Quick answer: A coaxial digital cord carries SPDIF audio as a 75 ohm electrical signal on an RCA plug. The cable must be specified as 75 ohm digital coaxial, not a generic audio RCA. Keep runs under 10 m, use a shield bonded to the shell at both ends, and expect dropouts rather than audible distortion when the cable is marginal.
Digital audio either locks or it does not, so a bad coaxial cord produces silence, clicks or a receiver that hunts for a signal. The cause is almost always impedance and shielding, two things a photo cannot show.
Why 75 ohm is not marketing
Impedance and reflections
The SPDIF signal is a square wave with fast edges. A cable whose characteristic impedance differs from 75 ohm reflects part of each edge back to the source. At one metre the reflection arrives late enough to be harmless; at ten metres it lands inside the bit window and the receiver loses lock.
Shield and return path
The shield is the return conductor, not just protection from interference. It has to be bonded to the RCA shell at both ends. A cable with a floating shield or a single-ended drain wire picks up hum and develops a ground offset that shows as jitter.
Specification and length
| Cable | Impedance | Practical length | Result |
|---|
| 75 ohm digital coaxial | 75 ohm | 10 m | Stable lock |
| Generic audio RCA | Unspecified | 1 m | Dropouts at length |
| Video RCA | 75 ohm nominal | 2 to 3 m | Usually works |
| BNC 75 ohm | 75 ohm | 30 m | Pro installs |
Connectors and adapters
RCA versus BNC
RCA is the consumer standard and relies on friction. BNC locks and holds a true 75 ohm geometry, which is why broadcast racks use it. A passive RCA to BNC adapter is fine because the signal stays electrical and the impedance is preserved.
Avoid 3.5 mm adapters
Adapters that convert a coaxial cord to a 3.5 mm jack change the wiring and usually the impedance. They are intended for analogue audio and will not pass SPDIF reliably.
Diagnosing a dropout
Swap the cord first, then the input. If the receiver locks on a short cable and fails on the installed one, the cable is the cause. If both fail, the source may be set to a format the interface cannot carry, such as multichannel PCM, and the receiver mutes rather than downmixing.
Terminations and repair
Plug quality
An RCA plug for digital audio needs a tight centre pin and a shell that makes firm contact. Loose shells produce intermittent ground, which sounds like clicks rather than hum. Gold plating helps in humid rooms but does not improve a joint that is already loose. When a plug wobbles in the socket, replace the cord rather than bending the shell inward to grip harder.
Field repair
A coaxial digital cord can be re-terminated with a screw-on RCA plug in an emergency. The impedance will not be exact and the shield contact will be poorer than a crimped joint, so treat it as a temporary measure. A soldered plug with heat shrink is more durable, and a crimped plug with a proper die is the closest to a factory termination.
Keep the original cord with the device when equipment is moved. Digital coaxial cords are often the first item lost in a house move, and the replacement bought locally is usually a generic audio cable with the wrong impedance. A labelled bag inside the equipment box prevents the problem entirely.
If the cord has to run past a power strip, cross it at a right angle rather than running parallel, which reduces the hum picked up by the shield on an unbalanced digital link.
Practical scenarios
- Streamer to DAC: a 1 m 75 ohm cord removes the conversion stage from the television.
- Rack to amplifier: a BNC run with RCA adapters at the consumer end keeps impedance intact.
- Second room: keep the coaxial run under 10 m or convert to optical.
- Desktop system: a short cord with a moulded boot avoids strain at the DAC.
FAQ
Will a coaxial digital cord improve sound quality?
It will not change the bits. A correct cable prevents dropouts and jitter, which is what makes the difference audible when a cheap cord is replaced.
How long can a coaxial digital cord be?
About 10 m with a proper 75 ohm cable. Beyond that, use BNC or convert to optical.
Does direction matter?
Not electrically. Directional markings on audio cables are a marketing convention for this interface.
Next step
Connector and cable types are explained in coaxial audio cables, digital cord construction in digital audio cord, and splitting options in coaxial cable splitter.
See the matching cables and adapters
