Forums

FFT Questions

Started by Maria March 31, 2004
Hey Rick,

Thanks for clarifying where she's coming from (processing
data already sampled).

r.lyons@_BOGUS_ieee.org (Rick Lyons) writes:
> [...] > "so all I know is that it > was recorded, filtered at 5kHz and > digitized at 10-20 kHz using a Digidata > 1200 A/D board. The data has been > downsampled to 2000Hz." > > That's sounds like lowpass analog filtering > with a cutoff freq of 5 kHz. I wonder how sharp > are the filter's skirts. Then the "digitized at > 10-20 kHz" phrase was also kinda "scary" because the > sample rate needs to be well-known and fixed > for FFT results to have meaning in terms of > spectral components' frequencies.
I agree - we don't know what her sampled data situation REALLY is.
> I didn't > know what her phrase "downsampled to 2000Hz" meant. > Wonder if that meant some sort of frequency > translation, or could it have meant, "decimated > to a sample rate of 2000 Hz"?
My first reaction is just that it was decimated - I don't know if she meant the sample rate came down to 2000 Hz or the data bandwidth.
> Her problem sure sounds interesting and one that > can be solved. I'll bet this is a university > research project. If she's in North Carolina > Randy, you could pay her a visit. If she's in Northern > California, I could stop by and look at her test > hardware. Sure sounds like any interesting project > to me.
You could probably get 10 usenet posts ahead if you could just talk to her interactively (i.e., on the phone). She could be at UNC/Chapel Hill since they have a biomedical engineering program and lots of medical stuff going on there (not to mention Duke). Of course she could be in Berkina Fasa, too. -- % Randy Yates % "Though you ride on the wheels of tomorrow, %% Fuquay-Varina, NC % you still wander the fields of your %%% 919-577-9882 % sorrow." %%%% <yates@ieee.org> % '21st Century Man', *Time*, ELO http://home.earthlink.net/~yatescr
Maria wrote:
> Hi, > > I am using Matlab to do ffts for some data collected in the lab of > electrophysiology recordings. When I plot the power spectrums, I get > huge peaks from 0-1 Hz. Are these peaks representative of the data or > artifacts carried over/created from the fft? > > Any help would be appreciated. > > Thanks.
Based on various posts in this thread I suggest a literature search for Dr. Edgar Gasteiger Veterinary Physiology Cornell University circa late 1960's I worked for him and Dr. Nangeroni as a student electronics tech. The problem description brings back vague memories but too few useful details. I've initiated personal contacts. If anything comes up, I will post.
> Maria wrote: >> Hi, >> >> I am using Matlab to do ffts for some data collected in the lab of >> electrophysiology recordings. When I plot the power spectrums, I get >> huge peaks from 0-1 Hz. Are these peaks representative of the data or >> artifacts carried over/created from the fft? >> >> Any help would be appreciated. >> >> Thanks.
Perhaps they are DC offsets? -- Toby asktoby.com BSOD VST & ME
"Toby Newman" <google@asktoby.com> wrote in message
news:Xns94C5846EA980Fgoogleasktobycom@127.0.0.1...
> > Maria wrote: > >> Hi, > >> > >> I am using Matlab to do ffts for some data collected in the lab of > >> electrophysiology recordings. When I plot the power spectrums, I get > >> huge peaks from 0-1 Hz. Are these peaks representative of the data or > >> artifacts carried over/created from the fft? > >> > >> Any help would be appreciated. > >> > >> Thanks. > > Perhaps they are DC offsets?
Toby, At the first spectral sample could be.... But not at any other spectral samples unless there's a window that will spread the spectrum of the dc component. Fred