HDMI to Optical: Getting S/PDIF Out of an HDMI Feed
Quick answer / TL;DR: HDMI to optical needs an active box. An HDMI audio extractor accepts the HDMI feed, passes the video through to the display and sends the audio out on optical, normally as two-channel PCM and often as Dolby Digital 5.1. An AV receiver does the same de-embedding internally and can route the result to an optical zone output. The box must be powered, and the source has to be set to PCM or bitstream to match what it can pass. No passive HDMI-to-optical cable exists.
HDMI to optical is the direction people ask for when the only output on the source is HDMI and the sound system only accepts light. It is not a cable, and no passive version of it exists at any price. HDMI carries video, audio and control in one high-speed stream with the audio locked to the video timing; an optical lead carries a two-channel or compressed surround S/PDIF stream as pulses of light. A wire cannot split one from the other, so the job always goes to a device with power inside it.
This page is about that device, the two forms it takes, and the format limit that decides whether the extraction works in a particular room.
Why a Passive Cable Cannot Do It
An HDMI stream is a serial digital link running at several gigabits per second, with video, audio and control packets time-multiplexed into the same lanes. The audio is not a separate wire inside the cable that could be peeled away; it is data interleaved with the picture, and it only becomes audio after the receiving end has decoded the packet structure. S/PDIF is a different format entirely: a framed stream at about 3 Mbit/s, built for a stereo pair or a compressed surround bitstream. Turning one into the other takes a demultiplexer, a buffer, an audio clock and an S/PDIF transmitter, and every one of those needs a supply.
The Two Devices That Do the Extraction
The extractor, for a projector or a monitor
An audio extractor is a small powered box with an HDMI input, an HDMI output and an optical output, sometimes with a 3.5mm jack beside it. The source plugs into the input, a cable runs from the output to the display, and the optical lead leaves for the sound system. Because the box sits in the video path it has to pass the picture intact, which is where the specification matters: a 4K extractor has to handle 4K at 60 Hz with HDR, and a cheap one will drop the display to 1080p or refuse the signal outright. It also reports its own capabilities to the source, which is the root of the format limit below.
The receiver, HDMI in and optical zone out
An AV receiver does the same work with more authority. The source goes into an HDMI input, the receiver decodes the audio itself and amplifies it for the room, and many models can also send an independent stream out of an optical port for a second zone – a kitchen, a patio, a pair of powered speakers elsewhere. Nothing extra is bought, the picture passes through to the television, and the extraction is a by-product of the receiver's normal job. The catch is that the zone output is usually fixed to two channels, so the far room gets stereo whatever the main room is playing.
The Path, Source to Speakers
| Source | Box in the middle | Output to the sound system | Typical room |
|---|---|---|---|
| Streaming stick | Extractor, HDMI pass-through | Optical, PCM stereo | Projector and a soundbar |
| Games console | Extractor, 4K pass-through | Optical, Dolby Digital 5.1 | Monitor and a 5.1 set |
| Laptop | Extractor, USB powered | Optical, stereo | Desk and a small DAC |
| Media player | AV receiver, HDMI in | Optical zone out, stereo | Living room plus a patio |
| Set-top box | AV receiver, HDMI in | Optical zone out, stereo | Whole-home audio |
Each row has the same three-part shape: a source that speaks HDMI only, a powered box that de-embeds the audio, and a sound system that speaks S/PDIF. The choice between the two boxes is about the picture as much as the sound. Where the display is a projector or an input-starved monitor, the extractor has to sit in the video path and must pass the resolution through. Where the display is a television and the receiver already belongs to the system, the receiver does the work and no extra box appears in the chain.
The Format Limit, PCM Against Bitstream
- PCM stereo – passed by every extractor, the setting that always works
- Dolby Digital 5.1 – passed by most, if the source is set to bitstream
- DTS – passed by some, downmixed to stereo by many
- Dolby Digital Plus – rarely passed, it belongs to the HDMI generation
- Dolby Atmos – never over optical, the format does not fit the pipe
The extractor reports its audio capabilities to the source over HDMI, and most report two channels. A source that respects that report decodes its own surround track and sends stereo down the link, which is why a film that is 5.1 in the menu arrives as two channels. Two things fix it. The extractor notes will say whether it passes Dolby Digital, and the source audio menu has a bitstream or passthrough setting that stops the player from mixing down. With both set correctly 5.1 survives the trip; with either left alone it does not.
What the Optical Side Cannot Carry
The optical output is the narrow end of the chain, and it is worth being clear about what is lost. No video passes, obviously. No control either – the volume and power commands that travel with CEC on HDMI stop at the box, so the television remote will not touch the sound system. The bandwidth ceiling excludes the newer formats: Dolby Digital Plus, TrueHD and Atmos all need more than S/PDIF provides. What remains is a solid pair of options, two-channel PCM at up to 96 kHz and compressed Dolby Digital or DTS at 5.1, which is exactly what a soundbar from the pre-HDMI era is built to accept.
Checks Before Buying the Box
Four specifications decide whether a particular extractor suits a particular room. The pass-through resolution and refresh rate: 4K at 60 Hz with HDR if the display is modern, plus a note on whether HDR survives when the box is powered. The audio formats it will pass: PCM for certain, Dolby Digital if 5.1 is wanted. The power source: a USB lead is fine at a desk but should come from a charger rather than the display's service port. And the outputs: a box with optical and 3.5mm both populated is more useful than one that forces a choice.
HDMI audio extractors are stocked with 4K pass-through, optical and 3.5mm outputs, USB or mains power, and the format table printed on the box; the catalog lists the pass-through resolution, the audio formats and the power input per part number.
Related reading: The receiving end of the optical leg, the amplifier side: optical cable for a receiver — optical-cable-receiver
Related reading: The signal question underneath the whole chain: is HDMI digital or analog — hdmi-is-digital-or-analog
Frequently Asked Questions
Is there such a thing as an HDMI to optical cable?
No. HDMI audio is interleaved with the video in a multi-gigabit stream and has to be demultiplexed, clocked and re-framed as S/PDIF before an optical lead can carry it, and that takes active electronics with a supply. Products sold under that name are either an extractor with a lead moulded on or a mislabelled item. If it has no power input, it cannot work.
Will an extractor pass 5.1 surround?
Usually Dolby Digital, sometimes DTS, and almost never anything newer. The extractor has to be told to pass the bitstream and the source has to be set to bitstream rather than PCM. If either side is left on automatic, the source sees a two-channel capability report and mixes down. Check the format table before buying if 5.1 is the point of the exercise.
Can a receiver send optical audio to a second room?
Many can. The zone output on the back panel is an independent optical port and can carry a different source from the one playing in the main room. It is normally limited to two channels, so the second room gets stereo even when the main room runs 5.1. The main room's speakers are unaffected, which is what makes the arrangement useful.
Does the extractor affect picture quality?
It can, and this is the part buyers underestimate. The box sits between the source and the display, so it must pass the resolution, the refresh rate and the HDR metadata through unaltered. An extractor rated below the display's capabilities forces a lower mode or refuses the signal. Match the box to the display and the picture arrives as it left the source.
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