Raspberry Pi TOSLINK transceiver hat

I've been informed that the WM8804 is now end-of-life. And so that puts the period at the end of the sentence for this project. I am not aware of a replacement that has built-in support in Raspbian. We'll see if that changes.

Note that to get the output to work, you have to set the TX source to "AIF" - that is, you have to configure the chip to generate the TX output from the i2s input on the chip. You can do this in amixer by using cset to set the "Tx Source" control to whichever value (probably 1) that decodes to "AIF". The default is "S/PDIF RX", which means that it comes from the receive input, but that will only work if the i2s clocks happen to match up with the S/PDIF received signal.

If you don't do this, then playing audio won't light up the output receptacle (which means it just won't work).

You don't need to do this for recording - it always sends the S/PDIF receiver input to the i2s receive pin. But you must insure that the parameters for the reception actually match up with the input source.