Reply-To: "Kevin Aylward"
From: "Kevin Aylward"
Subject: Re: Detecting original ultrasonic signal vs. reflected?
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Date: Wed, 25 Sep 2002 10:35:37 +0100
NNTP-Posting-Date: Wed, 25 Sep 2002 10:35:44 BST
"Winfield Hill" wrote in message
> John Rahn wrote...
> > Is it at all possible, to tell the difference, of a reflected
> > ultrasonic signal to an original signal.
> > I am starting a project that sends a signal from a transmitter to
> > separate receivers and want to try to figure out how to
> > out reflected signals, because I will be timing the received
> > and a reflected signal would cause an inaccurate reading.
> Generally reflected signals arrive after direct signals, because
> the path is longer. :-)
> But you asked, and yes, in the case of very short pulses in low-Q
> wide-bandwidth high-frequency systems, they do have very different
> signatures. Direct signals can be short and sweet, wellformed and
> ideal. By contrast reflected signals are generally stretched by
> the geometry and expanse of the reflecting surfaces. Furthermore,
> scattering, refraction and attenuation contribute to an overall
> deterioration of the stretched and delayed portions of a reflected
> waveform, creating regions of reinforcement and cancellation, both
> the signal phase and amplitude vary wildly throughout the pulse.
> Use wide-bandwidth drivers and receivers, sample your signal 5 or
> 10x per cycle. Cross-correlate the digitized signal with a stored
> version of the direct signal to reveal the dramatically different
> signatures for the two cases. The cross-correlation peak is also
> a very good way to determine an actual direct-path time delay.
> Simple examination for the presence of any reverberation energy
> after the beginning of the pulse and after it should have ended is
> a simplified method, but examining the entire cross-correlation
> waveform is an excellent way to evaluate the reverberation energy,
> thereby improving overall recognition robustness.
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