HDMI ARC or optical: getting the best sound from your TV
Two sockets on your television promise better sound than its own speakers: an HDMI port marked ARC, and a small square optical one. They are not equals.
Your television sounds thin, and behind it sit two ways to fix that: an HDMI socket marked ARC, and a small square socket with a flap for optical. Both send the television's sound to something better, a soundbar, an AV receiver, or a hi-fi amplifier and speakers. They look interchangeable and they are not: one carries far more than the other, and one brings a convenience the other simply cannot. Which you should use comes down to what you are feeding, and to one setting that people almost always miss.
what each connection carries
The difference between the two is not really about fidelity, it is about bandwidth and control. For ordinary stereo sound an HDMI ARC connection and an optical connection carry identical digital data and sound exactly the same. The gap opens up with surround sound and with convenience. HDMI ARC has the bandwidth to carry compressed surround, Dolby Digital and DTS, and in its newer eARC form the full, uncompressed Dolby Atmos and DTS:X that modern films use. Optical carries stereo and compressed 5.1 perfectly well, but it runs out of room before those high-bitrate formats, and it cannot pass lossless Atmos at all. HDMI also carries a control signal, which optical has no way to do.
hdmi arc: one cable, surround, and a remote that behaves
ARC stands for Audio Return Channel, and its trick is to send sound backwards down a single HDMI cable, from the television to the audio device, using the same lead that would normally carry the picture the other way. One HDMI cable does the whole job. Along with the audio it carries CEC, the control link that lets one remote handle both boxes, so your television remote adjusts the soundbar's volume and both power on together. Manufacturers brand it in their own way, Anynet+ on Samsung, Bravia Sync on Sony, but it is the same feature underneath.
For the best formats you want eARC, the enhanced version found on recent televisions and amplifiers, which carries the full lossless soundtrack that plain ARC has to compress. Two details matter. Plug into the one HDMI socket actually labelled ARC or eARC, not any of the neighbouring ones, and use a proper High Speed HDMI cable, or an Ultra High Speed cable if you are chasing lossless Atmos through eARC. The weakness of the whole arrangement is CEC: the handshaking between brands can be temperamental, occasionally selecting the wrong input or refusing to pass the volume, and when it misbehaves it is the first thing to switch off and on again.
optical (toslink): universal, isolated, unfussy
Optical is the connection almost every television has offered for twenty years, and almost every DAC, hi-fi amplifier and soundbar accepts it. It sends the sound as pulses of light down a thin fibre, carried by a Toslink cable, and because the two boxes are joined by light rather than copper, no electrical connection passes between them. That gives optical a genuine advantage: if your system develops a faint hum from a ground loop, an optical link cannot carry it, so the hum simply disappears. Optical is also gloriously simple, with no handshaking to go wrong. Its limit is bandwidth: it will happily pass two-channel stereo and compressed 5.1, but not the high-bitrate or lossless surround formats, and it carries no control signal, so you keep a separate remote unless your amplifier has its own volume in the chain.
and what about coaxial?
A third digital connection, coaxial, is worth a mention because it often sounds a shade better than optical. Coax sends the same data as an electrical signal down a single 75-ohm cable on an RCA plug, and it carries a little more bandwidth headroom, which is why many listeners prefer it on a dedicated two-channel source such as a CD transport. For television, though, the point is usually academic: very few televisions provide a coaxial output, so optical is normally the digital choice actually on offer. Remember too that coax, being an electrical link, does pass ground between the boxes, so it can carry the very hum that optical cures. If your source is a CD player or streamer rather than a television, our optical, coaxial or USB guide goes further into that choice.
so which should you use?
The decision follows what you are trying to do. If you want surround sound, a soundbar, an AV receiver, or anything that will decode Dolby Atmos, use HDMI ARC, and eARC where both ends offer it, because only HDMI carries those formats and the single-cable control is a real convenience. If instead you are feeding a two-channel hi-fi, a stereo amplifier or a DAC and a pair of speakers, optical is usually the better fit: it is what your television provides, its isolation keeps the system quiet, and for stereo it sounds identical to anything else. Bear in mind that most stereo hi-fi amplifiers carry no HDMI socket at all, the Arcam A5+ is a good example, while the step-up A15+ adds eARC, so for a great many hi-fi systems optical, or an optical feed into a DAC, is the only way in from the television. If you have not yet settled which kind of box belongs in the middle, stereo amplifier or AV receiver? is the choice sitting underneath this one.
getting the best sound from either
Whichever you choose, one setting decides whether you hear anything worth having. Deep in the television's sound menu is a digital output format, usually offering PCM, Bitstream, Dolby Digital or Auto. A two-channel hi-fi DAC or amplifier accepts only stereo PCM, so a television left on Bitstream or Dolby will hand it a surround stream it cannot read, and you will get silence or a burst of static. Set the television to PCM, or Stereo, when you are feeding a stereo system, and to Bitstream or Auto only when the far end is a soundbar or AV receiver that can decode surround. Beyond that, use a decent optical or HDMI lead rather than the very cheapest, keep optical fibres gently curved rather than kinked, and plug HDMI into the labelled ARC socket. Get the format right and the rest tends to look after itself.
the two-minute checklist
Surround sound, a soundbar, or an AV receiver? Use HDMI ARC, and eARC with an Ultra High Speed HDMI cable for lossless Atmos. Feeding a stereo hi-fi or a DAC? Use optical, and enjoy its freedom from hum. No HDMI on your amplifier? Optical, or an optical feed into a DAC, is your route in. Coaxial is worth having on a CD player or streamer, less so from a television. Above all, set the television's digital output to PCM or Stereo for a two-channel system, and to Bitstream or Auto for a surround one. And if you are unsure what your television and your amplifier can share, add both to the planner and it will find the connection between them.
HDMI ARC for surround and control, optical for a quiet two-channel life, and the right output setting for either. Match the connection to the job and the television finally sounds the way it should. Happy listening.