Re: Solutions for sending triggers in an AEP (EEG) experiment? (Bob Masta )

Subject: Re: Solutions for sending triggers in an AEP (EEG) experiment?
From:    Bob Masta  <audio@xxxxxxxx>
Date:    Wed, 10 Sep 2008 08:26:50 -0400

On 9 Sep 2008 at 11:30, Pawel Kusmierek wrote: > Hi all, > > Andreas: > thanks for the information. It is quite possible that it was me who > was imprecise or plainly wrong, probably because my knowledge of > windows sound system was insufficient. I thought that unless a Kernel > Streaming or ASIO driver is used, the delay from "playsound" to actual > sound being started at the soundcard output must be 30 ms. I think I > need to read about this more thoroughly - although only for fun, as my > current setup plays sounds and records neural (and other kind of) data > in the same piece of hardware, so the processes are very well > time-locked. So I don't have to worry about it anymore. > Note that the fact that the same piece of hardware is used does not automatically assure time-locking under Windows, although for sound cards under DirectSound and later it at least allows the possibility, if you don't also need to control sample rate. With the more primitive "wave" API functions synchrony typically depends upon sample rate, such that multiples or certain submultiples of the card's design rate (48000 Hz, usually) are synchronous, and other sample rates like 44100 may drift. Also, note that under Windows, using any Microsoft API tha I know of, it is not possible to set or determine the sync point... the input and output may be running at exactly the same rate, but the alignment is unknown. I have found that (with wave API methods, at least) it's possible to get +/- 1 sample alignment (and usually 0) by careful attention to the code that starts the input and output processes, along with a one-time calibration process that generates pulses on the output and watches for them on the input to determine the time alignment. Best regards, Bob Masta D A Q A R T A Data AcQuisition And Real-Time Analysis Scope, Spectrum, Spectrogram, Signal Generator Science with your sound card!

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