[PD] PD latency and timing accuracy with HID

[rubber glove]

> 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...

-Martin.
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