On Jan 27, 9:38�am, Jerry Avins <j...@ieee.org> wrote:> On 1/27/2012 6:47 AM, Rick Lyons wrote: > > > > > > > On Fri, 27 Jan 2012 03:50:08 -0500, robert bristow-johnson > > <r...@audioimagination.com> �wrote: > > >> On 1/26/12 9:58 PM, Rick Lyons wrote: > >>> On Tue, 24 Jan 2012 09:21:59 -0600, "marieclare" > >>> <marieclare.mccormick@n_o_s_p_a_m.uws.ac.uk> � wrote: > > >>>> I am using MATLAB to analyse EMG signals. �So far I have rectified the data > >>>> and performed a STFT (using a Hamming Window). �I am now looking to > >>>> calculate the median frequency of the power spectrum and would be grateful > >>>> if someone could advise me on the best way to do this. > > >>>> Thanks > > >>> Hello marieclare, > >>> � � If you have signal, let's call it signal "A". > >>> If you rectify signal "A" you have a different signal > >>> that we'll call signal "B". � I hope you > >>> realize that the spectrum of signal "A" will not > >>> be the same as the spectrum of signal "B". > > >> oh c'mon Rick, you're not saying that two signals need be at all alike > >> to have the same spectrum. �say A is a dirac impulse and B is white > >> noise. �two impossible signals with real-world approximations that have > >> nothing to do with each other except, um, i forgot. > > > Hi Robert, > > � �no, I sure didn't mean to imply that two signals need > > be at all alike to have the same spectrum. �Perhaps > > I misunderstood the original poster. �It just seemed odd > > to me that he said he's analyzing an EMG signal and I > > assumed by "analyzing" he meant spectrum analysis. > > And then he said he was rectifying the EMG signal, and > > that would produce a signal with a different spectrum > > than the spectrum of the original EMG signal. �I was merely > > trying to point that out to the original poster. > > > Being that I know nothing about analyzing EMG signals, > > perhaps I should just keep quiet. > > I don't know how modern analyses of EMGs are done, but I suspect that > the term "rectify" here isn't related to absolute values, but maybe to > taking the tilt out of the baseline. I've heard it used that way in > Auger spectrometry. > > Jerry > -- > Engineering is the art of making what you want from things you can get. > �����������������������������������������������������������������������- Hide quoted text - > > - Show quoted text -Hi Jerry, I know very little about this (my research advisor was into bio- medical stuff), but from what I understand, EMG is the electromyographic signal produced by a muscle. The signal is indeed rectified. The sequence, as I understand, is to rectify (absolute value), low-pass filter to get the envelope, and then integrate the filtered signal to determine the level of muscle activity. That's all my knowledge on the subject :>) Maurice
STFT and median frequency of EMG signal in MATLAB
Started by ●January 24, 2012
Reply by ●January 27, 20122012-01-27
Reply by ●January 27, 20122012-01-27
On Fri, 27 Jan 2012 10:41:09 -0500, Jerry Avins <jya@ieee.org> wrote:>On 1/27/2012 7:06 AM, Mac Decman wrote: > > ... > >> I don't think the OP is dealing with spectrums as much as he is >> dealing with IF of the signals. Not sure but from what I have read on >> the subject I believe this. I would be happy to hear the reply from >> the OP. > >I meant to ask before. It can't be "intermediate frequency". What does >IF mean here? > >JerryInstantaneous Frequency. Sorry I forget how many people are in communications. There's a little acronym overlap. I'm pretty sure that is what the OP really wants since he mentioned STFT. Mark DeArman
Reply by ●January 28, 20122012-01-28
On Jan 27, 4:57�pm, Mac Decman <dearman.m...@gmail.com> wrote:> On Fri, 27 Jan 2012 10:41:09 -0500, Jerry Avins <j...@ieee.org> wrote: > >On 1/27/2012 7:06 AM, Mac Decman wrote: > > > � ... > > >> I don't think the OP is dealing with spectrums as much as he is > >> dealing with IF of the signals. �Not sure but from what I have read on > >> the subject I believe this. �I would be happy to hear the reply from > >> the OP. > > >I meant to ask before. It can't be "intermediate frequency". What does > >IF mean here? > > >Jerry > > Instantaneous Frequency. �Sorry I forget how many people are in > communications. There's a little acronym overlap. �I'm pretty sure > that is what the OP really wants since he mentioned STFT. > > Mark DeArmanThinking out loud here. Take the FFT of the EMG signal. Once he gets the FFT, if he integrates the FFT, then he could get a spectrum envelope. Then take the final value and divide by 2. This would give the "half-spectrum energy" value. Find the frequency that corresponds to this "half-spectrum energy" value, and that would be the median frequency he's looking for. Maurice Givens
Reply by ●March 6, 20142014-03-06
On Tuesday, January 24, 2012 11:21:59 PM UTC+8, marieclare wrote:> I am using MATLAB to analyse EMG signals. So far I have rectified the data > > and performed a STFT (using a Hamming Window). I am now looking to > > calculate the median frequency of the power spectrum and would be grateful > > if someone could advise me on the best way to do this. > > > > ThanksHi, I am also using STFT to analyse my EMG data. Any guidelines for me to do on STFT? thank you :)
Reply by ●March 6, 20142014-03-06
Apparently there is a relationship between muscle fatigue and the frequency of the EMG signal. http://www.ncbi.nlm.nih.gov/m/pubmed/8785930/ So this might be why you would want to track the frequency. Bob
