Quick answer: A coaxial digital cord carries S/PDIF on a 75 ohm cable terminated with RCA or BNC. Using a generic audio RCA cable, which is often not 75 ohm, causes reflections that can make a receiver drop lock. Runs of a few metres are forgiving, and BNC holds up better than RCA over distance.
Two cables look identical, both have RCA plugs, and only one carries a digital signal without trouble. The difference is characteristic impedance and shielding, neither of which is visible. Buyers who substitute a spare audio cable usually get away with it at one metre and then blame the DAC when a three metre run clicks and drops.
What makes it a digital cord
S/PDIF over coax sends a square wave at roughly 0.5 V to 1 V peak to peak, with a bit rate that depends on the sample rate and channel count. The cable must present a consistent 75 ohm impedance from plug to plug, and it must be shielded well enough to keep the clock edges clean. A cable with the wrong impedance produces reflections at each end, and those reflections land on the receiver as timing uncertainty.
Analog audio cable is often built with a different impedance and looser shielding, because analog line level signals tolerate it. Nothing marks the difference on the jacket beyond a part number, which is why the specification matters more than the appearance.
RCA against BNC
| Attribute | RCA | BNC | Practical effect |
|---|
| Impedance control | Variable in cheap plugs | 65 and 75 ohm types exist | BNC is more consistent |
| Return loss | Worse at high frequency | Better with correct type | Fewer reflections |
| Retention | Friction fit | Bayonet lock | BNC stays put in racks |
| Consumer support | Universal | Pro and broadcast gear | Adapters usually needed |
For a consumer receiver and a one metre run, RCA is entirely adequate. For a rack with long runs and many devices, BNC removes a variable. Use a 75 ohm BNC, not the 50 ohm type that dominates test equipment, because mixing the two introduces a mismatch at the connector.
Length, sample rate and lock
Why short runs hide problems
Reflections take time to travel to the end of the cable and back. On a one metre cord the echo arrives well after the receiver has sampled the bit, so it causes little harm. On a ten metre cord the echo can land inside the sampling window, and the receiver either corrects with error concealment or drops lock entirely.
Higher sample rates demand more
A 192 kHz stream has a higher bit rate than a 44.1 kHz stream, so the same cable has less margin. A cord that works at CD quality may click at high resolution, which is a useful diagnostic: if the fault appears only at the higher rate, suspect the cable rather than the source.
Ground loops
The shield connects the chassis of both devices. Where those chassis sit on different mains circuits, a small current flows through the shield and adds hum. A transformer coupled input on the receiver, or an optical TOSLINK link, breaks the path.
What to check before buying
A cord that has been crushed under furniture or bent sharply at the plug belongs in the bin, even if it still passes audio. Look for a stated 75 ohm impedance, a foil plus braid shield and a solid centre conductor for permanent installs. Avoid cords sold only as audio cables without an impedance figure. If the run is long or the environment is electrically noisy, move to BNC or switch to optical, which removes the electrical path entirely.
Practical scenarios
- CD transport to DAC: a short 75 ohm RCA cord is enough and costs little.
- AV receiver to subwoofer: that is an analog line level link, so a digital cord is unnecessary but harmless.
- Broadcast rack: BNC with 75 ohm cable and proper termination is the only sensible choice.
- Long run to a second room: consider TOSLINK or a coax run with BNC rather than a stretched RCA cord.
FAQ
Can I use a video RCA cable for digital audio?
Usually yes, because composite video cable is also 75 ohm. It is not guaranteed, but it is a better substitute than an audio cable.
Does a coaxial digital cord need a direction arrow?
No. Direction markings are marketing on a coaxial cord, since the signal path is symmetric.
Why does the receiver show no signal at 192 kHz?
The cable or the source may not support the rate. Test at a lower rate to separate the two.
Next step
Check the impedance and the run length before choosing between coax and optical. The coaxial S/PDIF guide covers the protocol, the coax connector guide explains plug options, and the attenuation guide shows how loss rises with frequency.
