Computing electrode offset?

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Uli
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Computing electrode offset?

Post by Uli » Tue Mar 22, 2005 3:45 pm

Hi all,

does anyone know how the electrode-offset values in
Actiview are computed ?

Are these values calculated directly from the data that is coming from READ_MULTIPLE_SWEEPS (Labview.dll) or are there special parameters that need to be sent via the API?

Best Regards

Ulrich

Coen
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Post by Coen » Tue Mar 22, 2005 8:10 pm

There is not really any computation involved, the offset values are basically the values (in uV) of each sample from each analog-to-digital converter. The ADC has an range of -262 mV to + 262mV, the LSB is 1/32th uV, these values define the scaling.

The ADC values are easily available in the ActiView code (in LabVIEW) as an output array of subvi "Downsample.vi", also see viewtopic.php?t=29. For off-line display of the electrode offsets, you can simply use the sample values from the BDF file. The nice thing of the special BDF file format is that the full 24-bit ADC values can be stored directly on file, without having to resort to high pas filtering to reduce the dynamic range (HP filtering would make the offset value unavailalble from the file)

Best regards, Coen (BioSemi)

brroach
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Post by brroach » Mon Nov 03, 2008 5:43 pm

"For off-line display of the electrode offsets, you can simply use the sample values from the BDF file."

What does this mean? Before I start my experiment I might see for example -10 for some electrode in the "electrode offset" tab of the software. How is that value stored in the .bdf file? I just need a bit more detail.

thanks,
Brian

Coen
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Post by Coen » Tue Nov 04, 2008 2:39 pm

n your example, you will see a DC voltage of -10 mV on the offset page , and an AC signal in the range of 100 uV in the monopolar display. The ActiveTwo is a DC system, it will store the AC signal superimposed on the DC electrode offset in the same data sample. So, in your example, you will find a string of samples with values fluctuating between -9,050 uV and -10,050 uV.

Best regards, Coen (BioSemi)

brroach
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Post by brroach » Thu Oct 13, 2011 7:27 pm

I have a question about these electrode offsets values in biosemi recordings. I know that less than 20mV is ideal but less than 40mV is acceptable. Is it appropriate to think that greater absolute offsets lead to increased channel noise? In biosemi documentation, you state that the exact placement of the cms/drl circuit (or perhaps just DRL specifically) should not be too important, but generally should be placed in/around the recording electrodes. Is it reasonable/correct to think that electrodes further from the cms/drl circuit, such as an ear or nose electrode in the standard 64 channel setup, would have higher offsets because of the physical distance from cms/drl?

I am wondering if offset value can be used to assess noise/data quality. For one-bucket data, these values allow us to identify bad electrodes, but one-bucket data values probably don't make sense as covariates in human test data, do they?

Coen
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Post by Coen » Tue Oct 18, 2011 5:10 pm

High and/or unstable offsets are an indication of contact problems and/or polluted electrodes. Contact problems and electrode pollution are certainly causes of increased electrode noise. However, there is not a one-to-one relation between offset level and noise level.

Offsets are displayed with respect to the CMS electrode. The DRL electrode is within the CMS/DRL feedback loop, and therefore it's offset only plays a minor role. The physical distance between CMS electrode and other electrodes does not play a role, offsets are determined by skin, gel and electrode properties.

Values for noise and offsets are generally lower in salt water than on the skin, because the water-electrode interface is more stable than the skin-gel-electrode interface (where things like skin properties, respiration, pollution, etc. also come into play). The values seen in one-bucket test can be regarded as the lower limit of what can be achieved on the skin under optimal conditions

Best regards, Coen (BioSemi)

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