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?
I have begged ADI to consider S/PDIF in future (smaller then -1445) SigmaDSP chips. My dream chip would be -1772 with S/PDIF and a -1701 core. More voices in support would be merrier!
One possible reason to use AD1895 between the Rx and -1701 is so that -1701 doesn't have to reset & load a new program for incoming sample rate changes (for example if its analog inputs have to keep running while the digital inputs are muted while acquiring a new lock). It also lets you pretend to run at double-speed rates while retaining the single-speed instruction set in the -1701 core.