I'm not sure if there is any handshaking on an SPDIF connection — I suspect there's not — or, if there is, how it might work. What would happen if a different stream was sent? If the headphones assumed PCM, it would be noise. If it doesn't assume, then we're back to processing it at least enough to recognize whether it's PCM or not.

All of that may be cheap, but I don't know.
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Bitt Faulk