SPDIF Cables: Coaxial or Optical, and Does It Matter?

November 8, 2026

Quick answer: SPDIF carries digital audio over either a 75 ohm coaxial cable with RCA plugs or an optical TOSLINK cable. Both support stereo PCM and compressed 5.1. Coaxial is more robust over distance and cheaper per metre, while optical breaks ground loops. Plastic optical cable is limited to about 5 m in practice.

Both cable types carry identical data, so the choice is about the room, not the format. Hum, distance and the shape of the port decide which one keeps working after the novelty of a new receiver wears off.

Same protocol, different physics

Coaxial electrical

A 75 ohm coaxial cable carries the SPDIF signal as a voltage. It is tolerant of length up to about 10 m and cheap to terminate, but it connects the grounds of both devices, which is where hum comes from when the television and the amplifier sit on different circuits.

Optical light

TOSLINK converts the signal to light and back. Because there is no electrical connection, ground loops disappear. Plastic fibre is convenient but lossy and fragile; glass fibre performs better but costs more and does not tolerate tight bends.

Choosing between them

Factor	Coaxial	Optical
Maximum length	10 m typical	5 m plastic, 30 m glass
Ground loop	Possible	Immune
Cable cost	Low	Higher
Bend tolerance	Good	Poor on plastic

Formats and limits

What SPDIF cannot carry

The interface carries stereo PCM, Dolby Digital and DTS. It cannot carry uncompressed multichannel audio, Dolby TrueHD or Atmos. A source set to output those formats will either downmix or send nothing, which is why a receiver sometimes stays silent on a new disc.

Sample rate and jitter

Both cable types support 24-bit 192 kHz in principle. In practice, long plastic optical runs can introduce enough jitter for some DACs to lose lock. If the receiver drops out on high-resolution files, shorten the optical run or move to coaxial.

Faults and how they present

A coaxial cable with the wrong impedance works at short lengths and produces clicks or dropouts at longer ones. An optical cable with a cracked end shows an intermittent signal that changes when the plug is moved. In both cases the fault looks like a source problem until the cable is swapped.

Ground loops and hum

Diagnosing the hum

Disconnect the optical or coaxial cable and listen. If the hum stops, the loop runs through the audio cable. If it continues, the source of the noise is elsewhere. Running the television and the amplifier from the same outlet strip is the first test, because a shared ground removes most loops. Only then is a ground loop isolator or a switch to optical the right next step.

What an isolator cannot fix

An isolator breaks the electrical path on the audio line, which removes hum but also blocks anything else carried on that path, including some remote control signals and, on a coaxial link, the return channel. It is not a substitute for correct wiring, and it will not fix noise picked up before the source, such as a failing power supply in a set top box.

Choose the interface once and stay with it. Switching a television between optical and coaxial to solve a hum problem usually moves the fault rather than fixing it. Decide based on the room, then correct the grounding and leave the cable type alone, so the next person to touch the system knows what to expect.

If both interfaces are available on the receiver, connect both and switch between them during a quiet passage, then keep the one that stays silent when the refrigerator or the heating pump starts.

Practical scenarios

Television to amplifier: optical removes the hum that HDMI switching often introduces.
CD transport to DAC: a short 75 ohm coaxial cable is the simplest stable link.
Rack to second room: coaxial with a BNC termination holds up better than 15 m of optical.
Desktop DAC: a 1 m optical cable keeps the computer ground away from the audio chain.