Re: Doppler question...

[brian whatcott <inet@intellisys.net>]

At 17:10 1/23/98 -0500, John Ertel wrote:
> On Fri, 23 Jan 1998 crvhs_dks@ncocc.ohio.gov 
(That is to say, Dwight Ashland) 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.

I endorse John's first assertion - that the carrier frequency shifts.
 I am a little surprised he will not concede that this shift also 
affects information encoded in both FM or AM transmissions.

 Surely I have heard of pulsars where there is a rhythmic variation
 in the pulse rate associated with the orbital motion of a twin?
 This would amount to AM.

> 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.

I take the view that one is simply elongating the time axis of a signal 
- so that its information is now only available over a longer time period -
certainly this amounts to a loss of bandwidth in a sense - but not of
information - only its rate...


> 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 take the view that an increase of 10% on the carrier frequency of an AM
broadcast signal will similarly lift the pitch of its music content by 10%

> 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.
>
> ... John P. Ertel 

I too find this worrying. Though it's always a pleasure to contradict
a Physics prof - I suspect that there is a technical sense in which
John's friends are right.

  When all else fails, I fall back on practicalities however - if we 
disregard spread spectrum methods and touch on just broadcast AM and FM,
we can say that the modulation takes about 100 kilohertz of an (about)
100 MHz carrier signal - a modest 0.1 percent

 At the other end of the dial, a 4 kilohertz AM signal o a 200 kilohertz 
carrier is still occupying only a 4% bandwidth.

 This gives those technical objectors very little room for maneuver 
in these cases....


brian whatcott <inet@intellisys.net>
Altus OK