[PD] PD latency and timing accuracy with HID

[rubber glove]

Hi:

I am currently developing a system using PD will record users' responses to
audio and video files. Essentially, the subject (this is for psychology
experiments) will turn a knob indicating their like/dislike (or whatnot) of
an audio or video file. For input, I am trying to use a Griffin PowerMate
knob with the HID object ( http://at.or.at/hans/pd/hid.html ). I have
written a simple external which keeps track of timing (using a timer from
flext), and writes the data to a text file.

Timing is important to the researcher. She has a system called the 'black
box toolkit' http://www.blackboxtoolkit.co.uk/ and I have
done some tests to determine the latency and timing accuracy. For the test,
I have a puredata playing an audio file consisting of noise bursts at
regular intervals, while I periodically press the knob. The 'Black Box'
listens to the sound output and is also wired into the knob.
I expected to see the values recorded by my PD external to be offset by 50ms
(the latency setting) in comparison to the Black Box recorded values, but I
was a bit surprised to see the offset vary between 48 and 62 ms (especially
the 48ms!), which gives an uncertainty of 14ms. I was hoping for less than
10ms. The latency is not a problem at all, I am more concerned with accuracy
and consistency (so that I can simply compensate for the latency)

Where I should begin to try to improve this uncertainty (would I call it
jitter?). What are the likely and most significant factors?
Currently I have only tried the test under OS X, with the powermate knob. I
will try again under linux, when I can get a LiveCD that will install easily
on the old eMacs present in the lab. I will also try with a midi controller,
when I find one that I don't mind soldering some test leads onto.

Is the timing error most likely in PD, the HID object, the knob hardware (I
know, probably), or just usb?
Would a midi controller knob be expected to perform better?
And really, what would be the limit of this kind of timing accuracy? Is 10ms
as good as I can hope for?

Thanks a million.

-martin
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