Re: audio-video asynchrony with a USB soundcard (Pawel Kusmierek )

Subject: Re: audio-video asynchrony with a USB soundcard
From:    Pawel Kusmierek  <pawel.kusmierek@xxxxxxxx>
Date:    Fri, 5 Dec 2008 10:02:55 -0500

Massimo, 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:;8229235;/fileinfo.html 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 anyway. 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! Good luck, Pawel 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 > synchronous. > > 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 > (SD<4-ms). > > All the best, > m > -- 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

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