I have an older design using an ADAU1701, and we'd like to upgrade it to include consumer-grade TOSLINK (optical input), but I don't want to add too much cost. From my reading, this gets complicated, because the master clock for the ADAU won't be synchronized with the external source. So I assume I'll need an ASRC.
I see a few options -
1. Add a chip to convert S/PDIF to I2S as a slave to the source, then use an ASRC to sync the digital data into the ADAU1701. This seems quite complicated, but maybe there's a cheap IC somewhere that does all this?
2. Convert the S/PDIF into analog, which is mux'd or mixed to the ADAU analog input. This probably requires a receiver + DAC, so $$.
3. Switch to an ADAU1442 or similar (adds $5), which handles the digital side nicely, but requires a codec to retain my analog capability, so that's another $2 or so. The extra processing is nice but I don't really need it.
Given that this type of input is so common in inexpensive consumer gear, I'm surprised it's turning out to be so expensive - easily $5 plus the connector in quantity.
Any other options I'm overlooking?
Do you need -1701 to run continuously? If not, pull it into reset on transitions. The transceiver chips usually have lock flag pin for that purpose.
As you noted if you only need Rx and not Tx, those chips aren't as expensive.
I wouldn't bother with AC97 as they have a lot of other functions you don't need, you'd have to program them (vs. a Rx chip that can run in hardware mode), and the codecs are much worse than -1701. Also check those parts carefully as I think only S/PDIF Tx was mandatory, many of them don't have Rx. I only mentioned them because some consumer products might still use them as cheap SoCs. I believe ADI has made all of theirs obsolete, but a few other vendors still have them in their catalogs.