Sampling frequency in Acquisition Client

Obtaining data from various hardware devices
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Amna.gh
Posts: 2
Joined: Tue Mar 31, 2015 5:42 pm

Sampling frequency in Acquisition Client

Post by Amna.gh »

Hi all,

First of all, thank you for developing easy to use GUI software. I have just started working on the software. So, I am acquiring data from BrainProducts amplifier and processing it in Openvibe (simple filtering and power spectrum analysis) and then sending the final matrix to LabView for further calculations.
Everything is working good for me, but there is just one concept I want to clear is about 'Samples Count Per sent Block'.
I've searched the OpenVibe forums, if the sample count per sent block = 32 and sampling freq = 512 then 512/32--> 1 Block represent 1/16 = 62.5 ms
so it means that 62.5 ms ---> 1 block then in 1s Acq. Server is sending 16 blocks to complete 512 sample/s. Am I right?
I am using BrainAmp Series (amplifier) and it's Fs is 5KHz (I haven't change ), because I can't find in acquisition server any option to change the sampling frequency. Is there any possibility if I want to change the sampling frequency?
And how this samples count works for spectrum analysis? If my sampling frequency is 5K and I am acquiring data for 4 channels, after temporal filtering I am doing time based epoching of 1s epoch duration with 0.5s epoch Interval.Then I am doing spectrum analysis and I extract 8-12 Hz using freq band extractor box. The final array I got is of 1024 length for 4 channels. That means 1024/4 --> 256 samples/channel in frequency domain. It looks like my sampling frequency is 512 Hz. Isn't it? How can I find my sampling frequency in acquisition client? Spectral analysis box is processing every block right? Can anyone explain how I am getting 256 samples/ Channel?

Thanks much :roll:

Regards,
Amna

gserrier
Posts: 71
Joined: Tue Apr 01, 2014 8:22 am

Re: Sampling frequency in Acquisition Client

Post by gserrier »

Hi,

You are right for the sample count by send block. To be more precise, the server will store data until it have enough data to send a block over the network. The timing is certainly not as precise as the one you calculate due to the operating system scheduler. However, as every data are timestamped, this is not an issue.

The parametrization of the sampling frequency of a device is completely depending on the device itself. The one you use doesn't let us the choice but we provide you a decimation factor (this setting is specific to your device). The base frequency is divide by the decimation factor. The default value is 10.

According to the Nyquist-Shannon theorem, you need to take a sampling frequency that is at least two times the max frequency you want to observes. So if you have a signal with a 512 Hz frequency, you can only observes frequency up to 256 Hz without having aliasing.

Guillaume

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