Chair potentiometer; trigger duration recommendation Sp.Mo.4

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tom_campbell75
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Joined: Fri Dec 04, 2009 12:34 am
Location: Center for Visual and Cognitive Neuroscience, North Dakota State University

Chair potentiometer; trigger duration recommendation Sp.Mo.4

Post by tom_campbell75 »

Dear Coen and colleagues,

Please I would appreciate and answers to a question about the minimal trigger duration in speed mode 4.

We would like to record EEG with millisecond accuracy of event timing. We would like to send TTL codes that encode the position of a horizontally moving chair in which the participant is sitting whilst EEG is recorded. We have set up a potentiometer that can pass out a voltage that corresponds to the horizontal position of chair. To get the best record of the time series of the chair position in trigger bits 1-8, we would like to send triggers for as short an interval as possible. In speed mode 4, please what would be the recommendation of the minimal trigger duration? Please would this vary as a function of decimation?

Best regards,
Tom Campbell.

Coen
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Post by Coen »

I assume that you use an 8-bit ADC to encode the position of the chair. If yes, then the optimal setup would be to use the sample rate clock of the ActiveTwo (pin 32 of the trigger port) to control the output rate of the chair position ADC.

Down sampling files recorded with the above proposed setup is best done with an offline tool (with modified down sampling routine for the trigger channel)

Best regards, Coen (BioSemi)

tom_campbell75
Posts: 18
Joined: Fri Dec 04, 2009 12:34 am
Location: Center for Visual and Cognitive Neuroscience, North Dakota State University

Post by tom_campbell75 »

Dear Coen,
Thank you very much for your helpful advice. I take it that pin 32 of the trigger port on the USB receiver outputs a series of short pulses at 2048Hz in speed mode 4. This we should use to control the sampling of the position of the chair potentiometer by a separate ADC that then send a position byte to the trigger inputs of the USB receiver. This would reduce the chances of a sample missing the position byte. We will try this. Please correct me if I misunderstood.
Best regards,
Tom.

Coen
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Post by Coen »

Your summary is correct. Use the rising edge of the pulses on the sample clock output (pin 32) to change the output of your ADC to a new value.

Best regards, Coen (BioSemi)
Last edited by Coen on Wed Jan 11, 2012 6:27 pm, edited 1 time in total.

tom_campbell75
Posts: 18
Joined: Fri Dec 04, 2009 12:34 am
Location: Center for Visual and Cognitive Neuroscience, North Dakota State University

Post by tom_campbell75 »

Thank you. This is helpful.

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