View Full Version : Re: Linear enveloping of EMG

Dr. Chris Kirtley
08-30-2000, 12:43 AM
Dear Tony,

Yes - I was confused by this for a while, too!

The answer is in the rectifier. This is the component that is
responsible for the linear envelope - not the low-pass filter. By
rectifying the signal, you are, in effect, moving everything downwards
in the frequency domain. This is also how a radio set works - the high
frequency carrier wave is removed (demodulated) by the "detector" (which
these days is a diode, but used to be a crystal in crystal sets). When
you remove the negative going part of the signal the shape of the top
(positive) half is the linear envelope. Of course, a bit of smoothing
helps, but all is does really is fill in the bits between the
positive-going pulses.

Here's a nice page I found that describes it in radio terms:

Hope this helps!

Dr. Chris Kirtley MD PhD
Associate Professor
HomeCare Technologies for the 21st Century (Whitaker Foundation)
NIDRR Rehabilitation Engineering Research Center on TeleRehabilitation
Dept. of Biomedical Engineering, Pangborn 105B
Catholic University of America
620 Michigan Ave NE
Washington, DC 20064
Tel. 202-319-6247, fax 202-319-4287
Email: kirtley@cua.edu

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