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Butterworth Filter

Discussion in 'Electronic Design' started by [email protected], Jun 22, 2005.

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  1. Guest

    Hello,
    I am working on data acquisition. I have collected some EMG data and
    now according to the research papers, the EMG data has to be filtered
    using 2nd order dual pass butterworth filter with the cut-off frequency
    of 40 Hz. What is 'dual pass'? Can anyone elaborate on it.

    thank you
     
  2. probably means 2 pole. there are some "sallen and key" websites that
    will help



    martin
     
  3. Guest

    2nd order means 2 poles but what does 'dual pass' mean?
     
  4. Rich Grise

    Rich Grise Guest

    EMG: ElectroMyeloGraph?

    The EMG data has to be filtered, yes.

    Going for a "2nd order dual pass butterworth filter with the cut-off
    frequency of 40 Hz" tells us that you're not paying attention to what's
    important here.

    The guy who wrote the paper you're trying to copy used a certain filter.
    Fine. What are you trying to accomplish with this filter? EMG data
    changes relatively slowly, AFAIK, compared to sample rates of some
    modern-day ADCs.

    If the particular textbook/examination papers for that particular
    filter are what you're looking for, then please go make love to the
    books on your own. If it's a flagrant test question, then go find out
    if your d!ck reaches your a55.

    Otherwise, filtering for EMG could make a really fun topic for
    sci.electronics.design ! I'd bet there's some spikes below 1Hz,
    but wildly varying, which would make the tracking of them all
    that much more fun!

    I'd use a cap and a resistor, and fudge around with the values until
    I got a usable output.

    Anyone?

    Cheers!
    Rich
     
  5. Fred Bloggs

    Fred Bloggs Guest

    "dual pass" could possibly mean a notch filter. A notch has two
    passbands and a bandlimited stopband. The requirement is probably to
    notch the power line frequency.
     
  6. Mac

    Mac Guest

    Maybe dual pass means you have to put the data through the filter twice,
    once forwards, and once backwards.

    --Mac
     
  7. Ban

    Ban Guest

    Yeah, this would indicate a digital filter.
    But when an analog filter is meant, maybe it is a cascaded Butterworth,
    which forms a Linkwitz-Riley filter of 4th order. The word "dual pass" is
    meaningful only inthe digital domain.
     
  8. Guest

    Hey Rich,
    no one asked you for your a55 hole opinion.
     
  9. Rich Grise

    Rich Grise Guest

    Maybe not, but I do note that you've singled this out of several other
    options.

    So, I'm guessing that it really _was_ a test question, am I right?
    (or homework - same difference)

    And I really, really wish that googlegroupies would clue up to
    copying and pasting context, if they can't find that other google
    option, which supposedly automatically quotes context when you
    post a followup.

    Thanks,
    Rich
     
  10. ----- Original Message -----
    From: <>
    Newsgroups: sci.electronics.design
    Sent: Thursday, June 23, 2005 12:20 AM
    Subject: Butterworth Filter

    Hello Dan,

    I bet it means run the signal twice through this filter.
    So it's finally a 4th degree filtering.
    The trick is to run it backward through the filter in the
    second pass. This removes nonlinear phase and delay.
    It can be only done with with digital signal processing.

    http://www.abdn.ac.uk/~psy359/dept/Papers/obsavoid.pdf
    "The raw X-, Y- and Z-coordinates
    of each IRED were digitally filtered by a dual pass through a
    2nd-order Butterworth filter with a cut-off frequency of 20 Hz
    (equivalent to a 4th-order filter with no phase lag and a cut-off
    of ~16 Hz)."

    Best regards,
    Helmut
     
  11. Terry Given

    Terry Given Guest

    Hi Helmut,

    can you clarify "run it backward" please?

    do you mean take N samples, n = 0...(N-1)

    and feed through the filter, giving y0...yN-1

    Then starting with the last output YN-1 and working backwards to the
    first output Y0, feed them through the same filter to give yy0...yyN-1 ?

    So the second pass through the filter is in negative time, hence the
    phase lags cancel.

    Cheers
    Terry
     
  12. Hello Terry,

    yes I have exactly tried that. The only thing we additionally need
    is reversing the order of the samples after the second run through
    the filter. y(N)=y(0), Y(N-1)=y(1), ....
    I had a hard time with Scilab to check this, because I had no
    experience with it before.
    The result has been indeed a filtered signal with zero delay.

    The example below runs a stored data sequence two times
    through a 4th degree lowpass filter. The second pass
    is done with the sequence reversed(last sample first).
    The result is a zero delay dual pass 4th order Butterworth filter.

    Best regards,
    Helmut



    Scilab example
    --------------
    ! The for-loops reverse the data sequence.
    ! fg=0.02*fs fs=10kHz

    t=(0:1e-4:0.1);

    sigbase=sin(2*%pi*t*50)+0.5*sin(2*%pi*t*150);

    signoise=sigbase+0.5*sin(2*pi*t*1100);

    [hz]=iir(4,'lp','butt', [0.02 0], [0 0]);

    y1=rtitr(hz(2),hz(3),signoise);

    y2for=rtitr(hz(2),hz(3),y1);

    for j=1:1001, y1rev(1,j)=y1(1002-j);end ;

    y20rev=rtitr(hz(2),hz(3),y1rev);

    for j=1:1001, y2rev(1,j)=y20rev(1002-j);end ;

    xbasc
    plot2d(t,signoise,style=3);
    plot2d(t,sigbase,style=1);
    plot2d(t,y2for,style=4);
    plot2d(t,y2rev,style=5);
     
  13. Hello,
    sorry, I forgot the % in front of pi in one place.


     
  14. Terry Given

    Terry Given Guest

    Hi Helmut,

    thanks for that. I've archived it, its a neat idea, and has got me
    thinking.

    Cheers
    Terry
     
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