Science Questions

Does music radio get doppler shifted?

Sun, 2nd May 2010

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Question

Jeff, University of Pittsburgh asked:

I thought of this while listening to Naked Scientists in my car. We’ve all heard the way in which sounds change frequency as an ambulance passes us with its siren going. What I wonder is whether a conventional radio broadcast experiences Doppler shift when we’re driving toward or away from the transmitter. Does the pitch or more likely the tempo of a pop song go up if I'm driving toward the transmitter, even if I can't perceive it? I figured that the Doppler effect wouldn’t affect digital broadcast but I’d like to know about that as well. Thank you.

Answer

We put this to Professor Stafford Withington, from the department of physics at the University of Cambridge:

I think the question is quite subtle and actually has three parts to it.  The first part of the question is very straightforward - do radio waves exhibit the Doppler effect? And they certainly do.  And anyone who’s been caught speeding by a policeman using a microwave speed gun certainly knows that that’s the case.  The police speed gun uses the Doppler effect to determine your speed relative to the police car.

Waves emitted by a source moving from the right to the left. The frequency is higher on the left (ahead of the source) than on the right.The second part of the question is if I now encode some audio information on the radio wave, does the Doppler effect get imparted on to the radio signal?  That’s much more subtle and needs thinking about carefully.  Basically, the question is - let’s say, I have a radio program like Radio 3 playing an oboe and the oboe is playing a perfect A, if I put my radio to my ear and move towards the transmitter at high speed, do I expect to hear the oboe’s note increase?  The answer is closely related to the way in which the amplitude signal is encoded onto the radio signal and it can be done in two ways.  Traditionally, amplitude modulation (AM) was used and this is where the audio information is encoded on the radio signal by modulation of the amplitude of the radio signal, more laterly people use frequency modulation and that the audio signal is encoded in the form of the frequency of the transmitted wave.  And it can be shown through some rather delicate arguments that in both of those cases, in fact the audio signal does take on the Doppler effect.  So in other words, you would expect the oboe’s note to change by some very small amount.

Diana -   But how small will these differences be?

Stafford -   The third part of the answer however is whether the effect is detectable and the problem is that when you derive the appropriate equations and put the numbers into the equations, what you find is that for an audio signal, the shift in frequency is exceedingly small.  So, even if I was to put a radio to my ear then move towards the transmitter at something like 10 metres per second, then you can show very simply that the shift in frequency of the audio signal would be of the order of thousands of a Hertz.  And I'm afraid that the average radio receiver and indeed human hearing is not good enough to detect those changes, but it would be possible and in fact, quite straightforward to build an instrument which would detect that change.

The digital case is rather different again because the audio information is encoded in a very complicated way and so, the coupling between the listener and the radio wave that originally transported the information to the listener’s home, that coupling is a very, very weak one.  There’s all sorts of electronics between those two stages in the process.  And so, a digital signal will not display the Doppler effect.

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There would be a Doppler shift in the radio frequency: if the radio receiver or transmitter were travelling at very high speed,
(e.g. spacecraft) it would be necessary to retune the radio receiver because of the Doppler shift.

http://www.spacetoday.org/Satellites/Tracking/SatTrackingHowDopplerWorks.html RD, Thu, 29th Apr 2010

If the spacecraft was travelling that fast and it's occupants could observe the people on Earth (which seems to be impossible) they might be entertained by what they saw. Geezer, Fri, 30th Apr 2010

It will be Doppler shifted, but not by enough to matter much for basic AM and FM radio. The effect becomes more significant at higher radio frequencies.

GSM mobile phones interesting lock the frequency of the handheld unit to the received signal from the base-station... which means that any shift in the received signal due to motion will be automatically 'compensated' in its transmission and the base station will see all the received signals spot on the correct frequency (this also cunningly gets around the need for a accurate frequency-reference in the handset). GSM phones also lock the slower timing to the appropriate sub-multiple of the radiofrequency, which keeps everything in perfect step...


Modern digital modulation schemes such as those used for digital TV use a high number (thousands) of very closely spaced subcarriers, and these certainly can get mangled by Doppler shift - especially if the shift comes from a reflection off a moving vehicle so the shifted stuff gets mixed up with the non-shifted direct signal.

In digital, even if the signal doesn't get corrupted by differently-shifted reflections and so can be recovered, effectively the whole timing gets sped up or slowed down. To avoid the playback not keeping up, or getting 'gaps' when the signal doesn't keep up, it would normally be frequency=locked to the incoming signal so the audio would in fact come out Doppler-shifted (by an amount so small no-one would notice it). techmind, Fri, 30th Apr 2010

I suppose the Doppler effect must end up changing the audio frequencies with AM and FM, although the effect is unlikely to be detected.

The RF signal either gets compressed or stretched with respect to time, so there has to be a corresponding compression or stretching in the modulated audio with respect to time.

I think! Geezer, Fri, 30th Apr 2010

Yes, but you will not notice it with radio unless you are going at an appreciable portion of C. You notice it because sound travels at around 300metres per second, whilst radio waves travel at around 300 000 000 metres per second, ie around one million times faster. Thus the doppler effect is around 1 million times less, and thus disappears pretty much for speeds that you reach on the ground.

However the doppler shift is used in GPS satellitetes, that compensate the signals they transmit to negate the doppler shift seen by the ground receivers, as well as to compensate for the acceleration the radio waves get from gravity in transit to the earth. SeanB, Sun, 2nd May 2010

Editorial: "of the order of thousands of a Hertz" should read "of the order of thousandths of a Hertz". evan_au, Sat, 9th Jun 2012

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