Quick Answer / Summary: Coaxial digital cable: 75-ohm characteristic impedance, RCA or BNC plugs, robust over modest lengths and immune to nothing but bad impedance. Optical Toslink: complete galvanic isolation, immune to electrical hum, but limited to short runs and vulnerable to tight bends. Choose coax for most domestic links, optical where ground isolation decides.
Digital audio leaves a transport on one of two cables: coaxial electrical or optical light. Both carry the same bits, and the differences that matter are practical – ground isolation, bend radius, length limits and the connectors on each end.
Coaxial Digital: The Impedance Rule
S/PDIF coaxial cable is specified at 75 ohms, and the specification is electrical rather than cosmetic: a mismatched cable causes reflections that the receiver must resolve. Ordinary RCA audio cable is not 75 ohms and works over short runs by luck rather than design. Use a cable sold as digital coaxial, keep runs short, and the interface is bulletproof.
Why 75 Ohms Matters
Reflections at an impedance mismatch smear the digital waveform. Receivers tolerate a surprising amount, which is why an audio RCA lead often works – until a longer run or a marginal receiver makes the error visible as dropouts.
Connectors and Length
RCA is the domestic standard, BNC the professional one. Domestic runs of a metre or two are forgiving; beyond five metres, use a specified 75-ohm cable and avoid adapters that break the impedance.
Optical Toslink: Isolation and Its Limits
Toslink carries the same S/PDIF data as light, which makes it completely immune to electrical ground loops – the reason it solves hum between a TV and an amplifier on different circuits. The trade-offs are mechanical: plastic fibre tolerates gentle bends only, and long runs attenuate the light.
When Optical Wins
Any link between devices on different ground paths – a computer and a hi-fi, a TV and an amplifier on separate outlets. The isolation is real and cannot be matched by a copper cable of any price.
Handling and Storage
Keep the bend radius generous, cap the ends when unplugged, and avoid coiling tightly. A cracked fibre still passes light at reduced level and produces the intermittent dropouts that get blamed on the DAC.
Choosing Between the Two in Practice
Start from the devices: if either end sits on a different mains circuit or earthing regime, optical removes the hum question entirely. If both share a circuit and the run is short, coaxial is mechanically tougher and equally transparent. The choice is environmental, not sonic.
Digital Cable Comparison
| Line | Coaxial S/PDIF | Optical Toslink |
|---|---|---|
| Medium | Electrical, 75 ohm | Light in plastic fibre |
| Ground isolation | None | Complete |
| Practical length | Metres, robust | Short runs, bend-sensitive |
| Best for | Most domestic links | Hum-prone device pairs |
| Failure sign | Dropouts from mismatch | Dropouts from cracked fibre |
Practical Applications
Connecting a CD transport to a DAC, solving hum between a TV and amplifier on different circuits, choosing the correct lead for a soundbar, and replacing a generic RCA lead that drops out on long runs.
Frequently Asked Questions
Can I use a normal RCA cable for digital coaxial?
It often works over short runs, but an audio RCA lead is not 75 ohms and can cause reflections and dropouts at longer lengths. Use a cable specified as digital coaxial.
Is optical better than coaxial?
Optical wins where ground isolation matters – between devices on different circuits. Coaxial is more robust mechanically and equally transparent otherwise. Neither is inherently superior for sound.
Why does my Toslink link drop out intermittently?
Usually a tight bend or a damaged fibre: light still passes but at reduced level. Check the bend radius and substitute a known-good cable before suspecting the DAC.