Well, bad news for this idea (No! See update below...). According to Ian Canada, whose wide array of properly hi-fi rPi HATs suggest that he should know what he's talking about, HATs that provide an old-school digital input (S/PDIF, AES/EBU, TOSLink, etc.) cannot be routed to Linux running on the rPi host due to the clocking requirements of the interface. They can only be routed to other HATs - a DAC for example. This fits with Tim's explanation for the sole currently-supported digital-in HAT.
Ah well, it was a nice idea, but not one that can be implemented via the OS on an rPi.
UPDATE Feb 2024: Re-reading the thread and the links provided by others who've contributed, I think I misunderstood Ian Canada's offline explanation to me. I now think he was telling me that his S/PDIF input product (the ReceiverPi) cannot be used in the way I would like, not that S/PDIF input to rPi for DSP processing is entirely impossible. The corollary though, given the apparent technical quality of his products, is that the potential solutions will not be able to be reclocked to reduce/eliminate jitter, meaning that top-end audiophile quality will not be possible. This hypothesis is supported by HifiBerry's FAQ about their digital input products (see "Q: But I read a lot about jitter problems…") . Still, worth continuing to experiment..
Ah well, it was a nice idea, but not one that can be implemented via the OS on an rPi.
UPDATE Feb 2024: Re-reading the thread and the links provided by others who've contributed, I think I misunderstood Ian Canada's offline explanation to me. I now think he was telling me that his S/PDIF input product (the ReceiverPi) cannot be used in the way I would like, not that S/PDIF input to rPi for DSP processing is entirely impossible. The corollary though, given the apparent technical quality of his products, is that the potential solutions will not be able to be reclocked to reduce/eliminate jitter, meaning that top-end audiophile quality will not be possible. This hypothesis is supported by HifiBerry's FAQ about their digital input products (see "Q: But I read a lot about jitter problems…") . Still, worth continuing to experiment..
