FM transmissions and the Doppler effect.

C

Chris Nixon

Junior
Jul 23, 2012
299
0
0
Northern Ireland
I'm not sure why this popped into my head earlier, but it did. If you have a performer running around the stage with an FM transmitter, why doesn't the Doppler effect cause problems?

Doppler effect - Wikipedia, the free encyclopedia

From my basic understanding of FM radio, this should be causing DC offset by moving the "base line" frequency above or below the carrier frequency the receiver is tuned to. Is it the case that this DOES happen but it's filtered out in the receiver?

Is the amount of shift small enough to be negligible because of the speed of the radio waves relative to the performers movement?

Chris
 
Re: FM transmissions and the Doppler effect.

Thanks, I thought that might be it. Just to check my understanding, would DC offset be the result if there was a movement that fast between transmitter and receiver?

Chris
 
Re: FM transmissions and the Doppler effect.

Chris Nixon said:
Thanks, I thought that might be it. Just to check my understanding, would DC offset be the result if there was a movement that fast between transmitter and receiver?
Click to expand...

No, not only can't the performer keep moving that fast indefinitely (so it would be an AC distortion in the signal), but you're thinking in the wrong domain. Doppler affects frequency, not amplitude.
 
Re: FM transmissions and the Doppler effect.

Bennett Prescott said:
Doppler affects frequency, not amplitude.
Click to expand...

My thought was that because FM radio uses frequency variation to represent audio amplitude that shifting the base frequency higher or lower (with the Doppler effect) would produce an offset in the audio signal.

I'm not arguing that this is right, because I don't know, but I'm trying to understand it better.

I realise that this being a problem is a totally unrealistic scenario.

Chris
 
Re: FM transmissions and the Doppler effect.

Hi Chris,

Bennett is right on in his answers, but if you'd like to delve a little deeper into the subject, check out A Closeup of Doppler Shift. It contains some tables and graphs showing Doppler shift vs. frequency that we see on Earth when the transmissions originate from Low Earth Orbit Satellites traveling at a minimum of 17,500MPH. For comparison, there is also a table showing the values when the TX originates from a car traveling at 100KM/H.

Interestingly enough, the maximum shift from those high velocity sources is still within the deviation tolerance (usually +/- 25KHz) of our pro audio wireless gear at similar frequencies.

Here's an interesting thread about the subject where some of the contributors show the equations and explain the math; Doppler Effect on FM.
 
Last edited:
Top Bottom