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

[Martin Schied]

Hi!

On 19.11.2010 05:33, Mathieu Bouchard wrote:
> On Wed, 17 Nov 2010, Martin Schied wrote:
>
>> 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.
>
> Then can you explain what's wrong in my reasoning ? (the formulas I 
> wrote)
no, sorry, I can't on the quick. but I know for sure that frequency 
modulation only produces harmonics if the carrier and modulator are in 
"simple" relations like 1:5 or your example 200 to 1000.

The frequencies added by fm are multiples of the modulator added and 
subtracted from the carrier. (carrier; 1000Hz, modulator 11Hz --> pairs 
in order of strength: 989 + 1011, 978 + 1022 Hz ...)
If you have frequencies going below 0Hz they wrap around the y=0Hz axis. 
Result is a negative freuqency, or likewise the phase is reversed. (for 
example 1000Hz modulated by 600Hz will have 200Hz in the spectrum)

If you use not-harmonic related frequencies like 320 and 1000 Hz and you 
will have new frequencies which aren't multiples of 320 and 1000, but 
inbetween. (580 and 1320, ...), Also you will get frequencies below the 
320 Hz. However if you drive the modulation very strong, a patten 
similar to a harmonic signal will appear, possibly with a new base 
frequency, but also likely without.

I'm absolutely not familiar with the case of a signal modulating itself, 
but I'm pretty sure from experiments with your patch (or the new I mad 
and attached) that there occur very similar (if not identical) effects 
if you modulate a mix of 1000 + 320 Hz with itself or you use separate 
carriers for 1000  and 320 Hz and modulate them separately by the mix of 
1000 + 320. I'm not sure if this superposition is valid, but I'm pretty 
sure... at least it sounds relatively similar if I do it in pd. The vd~ 
sounds always "richer" possibly artifacts. (the spectrum looks very 
dirty too)

>
>> 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.
>
> You can add another [vd~] or [delread~] to fix that. I think I did 
> that in some other version of haut-parleur-doppler.pd, and if not, I 
> did similar things with other effects.
>
>> Like this it simulates a moving listener instead a moving sound source.
>
> But there is no difference between a moving listener and a moving 
> source, apart from the wind.
there is. but for the low speeds of the cone we are simulating the 
audible effects are not very different. (a moving sound source at the 
speed of sound produces infinite frequency, a moving listener at speed 
of sound hears only doubled frequency).
>
> What you mean is, regardless of whether it happens in the listener or 
> the source, the motion is out of phase with the actual signal.
>
exactly and this is why I'd like to move the write head. How did your 
correction using vd~ work?
>
>> Unfortunately I don't know of such externals already existing. 
>> However I don't expect the effects being very different from the vd~ 
>> method.
>
> What if they aren't _any_ different ?
it's only because of the slight delay I'd be interested to eliminate. 
But as you also said I don't expect any big difference between the delay 
free case and the vd~ case as the modulations occuring with a slight off 
phase will not sound any different. (however 2ms can be quite different 
for short percussive signals)
>
>> 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.
>
> But it depends on the gain you choose. Suppose you have a 
> weapons-grade giant speaker, with a membrane of a few hectares...
>
> I mean, play with the [*~] and you can get the delay variation to be 
> bigger than the period of a sinewave.
yep, that's also one of the effects I wanted to eliminate by the moving 
write head.

Martin
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