From obxwindsurf@yahoo.com Thu Oct 09 08:26:01 2003
Subject:Before anyone corrects me on my explanation of doppler...

My last explanation of Doppler in the "vibrato/chorus" thread should
be qualified by the context of doppler in the "human audible range".

Doppler is also used in radar technology and also results in a
similar frequency shift caused by a reflected radar pulse either
compressing or expanding the carrier frequency resulting from
relative physical motion between the source (the radar gun or dish),
and the destination (the target).

The classic doppler shift is demonstrated by a vehicle blaring the
horn in which the frequency increases to a maximum as the vehicle is
on an approaching attitude and decreases to a minimum as the vehicle
is departing relative to your position.

My explanation in the "vibrato/chorus" thread is limited to and is
valid for audible sound sources.

The point I was trying to make is that doppler requires a signal
source and RELATIVE PHYSICAL MOTION to occur between the source and
the destination, whether the source moves and the listener is
stationary or vice versa.

No such relative physical motion takes place inside of an electronic
circuit - but there a number of ways to approximate this but it is
only an approximation, not a true doppler effect.

This is a primary reason why no Leslie simulator will ever be as good
as the real thing. The amplitude modulation can be modeled or
synthesized and time-varying reflections that coincide with amplitude
modulation can be modeled or synthesized. Amplitude modulation is
easy, spacial reverberation (delay and reflection) is a little more
difficult but DSPs make it much easier to represent mathematically
based on assumptions of room geometry and the speed at which sound
travels through air. Doppler cannot because in the audible range it
is a purely acoustic phenomenon which changes with the distance
between listener and source as well as relative motion and rate of
relative motion.

Best regards,
Kevin