Scott Seidman wrote:
> Tim Wescott <[email protected]> wrote in
> news:[email protected]:
>
>
>>That is my understanding, too. Come to think of it, I'm not sure if it
>>would be as much of a case with EEG and polygraph machines, because they
>>don't have the same sort of sharp transient embedded in bumps the way
>>EKGs do.
>>
>
>
> EEG is much more random than ECG, and you can't really learn as much even
> from a perfectly recorded signal. From ECG, things like the width of the
> spike (the QRS complex) are critical for understanding of the heart
> physiology, as is the time delays between one part of the wave and what
> follows.
>
> The useful ECG passband only goes to 250 Hz or so. There are some really
> good noise limiting techniques that are often used, like 90dB or more
> common-mode rejection amplifiers, and active ground ("driven right leg")
> designs. It shouldn't be that tough to get useful analog schemes, as it's
> been done for decades on chart recorders (and maybe even kymographs!!). If
> I needed to do it accurately and digitally, I'd probably go with a sample
> rate of about 4 khz, a nice gentle anti-aliasing filter at about 400 hz,
> and maybe some nonlinear smoothing (median filtering and Savitsky-Golay
> would both be tested)
>
>
This pretty much confirms my understanding. I've spend the last two
hours of my day finding a pretty picture of an EKG waveform on the web,
synthesizing a fake one in SciLab, and filtering it a couple of
different ways for this article I'm writing. I'm laughing right now
because right before I read your post I found out that if I filter it at
about 400Hz it looks pretty good...

In light of what I've seen and what you've just said your nonlinear
smoothing certainly sounds interesting. I would add to that that should
I be tasked with storing an EKG recording in some storage-sensitive way
I'd probably look at some adaptive compression scheme. I'm really not
sure if I'd consider lossy compression without lots of research (and
indemnification!) behind me, but one could probably do some sort of
lossless compression that would vary the compression rate depending on
the signal (and consequently possibly run out of storage -- dangit, more
indemnification, please!).

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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