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I'm looking at the small Sonos optical audio (toslink) -> HDMI cable in front of me and I don't know how it works (https://www.sonos.com/en/shop/optical-audio-adaptor.html). It has an optical audio input and can be connected to a speaker via HDMI, so basically it looks like it converts the optical signal.

I can only find clunky HDMI audio splitters that also output optical, but I don't know how this cable works since it is so small - or is it and this is totally normal?

How do optical -> HDMI cables / splitter work anyways? Is there a small IR diode in there detecting optical signals directly converting them? Is there a small contraption inside this cable?

Dennis G
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    I suspect this is "HDMI ARC", but the spec isn't readily available. – pjc50 Jan 05 '19 at 20:26
  • My goal is to extract digital Audio from Optical TOSLINK from my Audio Interface and then send this digital audio signal into an HDMI input for further processing. I don't need video signal at all, just audio. I would like to skip the DAC<->ADC path to avoid signal degradation. I wonder wether this would work. – Tamas Kalman Dec 27 '22 at 09:23

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That adapter is meant to be used with a soundbar that can only receive audio from a television via HDMI Audio Return Channel (ARC) connection.

Therefore the adapter gets power from soundbar HDMI connector and it has a TOSLINK input to receive optical S/PDIF digital audio stream from the TV. The same S/PDIF stream is then sent to the soundbar over the UTIL pin in HDMI connector almost as if it were a coaxial S/PDIF signal, the electrical specs are only slightly different.

So the adapter does not need to handle HDMI streams at all. However it might have a specialized chip for the ARC physical interface to meet the electrical specs of the S/PDIF signal.

Justme
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HDMI's digital signal specification is packet based; there's packet types for audio, for video and for other data types.

The cable in question hence would probably contain an IC that takes the audio from the SPDI/F interface and packetizes it and sends it over HDMI. I don't see any standards-compliant other way.

Luckily, the uncompressed audio that TOSLINK typically transports should be fully compatible to HDMI, so there's no need for recompression. My guess is the same is true for most other audio formats toslink can transport – HDMI was definitely designed with reusability of existing en- and decoders in mind.

Marcus Müller
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