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View Full Version : Re: Linear enveloping of EMG



Jason Harrison
08-30-2000, 08:49 AM
>Dr. Chris Kirtley writes:
>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.

Actually, a rectifier is a non-linear device (or operation). In a AM
demodulator the "signal" is removed from the carrier because the
"signal" was added to the carrier by modifying the AMPLITUDE of the
carrier. In a Series-Diode Detector (very simple AM radio receiver),
the carrier which is Amplitude Modulated by the signal (this is what
the radio station broadcasts) is passed through a very narrow band
pass filter then half-wave rectified with a diode. The half-wave
rectification is what extracts the "signal" from the the carrier -- a
bit of smoothing then removes the carrier frequency.

See figure 3-5A and discussion at
http://www.tpub.com/neets/book12/51a.htm

-Jason

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