Audio observations

From a few audio captures I did yesterday, I found something peculiar. If you string a bunch of sample data to the DDA port (in DDA mode), with the volume something relatively loud and you wait for the DAC to ‘return to zero (centerline), and then you set the volume to silence ($c0) – you’ll get an interesting pop on the DAC line (assuming the last sample was far enough away from center line to give that effect).

This ‘pop’ has a return to zero phase like that if you write a sample and let it drift. And yet, no sample was written. And it’s not an audio spike and then quickly back to centerline. This pop seems to be only relative to a pan left/right that’s active (not silenced).

Now, $c0 is supposed to be the lowest volume – silence. But I don’t think it silences the DAC so much as it just prevents any sample output (be it DDA or normal channel mode). That’s a big difference. The weird thing is, is that the last sample written to DDA seems to get re-output on the DAC before is ‘off’. I’d go a step further and say that not only is the being silenced with the lowest volume setting, it’s probably just turning the DAC off all together. The equivalent to writing $00 to $804. IIRC, Air Zonk curiously puts a channel normal into DDA mode and writes $0f before shutting it off to update the channel buffer.

So with that, I have more tests to do today 😀


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