Very interesting, I'd love to see more info, like the numbers and the
patch you used to run these tests. Its a good start.
.hc
On Jul 19, 2006, at 10:35 AM, rubber glove wrote:
I managed to run some tests on the HID timing.
The contestants were: -regular HID object in OS X -regular HID object in OS X with blocksize=64 -dissected HID object in OS X (i basically tore the read functions out of the hid code, and
put them in my own object, which was recording the values. I did a
messy, quick job of it, but...) -ctlin midi object in os x -regular HID object in Linuxthe winner, with < 4 ms of timing error was:
linux... I was short on time in the lab (getting the eMac machines
booting off the livecd took a bit of time), so I wasn't able to try
out different latency settings, or try the midi controller in linux.the others were all approximately the same, at around or just above
a 10 ms spread in timing offsets (I don't have the numbers in front
of me.... but One interesting thing is the standard deviation. it
seems like mostly the timing error is very close to a mean value,
then periodically, there will be a value that is way off, which
throws off the average. I haven't made enough measurements to see
if there is some sort of periodicity to those 'way off' readings,
maybe due to some process switching/memory management by the kernel?)the smaller blocksize did slightly better than the default setting,
the dissected object did slightly better than that... so I would hazard the guess that the delays due to control message
passing are not a huge contribution.and for some reason, the midi object did even worse than the hid
object. not at all sure why that is, but it could have to do with
the midi devices I was using - who knows.One thing I realize I should have done is try pd in os x as
superuser, as this may give the process a higher priority? I ran
sudo pd in linux to have access to the hid devices...I'm not sure how much more experimentation I will be doing on this,
as they are pretty satisfied with < 5 ms error and probably won't
pay me to just play around with their expensive timing test
equipment....-martin
On 7/5/06, Hans-Christoph Steiner hans@eds.org wrote:
On Jul 4, 2006, at 10:08 AM, rubber glove wrote:
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...
I would also like to see the [hid] code be as low jitter as
possible. I haven't had a chance to measure it yet tho. Unless
things are really taxing the CPU, I don't think that the Pd
messaging will cause more than ~1.5ms of jitter (1 block of 64
samples divided by 44100 samples). Perhaps running Pd at 96KHz or
192KHz would reduce that jitter, provided that the block size
remains 64.I think that the first step is to devise some tests to find the
true source of the jitter. I did some quick searches about USB HID
jitter and didn't find much, except this:http://elists.resynthesize.com/max-msp/2005/08/1515529/
It seems that this person thinks that Mac OS X's USB MIDI driver
has lower jitter than the USB HID driver. Ultimately, if we find
the source of the problem, I would like to include it in the next
version of [hid]..hc
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If you are not part of the solution, you are part of the
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