I asked in their Discord and their tech support said it's consumer standard and only does 2 channel PCM or multichannel bitstream. Not even sure how multichannel PCM over SPDIF is possible anyway due to the bandwidth
The last part, I absolutely hate that. Its not 1983. What bandwidth does SPDIF have then? I can tell you it does not specify a bandwidth, there is not a limit.
The only thing that prevents multichannel SPDIF is the transmitter and receiver, again its not 1983. Got updated at some point.
That's like saying HDMI 2.X can only do the bandwidth of HDMI 1.0, well 2.X is an updated version ofc.
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HDMI and SPDIF using a HDA bus will be limited to ~37 Mbits/s, even your GPU is limited to ~37 Mbits/s, 125 Mbits/s is greater than ~37 Mbits/s.
So, to use the full amount of TOSLink high speed, you would need multiple HDA buses or USB 2.0 audio (480 Mbits/s).
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Another thing is, ~37 Mbits/s for audio is a tiny number, RGB optical can shift 10 channels, 200 million samples (ps), 256 bit as [2 Gbits/s RAW].
In PCM that would need, 512 Gbits/s in comparison, in both cases its uncompressed audio, just different methods.