Hi,
Iam a bit confused here regarding sampling rate vs no. of samples.
If the Sampling rate is set to 250 SPS then why do we get 256 samples in the SD card data ? I need to design my Filter and its important to know what is the exact sampling rate ?
The Cyton sample rate is indeed 250 Hz. What are you seeing in the SD card data that leads you to assume that the rate is 256 Hz? You may be seeing the one byte sample counter, which does cycle through 0 to 255, then repeats.
Hi wjcroft,
Thank you for your reply. I was considering the one byte sample counter as sample number which made me think that there are 256 samples . One more thing I wanted to ask is that after filtering the raw data, the values which we get as filter output can be considered as actual microvolts value? for eg. suppose we get a value of -6092.334 as filter output so it should be considered as -6092.334 microvolts?
Comments
Sumit, hi.
The Cyton sample rate is indeed 250 Hz. What are you seeing in the SD card data that leads you to assume that the rate is 256 Hz? You may be seeing the one byte sample counter, which does cycle through 0 to 255, then repeats.
Regards, William
Hi wjcroft,
Thank you for your reply. I was considering the one byte sample counter as sample number which made me think that there are 256 samples . One more thing I wanted to ask is that after filtering the raw data, the values which we get as filter output can be considered as actual microvolts value? for eg. suppose we get a value of -6092.334 as filter output so it should be considered as -6092.334 microvolts?
Sumit, the Cyton samples contain a DC offset, which is typically removed with a high pass filter at .5, or a bandpass from say .5 to 45 Hz.
https://openbci.com/forum/index.php?p=/discussion/201/large-millivolt-data-values-fbeeg-full-band-eeg