Adding TOSLINK / optical input to 1701 project?

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?

  • Nice work, thanks for sharing. 

    This might be the most expensive way to do it, though!  The AD1895 is $6, and the WM8804 is $7, so that's a lot of added cost.

    Curious - why didn't you have the ADAU1701 be a clock slave from the WM8804, so you wouldn't need the AD1895?

  • ADAU1445 is about $12, plus a $2 DAC (codec, really), so that's about a $10 increase in price.  A lot for a consumer item.  I was hoping to find a solution in the $2 range.

  • 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.

  • Well hopefully ADI is listening to my begging as well.

    I can see that benefit from the AD1895. I can live without continuous analog inputs briefly during a switch to digital.  (I don't expect many sample rate changes from consumer connections though.)  A reload is a bit of a hassle, but it's workable.

    I think I'll have to try the DIR9001.

  • jon71 wrote:

    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!

    You have my voice in support too!!

    AD should also release register control interface to the hardware configuration tab in SigmaStudio for ADAV4601.