Digital Coax: S/PDIF on Copper
Quick answer / TL;DR: Short answer: digital coax carries PCM stereo and compressed 5.1, not the lossless formats HDMI supports. Choose it over optical where the cable must be robust or the run long; choose optical where hum or ground loops are present. Both need the source set to the format the receiver decodes.
Three coaxial families: BNC for CCTV, F-type for TV, N for RF – each with its impedance.Digital coax is S/PDIF carried on a 75-ohm coaxial cable with RCA plugs. It sends the same bitstream as optical TOSLINK and differs only in the physical layer – which is why the two sound identical when both work, and why the choice is about environment rather than quality.
This is the digital-coax guide: what the interface carries, the format ceiling, and the practical differences from optical.
What It Carries
| The format | Supported | The note |
|---|
| PCM stereo | Yes | Always |
| Compressed 5.1 | Yes | Most implementations |
| Lossless | No | HDMI only |
| Sample rates | Up to 96 kHz typically | Per device |
The ceiling is the reason HDMI displaced coax for home cinema while coax remains useful for stereo.
Against Optical
- Coax is robust and tolerates handling
- Optical is immune to electrical interference
- Coax can carry ground loops; optical cannot
- Neither is inherently better sounding
The choice follows the installation: noisy electrical environments favour optical.
Standards and Verification
| The reference | The relevance |
|---|
| IEC 60958 | S/PDIF |
| 75-ohm impedance | The cable requirement |
| Field test | Receiver lock and format display |
Per-device format support is not provided in the knowledge base. The interface limits are standard; check the receiver's display for the locked format.
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 – a setting, not a cable fault.
Real-World Applications
The stereo system
A streamer into an integrated amplifier: digital coax at 75 ohms – a clean link with no analogue stage between the source and the DAC.
The hum
The same link humming: a ground loop between earthed devices. Optical substituted – the electrical path removed.
Related reading: digital coaxial cable — blogs/news/digital-coaxial-cable
Related reading: coax vs optical audio — blogs/news/coax-vs-optical-audio
Frequently Asked Questions
Is digital coax better than optical?
Neither is inherently better; the environment decides.
Does it carry surround?
Compressed 5.1 usually; not lossless formats.
What cable?
75-ohm coaxial with RCA plugs.
Why does it hum?
A ground loop between devices.
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