What S/PDIF Can Carry and What Only HDMI eARC Can
Both cables leave the same display, and they do not carry the same audio. Optical S/PDIF moves a two-channel frame with room for one compressed 5.1 stream, while HDMI ARC and especially eARC move multichannel and object audio. The choice is rarely about cable quality; it is about whether the stream fits inside the transport at all.
Quick Answer / Summary
Choose optical when the source sends PCM stereo, Dolby Digital, or DTS 5.1 and the run benefits from isolation. Choose HDMI eARC when the source uses Dolby TrueHD, DTS-HD Master Audio, or Atmos, or when the display must pass uncompressed multichannel. Optical cannot carry those formats at any cable grade, because the limit sits in the S/PDIF frame rather than the cord.
Bandwidth and Frame Limits
A S/PDIF frame carries two channels at up to 24 bits, and the standard supports sample rates to 192 kHz on coax and commonly 96 kHz on optical. The spare bits in the frame can hold a compressed 5.1 stream at roughly 640 kilobits per second, which is why Dolby Digital and DTS fit. Anything that needs more than that budget is rejected at the transmitter, not at the cable.
This is why a premium optical cord cannot rescue a TrueHD stream. The transport is full before the light leaves the port, and the source must either downconvert the stream or refuse to send it.
Where TrueHD, DTS-HD Master Audio, and Atmos Break
Lossless codecs need several megabits per second, and Atmos objects ride on top of a TrueHD or Dolby Digital Plus core. Neither fits a S/PDIF frame. A display set to bitstream over optical will downconvert to Dolby Digital or output silence depending on firmware, and a soundbar will show a locked signal with no sound. HDMI eARC exists to carry that bandwidth over the same link as video.
The ARC and eARC Handshake
ARC runs over the same HDMI link as video and needs CEC to negotiate, while eARC adds dedicated data pairs and a higher payload. If CEC is disabled, or the cord is an old high-speed type without the Ethernet pair, the audio return channel may not come up at all. Check the menu before replacing hardware.
Latency, Switching, and Control Differences
Optical carries audio only, so a display can send sound to a legacy amplifier while video goes straight to the panel, which avoids an HDMI handshake delay on every input change. HDMI audio shares the video link, which simplifies cabling and remote control but ties audio routing to the switch. Both approaches can produce lip sync error, and both need a delay setting on the decoder. Optical also avoids the moment of silence when a source changes resolution.
When Optical Is Still the Right Choice
Older amplifiers, soundbars, and AV receivers often have no eARC input, and optical is the only digital path they accept. Optical also breaks the ground connection between a display and an amplifier on a different circuit. In a retrofit where the HDMI run is long or marginal, keeping video local and sending audio over fiber is a reliable, low-cost fix. The link stays stable because it never negotiates with the video path.
Hybrid Layouts That Work
Run HDMI from the source to the display for video, and optical from the display to the amplifier for stereo or compressed surround. Set the output to PCM if the amplifier has no Dolby decoder. Label both ends of the fiber, because a swapped optical input is the most common commissioning error.
Comparison Table
| Audio format | Toslink S/PDIF | HDMI ARC | HDMI eARC |
|---|---|---|---|
| PCM stereo | Passes to 96 kHz common | Passes | Passes |
| Dolby Digital 5.1 | Passes | Passes | Passes |
| Dolby Digital Plus | Does not pass | Case by case | Passes |
| Dolby TrueHD | Does not pass | Does not pass | Passes |
| DTS-HD Master Audio | Does not pass | Does not pass | Passes |
| Dolby Atmos | Does not pass | Limited | Passes with a supported core |
Table: Audio formats by transport
Practical Application Scenarios
- A customer keeps a 2012 AV receiver with no eARC input, so the display sends Dolby Digital over optical and the lossless disc track is downconverted.
- A new soundbar supports Atmos and needs eARC, and the installer replaces an old high-speed HDMI cord that lacks the Ethernet pair.
- A bar install sends video over a 12 meter HDMI run and audio over fiber, which keeps lip sync stable when sources change resolution.
- A conference room uses optical from the display to a ceiling amplifier to break a ground loop that appeared after a projector upgrade.
Related Products and Further Reading
- optical and HDMI adapter range – Compare optical cords and HDMI audio extractors by format support.
- contact our engineers – Send your soundbar and display models for an eARC compatibility check.
- video adapter and converter guide – Explains how audio and video paths are split inside adapters.
- optical vs HDMI in surround setups – Walks through surround formats on an optical link.
Frequently Asked Questions
Can optical carry Dolby Atmos?
No. Atmos rides on a TrueHD or Dolby Digital Plus core that exceeds the S/PDIF frame. A display will downconvert to Dolby Digital or mute the output when optical is selected.
Is HDMI audio better than optical?
HDMI eARC carries more formats, including lossless and object audio. Optical is equal for PCM stereo and compressed 5.1 and adds ground isolation, which matters in some racks.
Why does eARC not work with my soundbar?
Check that CEC is enabled on both devices, that the cord supports Ethernet, and that the soundbar input is the ARC port rather than a plain HDMI input. Reboot both after changing the setting.
Can I use both outputs at once?
Many displays allow only one digital audio output at a time, and selecting optical often disables the internal speakers. Check the menu for a simultaneous output option before promising two zones.