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Can a PET audio technique work on a Lisa?

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stepleton:
Another potential advantage of modulating the volume:

In PDM, the representation of "no sound" (that is, a flat line at 0) is 101010101010101010101010: 50% on/off at the highest frequency possible. If we just play that at a fixed volume, there are a few possible consequences, not quite all exclusive of each other:

(bad:)
- This frequency is audible, or within the Lisa's audio system, it generates audible resonant tones. Annoying!
- Same, but maybe too high for humans, still audible to any pets.

(fine:)
- The audio system doesn't have adequate bandwidth for that frequency and so it essentially gets smoothed out and behaves like a DC bias on the speaker.

My money is on the (bad) outcomes being more likely, in which case being able to turn the speaker right down is going to be a big quality of life improvement. But even if things are (fine), I'm still pretty sure that relieving PDM of the need to recreate the audio's dynamic range will be an important quality win.

sigma7:
When the VIA is shifting out bits, the maximum shift rate available is phi2/2

On the 2/10 I/O Board, the VIA phi2 is CPUCLK/4, so about 1.25 MHz.

On the Lisa 1/2 I/O Board, the VIA phi2 is the 68K's E clock, which is CPUCLK/10 (with a 60/40 duty cycle IIRC, but that doesn't matter here).
With a stock CPU Board, E is about 500 KHz, and with an original 16 MHz XLerator, E is 1.6 MHz.

So the slowest hardware configuration's maximum shift rate is 250 KHz per bit, making the byte rate about 31 KHz. So that suggests the output in the audible spectrum will essentially average to the count of high bits - one of nine amplitudes corresponding to 0 bits high to 8 bits high.

The shift register can be set to shift bits slower than that using the T2 counter, but then you'll hear individual bits.

I expect one can manage to write a new byte at the same rate as it is shifted out, so you end up with performance of a 3 bit 31 KHz DAC.

If you simultaneously change the volume, I suppose one might get close to a 6 bit DAC.

Good luck!

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