- From: Bernd Porr <BerndPorr_at_f2s.com>
- Date: Fri, 04 Mar 2005 09:22:20 +0000
The reason for using a notch filter is that I don't want to throw away
all the frequencies above 50Hz. In my case it was ECG. You can use a low
pass at around 40Hz to filter an ECG but then the peaks of the ECG
become much smaller. Bad for the patient. ;-) The full spectrum reaches
up to 100Hz. When I sample with 1kHz I don't need an anti alias filter,
thus no soldering, etc.
The notch filter takes the raw data from comedi and filters it.
Afteraging comes after the notch filter. I think the only thing to
change is the frequency to 60Hz. Or you could just add a text field so
that everybody can change it. It also works pretty good as an DC filter
with f is set to zero.
/Bernd
www: http://www.berndporr.me.uk/
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Daniel Nilsson wrote:
> On Thu, Mar 03, 2005 at 09:36:42AM +0000, Bernd Porr wrote:
>
>>You can use the program
>>http://www.linux-usb-daq.co.uk/software2/comedi-record/
>>to sample quite slowly. It internally always samples at 1kHz and then
>>averages on the fly to get a lower rate. It also has a 50Hz notch filter
>>(sorry, American colleagues). However, the filter frequency can be
>>changed in the program. I used it also with thermocouple sensors.
>
>
> Bernd,
>
> Thanks for the pointer, this is very close to exactly what I was
> looking for. From looking closer at your filter characteristics in the
> code I see that you are creating a pretty sharp notch filter. From the
> description above I take it that the purpose of this notch filter was
> to only take care of the 50Hz noise ? Is there a specific reason why
> you decide to do a notch filter instead of a low-pass filter in your
> application ? Is the notch filter applied before the averaging
> function ?
>
> Thanks
Received on 2005-03-04Z09:22:20