Re: Doppler question...

["Prof. John P. Ertel (wizard)" <jpe@nadn.navy.mil>]

On Fri, 23 Jan 1998 crvhs_dks@ncocc.ohio.gov wrote:
....
> 	If you were traveling at an extremely fast rate towards or away
> from a radio station antenna and turned on the radio, would the music you
> are listening to be affected?
....

  Information is "encoded" onto a carrier either as AM or FM and is
"decoded"  appropriately.  The Doppler Effect would certainly change the
frequency of the carrier received.  It should not really affect the
encoded information correspondingly due to the Doppler Shift.

There could be an effect if the observer was moving AWAY from the source.
This would "down-shift" the carrier and thereby reduce the "band-width" of
frequencies that can be supported on the carrier.  In this way, some
frequency information could be lost.  Whether this would affect the music
to which you were listening, would have to be calculated.

It would not require a speed that was "an large fraction" of the speed of
light to have an effect.  If the information(music) modulated onto the
carrier happened to have a frequency content that "pushed the limits" of
the band-width, you would have some loss of frequency content.  Would it
be noticable to the ear?  Again, I don't know.

I cannot think of how the information(music) would be affected if the
receiver were to be moving towards the transmitter as the question was
originally posed.  It would seem that this would only serve to increase
the effective band-width which should not cause any loss.

I should note that all of my friends that do "Information Theory" take it
as a given that "any change in the carrier frequency can cause a loss of
frequency and amplitude content."  I don't really understand this (see
above), but that's what they say and that is their area of specialization.


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