Found out something about the ADPCM commands on the AWE64.
The maximum sample rate possible is quite a bit higher than the official sample rates published in Creative's official documentation.
You can go up to 25KHz with 2-bit ADPCM playback.
However if you exceed the maximum possible on AWE64 the decoded output sounds jittery, as if the DSP can't keep up decoding.
The 20-byte (8-bit) / 40-byte (16-bit) FIFO load is consistent although it's only in samples, not sample pairs. Stereo 8-bit playback doesn't load 40 bytes and 16-bit stereo playback doesn't load 80 bytes.
There is one graph in the commit that suggests this 40-byte FIFO only loads about as 2x as fast as playback when playing something as high bandwidth as 16-bit stereo PCM at 45454Hz.
This was done with the 16-bit DMA on a 16-bit wide DMA channel. I'll do another test in the future to see if DMA timing changes if I use the same 8-bit DMA channel for both 8 and 16 bit PCM.
EDIT: Yes, apparently 16-bit stereo PCM at 45454Hz is fast enough that when you assign both DMA channels to an 8-bit DMA channel on the ISA bus, the data rate it reads over ISA is hardly faster (about 1.1x) than the data rate needed to play 16-bit PCM at 45454Hz.
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