Optical Audio vs HDMI: Two Paths for One Signal
Quick answer / TL;DR: Short answer: optical audio carries S/PDIF at 3.1 Mbps, enough for stereo PCM to 96 kHz and compressed Dolby Digital or DTS 5.1. HDMI carries audio alongside video and, with eARC, supports Dolby Atmos TrueHD and DTS-HD MA. Choose optical for isolation and simplicity, HDMI for modern surround formats.
Four audio lines: balanced XLR, TRS, unbalanced RCA, and speaker cable.Both carry digital audio from a source to a decoder, and both do it without converting to analogue in between. The difference is capacity, and capacity decides format support. Optical tops out at 3.1 Mbps; HDMI carries audio inside a link that can exceed 40 Gbps.
That gap is the whole story for home cinema, but it is not the whole story for audio generally. Optical's electrical isolation solves problems that HDMI cannot, and those problems are common in studios and in rooms with poorly earthed equipment.
Capacity Sets the Format List
| Format | Bitrate | Optical | HDMI ARC | HDMI eARC |
|---|
| Stereo PCM 48 kHz | 1.54 Mbps | yes | yes | yes |
| Stereo PCM 96 kHz 24-bit | 4.6 Mbps | no | no | yes |
| Dolby Digital 5.1 | 0.64 Mbps | yes | yes | yes |
| DTS 5.1 | 1.54 Mbps | yes | yes | yes |
| Dolby Digital Plus | up to 3 Mbps | marginal | no | yes |
| Dolby TrueHD Atmos | up to 18 Mbps | no | no | yes |
| DTS-HD MA | up to 24.5 Mbps | no | no | yes |
The Dolby Digital Plus row explains a common confusion. Some televisions will pass it over optical and some receivers will decode it, but the behaviour is not specified and varies by model. Treat optical as a Dolby Digital 5.1 path and the results become predictable.
The Isolation Advantage
Optical cable is a plastic fibre. It carries light, not electrons, so it cannot create a ground loop between two devices on different circuits. In a room where a computer and an amplifier are on separate outlets, that removes the 50 or 60 Hz hum that no amount of cable swapping fixes.
HDMI carries a ground conductor. It does not create ground loops by itself, but it does provide a path, and a difference in earth potential between two devices will flow through the HDMI shield. This is why some installations hum only when the HDMI cable is connected.
The isolation also works as protection. A faulty power supply in one device cannot push current through an optical link into another device, which is a real consideration with inexpensive external supplies.
Latency and Synchronisation
Both paths introduce delay, and the delay is dominated by the decoder rather than the cable. Optical has a slight advantage in that there is no video pipeline to synchronise with, so an audio delay setting is rarely needed. HDMI carries audio and video together, and a television that processes video adds latency that the audio path does not share unless the receiver applies a matching delay.
- Optical to a DAC: lowest complexity, no lip-sync setting needed
- HDMI ARC to a soundbar: audio delay usually handled automatically
- HDMI eARC to a receiver: manual audio delay may be needed for video processing
- Optical from a television: the television's own processing delay applies
When Each Wins
Optical wins
Studio monitoring where ground isolation matters, legacy receivers with no HDMI, televisions whose HDMI ports are all occupied, and any installation where simplicity beats format range.
HDMI wins
Anything involving height channels, lossless surround, or a soundbar that expects CEC volume control. Also any case where a single cable for audio and video simplifies routing.
Real-World Applications
Television with an older soundbar
Optical. The soundbar decodes Dolby Digital 5.1, the television passes it, and there is no CEC to misconfigure. The result is stable and audibly good for most content.
Desktop computer feeding a DAC and amplifier
Optical from the motherboard or a USB-to-optical converter. This breaks the ground path between the computer's switching supply and the audio system, which is the usual cause of audible noise at idle.
Media room with a Dolby Atmos receiver
eARC over HDMI. Height channels only exist in the TrueHD or Dolby Digital Plus containers, and only eARC carries them reliably from a television.
Conference room with a ceiling speaker system
Optical from the display or codec to the amplifier. The run is short, the format is stereo or Dolby Digital, and the isolation avoids hum from the room's lighting circuits.
Related reading: HDMI ARC versus optical — blogs/news/hdmi-arc-vs-optical
Related reading: Optical audio extractor setup — blogs/news/optical-audio-extractor
Related reading: TOSLINK optical cable guide — blogs/news/toslink-optical-cable
Standards and Verification
S/PDIF is defined in IEC 60958, with part 3 covering consumer applications and the channel status bits that declare sample rate, format and copy protection. The optical variant uses the TOSLINK connector defined in JEITA CP-1212.
HDMI audio is defined within the HDMI specification, with the audio return channel added in HDMI 1.4 and eARC in HDMI 2.1. eARC uses the HDMI Ethernet and Audio Return Channel conductors, which is why a cable must include them.
Verification: play a known multichannel test file and read the decoder's front panel. It reports the format it locked to, which is more reliable than any setting menu. If it shows stereo where surround is expected, the source is set to PCM output rather than bitstream.
Frequently Asked Questions
Is optical better sound quality than HDMI?
No. Both are digital and lossless within their capacity. Optical is limited by bandwidth, not by quality.
Does optical support 7.1?
Only in compressed formats that fit within 3.1 Mbps. Uncompressed 7.1 does not fit.
Can I use both at once?
Not usefully. Most sources output to one path at a time, and running both causes duplicate or conflicting audio.
Why does optical cut out?
Usually a sharp bend, a dirty connector end or a cable longer than about 10 metres. Check the routing first.
Which is better for a PC?
Optical, if hum is the problem. HDMI, if the PC drives an AV receiver with surround content.
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