Re: Solutions for sending triggers in an AEP (EEG) experiment? (Andreas Widmann )


Subject: Re: Solutions for sending triggers in an AEP (EEG) experiment?
From:    Andreas Widmann  <widmann@xxxxxxxx>
Date:    Mon, 8 Sep 2008 13:21:00 +0200
List-Archive:<http://lists.mcgill.ca/scripts/wa.exe?LIST=AUDITORY>

Dear Pawel, > That's very interesting, would you please elaborate a bit more on > that? What do you mean by "very low millisecond range"? Do you by-pass > the Windows kmixer, whose latency is 30 ms? I thought than only using > ASIO drivers or so-called "Kernel Streaming" which is available in > certain audio player application can by-pass the kmixer, and sound > produced with DirectSound has to have latency of ~30 ms. Sorry, if was imprecise here: This solution does not provide any streaming support, but only plays sounds from (pre-loaded) buffers. With "latency" I referred to the delay between "play sound" command and actual sound onset (and not streaming input to output latency as used in relation with ASIO). I have measured the delays this morning as I didn't remember the exact numbers: playsound command itself takes about 350 Ás to execute, sound onset is about 400 Ás after playsound returns (including some small trigger mex api overhead). Jitter is about 50 Ás and clearly never above 100 Ás (I did not see a single outlier in several thousand measurements this morning, but one day of course there will be one ;) That is, in our setup the trigger onset precedes sound onset by 400 Ás with virtually no jitter (relative to our EEG sampling rates more than an order of magnitude longer). As Cogent is closed source I do not know whether and how they bypass the kernel mixer, sorry (I asked them for source code access some years ago in order to add multichannel support, but did not get a positive response). In principle it should be possible to bypass kmixer also with DirectX (see e.g., http://msdn.microsoft.com/en-us/library/bb219700(VS.85).aspx), but I'm not sure whether this approach is really used in Cogent. Are you sure that the 30 ms latency refers to all (non-ASIO) hardware? Hope this helps, best Andreas > Daniel, > > As for "synchronizing" the parallel port with sound onset: I don't > know if it will help you, but in my (previous) setup (which was for > single unit recordings, actually), I used a feedback solution similar > to Daniel's: a square pulse combined with the audio stimulus into a > stereo file to be played together. The pulse was then converted to > TTL with an optocoupler, and read into the same computer via the > printer port, which was also used to read TTL inputs from the spike > discriminator. I was able to poll the port frequently enough to have >>90% of poll-to-poll periods below 1 ms, and all (or close enough to > all) below 2 ms. This was done with a custom-written software under > windows XP. So you could basically keep checking your printer port for > sound onset and upon detecting it immediately output your 8-bit > trigger via output lines of the printer port, if ~1ms accuracy is good > enough for you. > > Pawel >


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