[PD] The (not) doppler distortion (was: overdriven speaker)

Martin Schied crinimal at gmx.net
Wed Nov 17 03:56:45 CET 2010


On 14.11.2010 03:40, Mathieu Bouchard wrote:
> On Sat, 13 Nov 2010, Mathieu Bouchard wrote:
>
>> On Wed, 10 Nov 2010, - wrote:
>>
>>> haut-parleur-doppler.pd is the original file from Martin,
>> [...]
>>> Please correct me if I'm wrong somewhere.
>>
>> Yes, that file is from me and not from Martin. (but that's just a few 
>> kilometres off)
>
> (Doh, I was thinking about a different Martin. In any case, the file 
> comes from me.)
thought you were joking...

in your previous mail you wrote
> It remains consonant to the ear so easily simply because it only 
> produces harmonics.
did you speak about the "doppler" distortion? true doppler distortion is 
harmonic for a single sine wave, but not for 2 or more sines of 
different frequencies. It's the same effect known as frequency 
modulation, in this case a signal being its own carrier. The difficulty 
in prediction of a spectrum is that the carrier is an always changing 
mix of frequencies and not a single sine wave like for a radio station 
transmitter or in the most simplest case of frequency modulation 
synthesizers, but it is pretty easy to find out it is non harmonic by 
listening...

I want to add that the vd~ approach is not the "perfect implementation" 
for the above described frequency modulatuon. the carrier is delayed 
against the modulating signal a bit. Like this it simulates a moving 
listener instead a moving sound source. Also for a single sine wave the 
change of the spectrum differs by variation of the delay.

A more close to reality simulation which does only fm without delay 
requires a variable (interpolating?) write into the delayline. 
Unfortunately I don't know of such externals already existing. However I 
don't expect the effects being very different from the vd~ method. A 
slow 20Hz sinewave modulating some high frequencies will not sound very 
different if the 20Hz is delayed or not. A 20 hz sound will have a 
duration which is much longer that the delay.

Martin


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