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Re: audio-video asynchrony with a USB soundcard
Did you try changing USB poll rate? By default it's 125 Hz, but can be
increased to 250, 500 or probably even 1000 Hz (though this seems to
be not recommended). From what I found out, this is sometimes done by
gamers to improve mouse responsivity, but the change should affect all
USB devices. So I guess this could possibly reduce the A-V asynchrony
with a USB card.
I found a program to change USB poll rate here:
If you want to try it, make sure that disable "Apply realtime" in
Settings. With this setting off, you will have to reboot to apply, but
every time I tried with the setting on, the system stopped responding
One more important thing: changing USB poll rate is considered a
strong tweak, with possible negative influence on the system or your
USB device(s). I tried it and my systems still work, as does my Fast
Track Pro, and other USB devices. But please don't hold me responsible
should your system were damaged in any way!
However, if you dare trying it and measuring the asynchrony, I'd be
happy to know the result!
2008/12/5 Massimo Grassi <massimo.grassi@xxxxxxxx>:
> Dear list members,
> I measured yesterday the audio-video asynchrony with an USB sound card
> (M-AUDIO Fast Track Pro).
> In practice, I was running a visual display and, in correspondence of
> one particular video-target-frame, I was switching on a 1-cycle sine wave
> (1000 Hz). I checked whether the video-frame and the audio signal were
> In general, the video preceded always the audio (although, if any, I had to
> expect the opposite result because in the program the soundplay command
> preceded the play the video-target-frame command). The average
> video-anticipation was of 6.3-ms (SD=3.0, maximal video anticipation 12-ms,
> min=2-ms). I forgot: the video was running at 100-Hz refresh rate.
> To summarise:
> 1) audio and video signals are often asynchronous;
> 2) the asynchrony (unfortunately) does not have a fixed value but changes
> within a certain range.
> It is not as bad as I was expecting but with a PCI sound card (a very cheap
> one) and identical set-up I observed an average asynchrony of 2-ms
> All the best,
Pawel Kusmierek PhD
Department of Physiology and Biophysics
Georgetown University Medical Center
The Research Building WP23
3970 Reservoir Road NW
Washington, DC 20007
phone: +1 202 687-8851 or 8028, fax: +1 202 687-0617