A coaxial digital audio cable looks exactly like an analog audio lead, uses the same RCA plugs, and is a different component. One carries a low-frequency analog signal on a screened cable; the other is a transmission line carrying a signal whose content reaches well into the radio band.
The confusion between them is the reason this cable is so often the wrong one.
Quick answer
| Question | Answer |
|---|---|
| What is coaxial S/PDIF? | An electrical digital audio interface on 75 ohm coaxial cable |
| What connector? | RCA in consumer equipment, BNC on professional gear |
| Can I use an ordinary RCA audio lead? | It will often work, and it is not the right cable |
| Why not? | Wrong characteristic impedance and higher capacitance, which causes reflections |
| Is coax better than optical? | Different, not better. Each has a case |
| Does cable length matter? | Less than in video, but the same principles apply |
What the signal actually is
S/PDIF carries a digital audio stream as a serial electrical signal, and because it is a digital stream at a substantial bit rate, its spectral content extends far above the audio band. That is the whole reason the cable is specified as a 75 ohm transmission line: at those frequencies, the cable is behaving as a transmission line rather than as a simple wire.
An analog audio cable is built for a different job. It is screened to keep interference away from a low-level signal, and it is designed with little regard for characteristic impedance, because at audio frequencies nobody needs it. Its capacitance is often higher than a video cable’s.
What goes wrong with the wrong cable
Two mechanisms, and the severity depends on the system.
Reflections. A cable whose impedance does not match the source and load produces reflections at the transitions. In a digital system, reflections distort the pulse shape, which makes the receiver’s job harder. The symptom ranges from nothing audible to intermittent dropouts, depending on how much margin the receiver has.
Jitter. The timing of the transitions carries information in a digital audio stream. A distorted pulse edge moves the point at which the receiver decides a bit is a one or a zero, and that timing uncertainty is jitter. In the worst cases the receiver fails to lock.
The practical point is not that an audio lead will always fail. It is that a digital audio link benefits from a cable built for the impedance, and that a cable specified as a 75 ohm digital coax is a cheap way to remove a variable.
Coaxial or optical
Both carry the same digital stream, and the choice is environmental rather than about sound quality.
| Coaxial | Optical | |
|---|---|---|
| Signal | Electrical | Light |
| Immunity to electrical interference | Good, but the shield must be intact | Complete — there is no electrical path |
| Ground loops | The shield is a possible path | Breaks the ground path entirely |
| Connections | RCA or BNC, and they tolerate repeated handling | Needs care; the connector must be clean and fully seated |
| Bend radius | Standard | More constrained, and a tight bend can damage the fiber |
| Length | Plenty for domestic runs | Adequate, with the same caveat about handling |
The decisive factor is usually ground isolation. Where two pieces of equipment sit at different ground potentials and produce hum through a coaxial connection, an optical link removes the path entirely. That is a structural advantage, not a tonal one.
Where the environment is electrically quiet and the cable route is convenient, coaxial is simpler, and its connectors tolerate handling better.
Connector practice
Consumer equipment uses RCA for coaxial S/PDIF, and RCA is a compromise as a 75 ohm connector: it was designed for audio and its impedance is not well controlled. It is what the equipment has, so it is what gets used, and a properly made digital coax cable with RCA ends is entirely adequate for a domestic run.
Professional equipment uses BNC, which is a genuine 75 ohm connector and is available in a form designed for the impedance. Where the equipment offers BNC, use it. The connector comparison is here, including the trap that BNC comes in both impedances.
Because an RCA digital coax cable and an RCA analog audio cable look identical, labeling matters. In a rack with both kinds of lead present, an unlabelled cable will eventually be plugged into the wrong socket.
Scenarios
A disc player feeding an amplifier with a coaxial input. A cable specified as 75 ohm digital coax, not a spare analog lead from the drawer.
A computer feeding a DAC over coax. Short run, and a digital coax cable removes any question about impedance.
A system that hums through a coaxial digital connection. Try an optical link. The hum is a ground loop, and breaking the electrical path is the fix.
A rack with several digital audio connections. Label the cables. An analog audio lead in a digital socket will usually work and is a latent fault.
FAQ
Is a coaxial digital audio cable the same as an RCA audio cable? The connector is the same. The cable is specified differently, for a 75 ohm transmission line rather than for a low-frequency analog signal.
Will an ordinary RCA cable work for digital audio? Often yes. It is the wrong cable, and a digital coax cable costs little.
Is optical better than coaxial for digital audio? Neither is inherently better. Optical is immune to electrical interference and breaks ground loops; coaxial has sturdier connections and is easier to handle.
Does a longer digital audio cable degrade the sound? The signal either arrives intact or it does not. A cable that is too long or too lossy produces dropouts rather than a gradual loss of quality.
Why is BNC used on professional gear? It is a better 75 ohm connector than RCA, with a bayonet lock.
Does the cable change the sound of digital audio? As long as the link is reliable, the data arrives unchanged. Cable choice is about reliability, not tone.