So I am doing a digital audio mod on one of my systems. Now I know the DC accepts/uses 11/22/44k no issues. I was thinking of using audio from PS2/XB/GC games that were also on the DC like Sonic Adventure or Space Channel 5 Part 2. I know it wouldn't be to much of a difference even with digital audio out and my high end speaker setup but it would be fun to do. As well as using some of these 900mb 100min CDR's I have laying around. Can't seem to find any info and if need be I can just test once the parts come in and the mod is complete. More less curiosity ATM for me. But I think it would be worth it with my xRGB and optical audio out. Besides would be a fun project if it works.
The audio clock comes from the GD-ROM drive, and it's 33.8688Mhz - this is a precise integer multiple of the 44.1kHz sampling rate, and I don't think this is a coincidence - especially as most DACs require either a 256Fs or 384Fs clock, and this can be obtained with a /3 or /2 of that reference clock at 44.1kHz. You could try lifting that pin and feeding in 36.864MHz - this should bump the audio rate up to 48kHz, but will probably screw up the audio pitches and timing, too.
The clock from the GD-ROM seems to be the master clock. There are actually a couple of other clock sources on that chip (HCLK from the G2 bus and a 32kHz xtal), but if you lose the 33MHz clock (say by unplugging the GD-ROM drive) then the chip stops responding entirely and the DAC clocks stop. My guess is that the GD-ROM clock is the main one, HCLK is just being used for the G2 bus interface and the 32kHz is used for the RTC (the fact that it's in the backup power domain supports this).
Anything that replaces the GD-ROM drive has to provide that clock - without it the audio simply doesn't work. That's also why the G1-ATA adapter board has a space to solder an oscillator.
