The main factor seems to be that pd only processes control messages in
between audio frames. Reducing the audio latency seems to bring the
jitter down to about 5ms but no further when I try it on WinXP or Linux.

Aha... that makes sense to me. So it is not so much just the 'hardware to pd' step, but also the control message passing within PD that is throwing off the timing here.

 

I would expect USB to add more jitter, as it passes through a more
complex path than serial or MIDI data before being handed to pd.

I think I am basically stuck with USB no matter how I go, as the computers involved here are all eMacs. Using midi or serial would involve a USB interface anyway.

 

If you need better timing, why not record the audio into one channel
along with the button on the other channel? There is essentially no
jitter with the audio stream because it passes through the sound
hardware at a fixed rate. The button could just emit a click when
pressed. Then you have no jitter beyond the time it takes to push the
button.

That's an interesting possibility, but I am only really using the button clicking as a simple way to test the timing. In actual use, the input would be a range of values from the rotary encoder of the knob... converting that into audio and back into numerical values seems altogether too complicated (frequency modulation?)
 

However, it brings to mind other possibilities:

If I modify the HID object to convert it into a signal-based external, then pass the values as signals, the timing should be much tighter? This seems to have the advantage that the input values will always be automatically lined up with the audio, no matter what the latency is set to.

Alternatively, If I incorporate the HID code into my external, and process the knob events myself, I would avoid any additional timing errors and be left with only the jitter inherent to the device, no?

Thanks much for the info...