphase stream over supported electrical or optical physical layers. It commonly transports two-channel PCM or non-PCM encoded bursts. A compressed multichannel payload must be detected and decoded before a conventional PCM DSP can process its individual channels.

Keep Three Layers Separate

Layer	Examples
Physical connection	coaxial electrical or optical TOSLINK
Framed interface	IEC 60958 / S/PDIF
Payload	stereo PCM or an encoded surround burst

Coaxial and optical links can carry the same logical S/PDIF stream. The connector alone does not reveal whether the payload is PCM or encoded surround.

Receiver And Decoder Are Different

S/PDIF input
→ digital interface receiver and clock recovery
→ PCM or non-PCM detection
→ compatible surround decoder when needed
→ discrete PCM channels
→ I2S, TDM, multiple lanes, or DACs
→ DSP processing

A digital-interface receiver can recover the framed stream and output serial data. It does not necessarily decode a compressed surround format. Decoding is a separate, format-specific stage.

What “5.1 Over Coax” Means

In common consumer S/PDIF systems, 5.1 content is often carried as a supported compressed payload rather than six independent PCM channels. The source must provide the intended output mode, the receiver must identify the payload, and a compatible decoder must produce the six PCM channels.

When the source is set to PCM, the connection may provide a stereo downmix instead. Verify the actual detected stream rather than relying only on a source-menu label.

ADAU1701 Architecture

The ADAU1701 accepts decoded serial PCM through its I2S/TDM-capable serial input. It is not a direct S/PDIF biphase receiver and is not a general surround decoder. A multichannel chain therefore needs a receiver, payload detection, decoding when required, a multichannel PCM bus, DSP channel mapping, converters, and outputs.

Verification

keep outputs muted until PCM or non-PCM status is known
verify receiver and decoder lock through source changes
test one decoded channel at a time
record channel order, sample rate, word length, clocks, downmix, and bass-management settings
route encoded payloads only to a compatible decoder or bypass path