appropriate SampleBlockSize

General information about brain-computer interfaces
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Ramiro
Posts: 7
Joined: 23 May 2013, 13:42

appropriate SampleBlockSize

Post by Ramiro » 28 Sep 2014, 23:13

hi,
I have plans to acquire EEG signals with g.MOBIlab+ 8-channel device, which has a sampling frequency of 256 Hz.
The question is: When configuring BCI2000 parameters , if SampleBlockSize = 8, I am limited to a maximum frequency of 32 Hz? in other words, the signals can not be more than 15 Hz?
Therefore, if I want more frequency I must decrease SampleBlockSize?

Thank you.

boulay
Posts: 382
Joined: 25 Dec 2011, 21:14

Re: appropriate SampleBlockSize

Post by boulay » 29 Sep 2014, 08:41

The maximum frequency content of your digitized signal is determined by the sampling frequency, not the SampleBlockSize. The SampleBlockSize determines how many samples are transmitted together for each iteration of the signal processing pipeline. It is often too demanding on the CPU to run the processing pipeline after the acquisition of each individual sample (i.e. every 1/256 = ~4 msec). The signal processing simply cannot complete its tasks in 4 msec before the next sample arrives. If you set the SampleBlockSize to 8 then the signal processing has 31.25 msec to complete its tasks. The tradeoff is that your feedback will only get updated once per block, so you want to keep your SampleBlockSize low enough that the feedback updates in an apparently smooth manner.

Ramiro
Posts: 7
Joined: 23 May 2013, 13:42

Re: appropriate SampleBlockSize

Post by Ramiro » 30 Sep 2014, 01:00

Thanks, now I understand.

Greetings.

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