S/PDIF Cable: One Standard, Two Cables
Quick answer / TL;DR: Short answer: S/PDIF over coaxial needs a 75-ohm cable, not an audio lead; over optical it needs a TOSLINK fibre cable. Both carry PCM stereo and, in the coaxial and optical consumer forms, compressed 5.1 – but not the lossless formats that HDMI carries. Match the cable to the socket and set the source to the format the receiver can decode.
S/PDIF is the consumer digital audio interface, carried either on coaxial cable with RCA plugs or on optical fibre with TOSLINK connectors. The protocol is the same; the physical layer is not, and each has a requirement that buyers miss.
This is the S/PDIF guide: the two physical versions, the 75-ohm rule for coaxial, and the formats the interface carries.
The Two Physical Versions
| The version | The connector | The requirement |
|---|---|---|
| Coaxial | RCA | 75-ohm cable |
| Optical | TOSLINK | Fibre, gentle bends |
| Protocol | Identical | The same bitstream |
The 75-ohm rule is the one that is routinely ignored, and the one that produces dropouts at longer lengths.
The Formats
- PCM stereo: always
- Compressed 5.1: usually
- Lossless and object-based: no – those need HDMI
- Sample rates: up to 96 kHz on most consumer implementations
The format ceiling is the reason HDMI replaced S/PDIF for home cinema, while S/PDIF remains useful for stereo.
Standards and Verification
| The reference | The relevance |
|---|---|
| IEC 60958 | The S/PDIF interface |
| IEC 61169 | Coaxial connectors |
| Field test | Receiver locking to the stream |
Per-device support is not provided in the knowledge base. The interface limits are standard; check the receiver's display for the format it has locked.
Practical Notes
The receiver's front panel usually reports the incoming format. If it shows PCM when a film is playing, the source is set to stereo output – a setting, not a cable problem.
Real-World Applications
The stereo system
A streamer to a stereo amplifier: coaxial S/PDIF at 75 ohms – a clean digital link with no analogue stage in between.
The audio lead substitution
An ordinary RCA audio lead used for S/PDIF over 6 m: intermittent lock. A 75-ohm cable fitted – the impedance had been the issue.
Related reading: Coaxial against optical — blogs/news/coax-vs-optical-audio
Related reading: Optical to analogue conversion — blogs/news/optical-to-rca-converter
Frequently Asked Questions
What is S/PDIF?
A consumer digital audio interface, carried on coaxial or optical cable.
Does it need a special cable?
Coaxial needs 75 ohms; optical needs a TOSLINK fibre cable.
Does it carry surround?
Compressed 5.1 usually; not lossless formats.
Why does the receiver show PCM?
The source is set to stereo output.
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