FFT Questions

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