Optical vs Coaxial Digital Audio: TOSLINK and S/PDIF

Both optical and coaxial digital audio carry the same S/PDIF signal, and for two-channel PCM or compressed 5.1 surround, either works. The difference is the cable and the connector: TOSLINK sends light down a fiber, coaxial sends a 75 ohm electrical signal down an RCA cable. Neither carries lossless multichannel formats such as Dolby TrueHD or DTS-HD Master Audio; that needs HDMI. Choose optical when you need electrical isolation between two mains-powered devices, and coaxial when the run is short and the sockets are RCA.

What TOSLINK and coaxial S/PDIF actually carry

S/PDIF is the consumer digital audio interface. It carries two-channel PCM and compressed 5.1 (Dolby Digital, DTS). It does not carry lossless multichannel formats, and it does not carry the higher bitrate object-audio streams that ride inside them. For a soundbar, a stereo DAC or an older AV receiver, that is enough. For a 4K disc player feeding a modern receiver with height channels, it is not.

TOSLINK uses a fiber optic cable with a square-ish connector, usually called the TOSLINK connector. The signal is light, so the cable is immune to electrical interference and to ground-potential differences between devices. That last point matters: if a TV and an amplifier sit on different circuits, or one is a laptop on a charger and the other is a mains-powered receiver, an optical link breaks the ground loop that would otherwise cause hum.

Coaxial S/PDIF uses a 75 ohm RCA cable. The connector is an RCA plug, but the cable is not the same as an analogue audio RCA cable: it should be 75 ohm coaxial, the same impedance used for video and antenna feeds. A standard analogue RCA cable will often work over a short distance, but it is not the right cable and can cause dropouts on longer runs.

Choosing between optical and coaxial

For a short run between a TV and a soundbar, either cable works. The practical differences are the sockets on the equipment and the electrical environment.

Use optical when:

• The two devices are on different mains circuits and you hear hum.
• The run is longer than a typical RCA cable handles well.
• You want to avoid any electrical path between a TV and an amplifier.

Use coaxial when:

• Both devices have RCA digital audio sockets and the run is short.
• You already have a 75 ohm coaxial cable, such as a spare video cable.
• The equipment has only a coaxial digital output.

Many TVs and soundbars have both. If both are present, optical is the safer default for a TV-to-soundbar link because it removes the ground loop. If the run is very short and both devices share a power strip, coaxial is fine.

For analogue connections, such as a turntable or a headphone output, the rules are different. See audio cable types: XLR, TRS, TS, RCA and 3.5 mm for the analogue side.

When HDMI eARC replaces both

HDMI ARC (Audio Return Channel) was introduced with HDMI 1.4 and returns audio from the TV to a soundbar or receiver over the HDMI cable. It carries stereo PCM and compressed 5.1, the same formats as S/PDIF. That makes it a direct replacement for optical or coaxial in a simple TV-to-soundbar setup, with one cable instead of two.

eARC (Enhanced Audio Return Channel) arrived with HDMI 2.1. It carries uncompressed multichannel PCM (up to 8 channels) and lossless formats such as Dolby TrueHD, DTS-HD Master Audio and Dolby Atmos within them, at up to 37 Mbps. eARC uses the HDMI Ethernet Channel wires, so it needs a High Speed cable with Ethernet or an Ultra High Speed cable. If your TV and soundbar both support eARC, and you want lossless surround, eARC is the correct link. S/PDIF cannot carry those formats at all.

The trade-off is compatibility. Older receivers and soundbars often have only optical and coaxial inputs. A TV with eARC can usually output to an older soundbar over optical, but it will downmix lossless formats to compressed 5.1 or stereo. For a comparison of the two HDMI audio return standards, see HDMI ARC vs eARC.

Connectors, cables and what to look for

TOSLINK connectors come in a few shapes. The standard square connector is the common one. Some portable devices use a mini-TOSLINK plug, which looks like a 3.5 mm jack and often doubles as an analogue headphone socket. A mini-TOSLINK to standard TOSLINK adapter cable is available for those.

Coaxial S/PDIF uses an RCA plug. The cable should be 75 ohm coaxial. RG6 is 75 ohm with an 18 AWG centre conductor, and RG59 is 75 ohm with a 20 to 22 AWG centre conductor. Either works for digital audio; RG6 is the heavier cable. F connectors are for TV coax, not for S/PDIF, so you need an RCA plug on each end.

Optical cables are more fragile than coaxial. A tight bend can crack the fiber and cause dropouts. Coaxial cables are more robust but can pick up electrical noise if run alongside mains cables. Keep both away from power cords where possible.

What goes wrong

• Using an analogue RCA cable for coaxial S/PDIF. It may work over a short distance, but it is not 75 ohm and can cause dropouts or jitter. Use a 75 ohm coaxial cable.
• Bending an optical cable too tightly. The fiber can crack and the signal will drop out. Keep the bend radius generous.
• Expecting lossless surround from S/PDIF. Dolby TrueHD, DTS-HD Master Audio and Dolby Atmos in their lossless form do not fit. The link will downmix to compressed 5.1 or stereo.
• Plugging a mini-TOSLINK into a headphone socket. The socket may be both, but the device must be set to output digital audio, not analogue. Check the menu.
• Assuming all RCA digital outputs are coaxial S/PDIF. Some are analogue stereo outputs. Check the label: it should say coaxial, digital or S/PDIF.
• Ignoring the HDMI cable version for eARC. eARC needs a High Speed cable with Ethernet or an Ultra High Speed cable. An older Standard HDMI cable may not carry the Ethernet channel.
• Ground loops with coaxial. If you hear hum, try an optical cable. It breaks the electrical path between devices.

Quick selection guide

For most home setups, optical is the safe choice for a TV-to-soundbar link, and coaxial is fine for short runs between components that share a power source. When both devices support eARC and you want the best surround formats, HDMI eARC replaces both.