Digital Coaxial RCA Cable: An RCA Plug Carrying S/PDIF
Quick answer / TL;DR: Short answer: use a 75 ohm cable with RCA plugs for digital coaxial audio, and keep the run reasonably short. A composite video cable is electrically suitable because it is also 75 ohm, while a generic analog audio lead may work but is not specified for the impedance. Digital coaxial and optical TOSLINK carry the same S/PDIF data, so choose on convenience and on run length rather than on sound quality. Coaxial generally tolerates longer runs than plastic optical fibre.
Three coaxial families: BNC for CCTV, F-type for TV, N for RF – each with its impedance.Digital coaxial audio uses an RCA connector to carry an S/PDIF bitstream over copper. It looks identical to an analog RCA cable, which causes persistent confusion and a great deal of unnecessary spending.
The signal is a high-frequency digital stream rather than a low-level analog voltage, so the requirements differ: impedance matters, and the cable should be 75 ohm rather than the arbitrary impedance of a generic audio lead.
Why Impedance Matters Here
The S/PDIF stream has significant high-frequency content, and impedance discontinuities cause reflections that corrupt the timing of the bitstream. A 75 ohm cable with 75 ohm connectors avoids that.
| Cable type | Impedance | Suitable for digital coaxial | Note |
|---|
| Digital coaxial, 75 ohm | 75 ohm | yes | the correct specification | Composite video cable | 75 ohm | yes | electrically equivalent | Generic analog RCA | unspecified | often works | not specified for the purpose | TOSLINK optical | not applicable | carries the same data | different medium |
In practice a short generic RCA cable frequently works, which is why the requirement is widely ignored. The failures appear on longer runs or with marginal sources, as dropouts rather than as audible distortion.
Coaxial or Optical
Both carry the same S/PDIF data, so the choice is practical rather than sonic.
- Coaxial: tolerates longer runs, no optical attenuation, but creates a ground connection
- Optical: breaks ground loops, but plastic fibre limits length to about 10 m
- Coaxial is more robust to bending and to handling
- Neither carries lossless or object-based formats
- Choose optical where hum has been a problem
Real-World Applications
Television to an older AV receiver
The receiver has coaxial and optical inputs and the television offers one of them. Either works for compressed 5.1, and coaxial is the more tolerant of the two over distance.
Soundbar with a coaxial input
A short 75 ohm cable is sufficient. A generic analog lead will usually work at this length, but a specified cable removes a variable.
Long run to a second room
Coaxial, because plastic optical fibre attenuates quickly. Keep the run within a few metres and avoid running parallel to mains cable.
Where a ground loop makes optical preferable
If hum appears through the coaxial connection, optical breaks the electrical path and usually removes it. That is the main practical reason to choose optical.
Related reading: Optical against coaxial digital audio — blogs/news/coax-vs-optical-audio
Related reading: Digital coaxial cable selection — blogs/news/coax-cable-digital
Standards and Verification
S/PDIF is specified in IEC 60958, which defines the data format for both the coaxial and optical variants. The 75 ohm impedance requirement comes from the electrical characteristics of the coaxial interface rather than from the RCA connector itself.
Verify by testing with content that carries the formats you use. A cable that passes stereo may fail on a compressed 5.1 stream, and the failure appears as dropouts rather than as distortion.
Check the connector fit at both ends. An RCA plug that sits loosely produces intermittent contact, which is more common than a cable failing electrically and is fixed by replacing the plug rather than the cable.
Label digital and analog RCA cables differently in a system that uses both. They look identical, and connecting a digital output to an analog input produces either silence or noise depending on the equipment.
Frequently Asked Questions
Can I use a normal RCA cable for digital coaxial?
Often at short lengths, but it is not specified for 75 ohm impedance. A proper coaxial or composite video cable is the correct choice.
Is coaxial better than optical?
They carry the same data. Coaxial tolerates longer runs; optical breaks ground loops. Neither is inherently better for sound.
Does digital coaxial carry surround sound?
Yes for compressed formats such as Dolby Digital 5.1. It cannot carry lossless or object-based audio.
How long can a coaxial digital run be?
Practical runs of several metres are fine, and longer than plastic optical fibre tolerates. Keep it away from mains cable.
Why does my coaxial connection drop out?
Usually a cable without proper impedance, a loose plug, or a run that is too long. Substitute a known-good 75 ohm cable to confirm.
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