Hello folks,
Quick Introduction - I'm the systems engineer who adopted these parts! They are getting a little old now, with little possibility for any silicon changes at this point.
I too asked about the S/PDIF output control a number of years ago with regards to these parts.
It was pointed out to me that it's expected that the following DAC and Amplifier for S/PDIF out is responsible for volume control. Similar to what previous posters have said. However, I think it makes a lot more sense if you start to worry about compressed audio data as well. As soon as you touch a compressed audio datastream, you corrupt the data. In fact, many other systems such as TV's and DVD players with S/PDIF out, don't modify their S/PDIF outputs either. I would need to go digging through S/PDIF specifications to see if there's any mention of volume control pre-transmit.
From what I've read here, the datasheet is rather clear on the impact of the standard windows mixer on the PCM29xx S/PDIF output. I understand that is a frustration (it was for me when I spotted this functionality too!), however, at this time, there is little we can do to change the functionality.
Thank you again for evaluating TI silicon. Rest assured that as we begin to investigate USB devices in the future, I'll add pre-transmit-volume-controls as part of the feature set! :)
Thanks again
Dafydd Roche