Quick Answer / Summary: Use a cable specified as 75 ohm digital coaxial, not a generic RCA audio lead. Keep runs under about 5 m for reliable S/PDIF, use BNC where the equipment offers it, and avoid adapters that change impedance. If a link drops out intermittently, suspect the cable before the DAC.
A digital coaxial cable carries S/PDIF audio as a 75 ohm electrical signal on an RCA or BNC connector. It looks like an ordinary audio cable and is not one. The difference is characteristic impedance, and it decides whether the receiver sees clean edges or reflections it has to guess at.
Why 75 Ohms Is Not Marketing
S/PDIF coaxial is a transmission line problem, not an audio problem. The driver expects a specific impedance; a cable with the wrong impedance reflects part of the signal back. At the data rates involved, those reflections can push a receiver past its decision threshold, and the result is a dropout rather than a subtle change in sound.
What Happens With an Audio RCA Lead
A standard audio RCA cable is typically around 50 ohms or simply unspecified. It often works at 1 m because the reflections are small relative to the margin. At 5 m or with a marginal receiver, the same cable produces clicks, dropouts or a failure to lock.
Impedance Is Geometry
Characteristic impedance comes from conductor diameter, dielectric material and spacing. It is not improved by thicker copper or gold plating. A cable sold as digital coaxial is built to a 75 ohm geometry, and that is the property to look for.
Connectors: RCA and BNC
Consumer equipment uses RCA; professional and some prosumer gear uses BNC. BNC is a constant-impedance connector with a positive lock, which makes it the better engineering choice. RCA is not a 75 ohm connector by design, which is one reason short runs are more forgiving than long ones.
Avoid Adapters
RCA-to-BNC adapters introduce a small impedance discontinuity and an extra joint. If both ends offer BNC, use a BNC cable. If you must adapt, keep the adapter at the source end and the run short.
Contact Quality
A loose RCA centre pin produces exactly the intermittent faults people blame on jitter. Gold plating helps corrosion resistance, not conductivity. Fit matters more than plating.
Length and Real-World Limits
S/PDIF over coax is robust to tens of metres in professional installations using proper 75 ohm cable and BNC. In a domestic system, the practical concern is not distance but cable quality and the number of joints. A 1 to 3 m run with a specified cable is trouble-free; a 10 m run with a generic lead and two adapters is not.
Coax vs Optical
Coaxial avoids the electrical isolation that Toslink provides, but it is more robust mechanically and handles longer runs and tighter bends. Use optical where ground loops cause hum; use coax where the cable must survive being moved.
Sample Rates
Higher sample rates and multichannel formats raise the data rate and shrink the timing margin. A cable that is marginal at 44.1 kHz stereo can fail at 192 kHz. If a format change causes dropouts, the cable is a prime suspect.
Practical Setup
Connect the transport’s coaxial output to the DAC or receiver’s coaxial input, select that input, and confirm the sample rate on the display. If the DAC reports no signal, check that the source is set to PCM rather than a format the input cannot accept, then swap the cable.
Ground Loops
Coaxial carries a ground connection between devices. If a hum appears when the cable is connected, the two devices sit on different ground potentials. An optical link breaks that path; a ground-loop isolator is the alternative.
Direction and Marking
Digital coaxial cable is not directional electrically. Arrows printed on the jacket are a marketing convention. What matters is that the cable is 75 ohm and the connectors are sound.
Digital Coaxial Cable Options
| Cable | Impedance | Connector | Practical length | Notes |
|---|---|---|---|---|
| Generic audio RCA | Unspecified | RCA | 1 m, unreliable beyond | Works by luck, not design |
| Digital coaxial RCA | 75 ohm | RCA | to ~5 m | Standard consumer choice |
| Digital coaxial BNC | 75 ohm | BNC | to ~10 m | Best contact and impedance |
| RG-6 with RCA | 75 ohm | RCA | Long runs | Stiff, suited to fixed installs |
| Optical Toslink | Not applicable | Toslink | to ~5 m | Full galvanic isolation |
Practical Applications
Common setups: a CD transport feeding an outboard DAC, a streamer connected to an AV receiver, a TV’s coaxial digital output feeding a soundbar or amplifier, a studio interface clocked from a house reference, and a legacy DVD player kept in service because it still plays the collection. In each case the cable carries a continuous S/PDIF stream where dropouts are audible as clicks rather than visible as artefacts.
Frequently Asked Questions
Can I use a regular RCA cable for digital coaxial?
It often works over a metre, but it is not 75 ohm and can cause reflections and dropouts at longer lengths or higher sample rates. Use a cable specified as digital coaxial.
Is a digital coaxial cable directional?
No. The signal is electrical and passes either way. Arrows on the jacket are conventional, not functional.
What is the maximum length for coaxial S/PDIF?
With proper 75 ohm cable, runs of 10 m and beyond work. Domestic installs are usually limited by cable quality and connectors rather than distance.
Coaxial or optical for a DAC?
Optical if you need ground isolation or run the cable around corners tightly. Coaxial if you want mechanical robustness and slightly simpler connectors.
Why does audio cut out at 192 kHz but not 44.1 kHz?
Higher sample rates need more bandwidth and timing margin. A marginal cable or a long adapter chain fails first at the higher rate.