The TV is mounted on the wall, the soundbar sits below it, and on the back of both devices you find three different audio outputs that all claim to deliver “5.1 surround.” Optical Toslink, coaxial RCA, and HDMI ARC each connect to a different cable, each costs a different amount, and each carries a different audio ceiling. Pick the wrong one and your Dolby Atmos system silently downmixes to two channels. Pick the right one and you cannot tell the difference between $5 of cable and $40. This guide walks through what each output actually supports, where the bottlenecks are, and which one to use for the most common home theater configurations in 2026.
The two old standards: optical and coaxial S/PDIF
Optical and coaxial outputs are two physical implementations of the same digital audio standard: S/PDIF (Sony Philips Digital Interface), defined in 1989. They carry identical data at the protocol level.
S/PDIF bandwidth maxes out at roughly 1.5 Mbps. That is enough for:
- Two-channel PCM, uncompressed stereo
- Dolby Digital 5.1, the lossy format used on DVDs and broadcast TV
- DTS 5.1, the lossy DTS multichannel format
S/PDIF cannot carry:
- Dolby Digital Plus (the streaming Atmos carrier)
- Dolby TrueHD (lossless multichannel)
- DTS-HD Master Audio (lossless multichannel)
- Dolby Atmos in any form
- PCM beyond stereo
The format ceiling is hard. Even with a perfect cable, S/PDIF cannot carry Atmos object metadata because the bandwidth does not exist.
Coaxial S/PDIF runs over a 75-ohm RCA cable, the same type as analog composite video. Optical S/PDIF runs over a Toslink fiber cable. Both deliver identical audio quality. Optical is immune to ground loops and electrical interference; coaxial is more tolerant of imperfect cable runs and dirty connectors. For a typical 1 to 2 meter run between TV and soundbar, either works.
What HDMI ARC adds
HDMI ARC (Audio Return Channel) was added in HDMI 1.4 in 2009. ARC routes audio backward from the TV to a soundbar or AV receiver on the same HDMI cable that carries video forward.
ARC’s bandwidth ceiling is roughly 1 Mbps for compressed audio. That carries:
- Stereo PCM
- Dolby Digital 5.1
- DTS 5.1
- Dolby Digital Plus, including the Atmos metadata layered on top
The Atmos compatibility is the key difference from S/PDIF. Netflix, Disney Plus, Max, Apple TV Plus, and Amazon Prime Video all stream Atmos as DD+ Atmos. That fits inside ARC and not inside optical or coaxial.
ARC cannot carry lossless Dolby TrueHD or DTS-HD MA. For those, you need eARC.
What eARC adds beyond ARC
HDMI eARC (enhanced Audio Return Channel) shipped with HDMI 2.1 and reached consumer TVs in 2019. The bandwidth jumps to roughly 37 Mbps, carrying:
- Dolby TrueHD, lossless multichannel up to 7.1
- DTS-HD Master Audio, lossless multichannel
- Dolby TrueHD with Atmos, the lossless Atmos on 4K Blu-rays
- DTS:X over DTS-HD MA
- Up to 32 channels of uncompressed PCM
eARC also mandates lip-sync correction that ARC made optional. The audible benefit shows up on 4K Blu-ray Atmos played through a discrete surround system. For streaming-only households, ARC and eARC sound identical because the source never exceeds DD+ Atmos either way.
Which to use, common scenarios
Scenario 1: Modern TV, modern soundbar, streaming Atmos. Use HDMI ARC or eARC. Optical caps the system at lossy 5.1 with no Atmos. Streaming Atmos requires the HDMI path.
Scenario 2: Older TV (pre-2018) with only optical output. Use optical. You are capped at 5.1 Dolby Digital, no Atmos. This is the best the TV can output.
Scenario 3: Modern TV with eARC, 4K Blu-ray player, high-end Atmos receiver. Use HDMI eARC. The Blu-ray sends lossless TrueHD Atmos to the TV via HDMI input, then the TV sends it onward to the receiver via eARC. Optical would silently downconvert to lossy 5.1.
Scenario 4: Two-channel stereo soundbar. Optical, coaxial, and HDMI ARC all sound identical. Pick whichever cable you have on hand. ARC adds remote-control integration through CEC, which is the only real reason to prefer it for stereo.
Scenario 5: AV receiver with both inputs available. Use HDMI eARC from the TV. The receiver is the audio brain, and you want every format the TV can pass through to reach it. Optical limits the receiver to Dolby Digital and DTS even when the source is capable of more.
Lip sync and CEC
S/PDIF outputs have no inherent lip-sync correction. TVs and soundbars handle the offset internally with a manual delay setting in the audio menu. If dialogue is out of step with mouth movement on an optical setup, the fix is to nudge the delay value 30 to 80 ms until lips and audio align.
HDMI ARC carries CEC, which most TVs use to control soundbar volume from the TV remote. eARC includes a stronger sync protocol that reduces drift to imperceptible levels in most setups. This is a real quality-of-life advantage that does not appear on a format spec sheet.
Cable cost and length
S/PDIF cables are cheap. A good 6-foot Toslink runs $8 to $15. A good coaxial digital cable runs $10 to $20. Both work reliably up to 10 meters. Optical Toslink has more brittle connectors and benefits from careful handling.
HDMI cables for ARC and eARC are pricier. ARC works on essentially any HDMI cable. eARC at full bandwidth wants a certified Ultra High Speed HDMI cable in the $15 to $25 range. For runs over 3 meters, an active or optical HDMI cable is more reliable.
The simple rule
If both ends of the chain support HDMI ARC or eARC, use it. Optical is the fallback when the TV cannot output ARC or the soundbar does not accept it. Coaxial is a fallback when optical is unavailable, which is rare in 2026. The format ceiling that matters in a modern streaming household is whether the path can carry DD+ Atmos, and only HDMI ARC and eARC can.
For more on the HDMI side of this routing, see our HDMI ARC vs eARC explainer and the HDMI cable quality myth piece.
Frequently asked questions
Can optical or coaxial carry Dolby Atmos?+
No. Both S/PDIF connections (optical Toslink and coaxial RCA) max out around 1.5 Mbps, which is enough for lossy Dolby Digital 5.1 and DTS 5.1 but not for Dolby Digital Plus, the carrier format for streaming Atmos. If you want any flavor of Atmos, you need HDMI ARC or eARC.
Is coaxial better than optical?+
In format support they are identical: both are S/PDIF with a 1.5 Mbps ceiling. Coaxial uses copper RCA cable, optical uses fiber Toslink. Coaxial is slightly more robust over long runs and is less prone to dirty-connector issues. Optical is fully immune to electrical interference. In practice the audio sounds identical.
Why does my TV only have optical, not HDMI ARC?+
Many budget TVs from 2018 to 2022 included only optical for audio output. Some 2026 entry-level TVs still ship that way to keep cost down. If your TV is in that category, your soundbar choices are limited to optical-input bars or you accept 5.1 Dolby Digital as the ceiling.
Can I use both HDMI ARC and optical at the same time?+
Most TVs let you select one or the other, not both. The audio output menu picks the active path. A few TVs allow simultaneous output for use cases like a soundbar plus headphones, but it is not common.
Will an HDMI ARC to optical converter let me use an old soundbar with a new TV?+
Yes. A small HDMI ARC to Toslink converter pulls audio off the ARC channel and outputs S/PDIF. The result is capped at 5.1 Dolby Digital, the same as a direct optical connection. Converters run $20 to $40 and work reliably for stereo and lossy 5.1 sources.
