Re: making a sound unrecognizable

[Nicolas Grimault <ngrimault@xxxxxxxxxxxxxxxxxxx>]

Dear Ursula,

Few years ago, I tried a MATLAB code to prevent a speech sounds to be 
recognizable. The overall spectrum and the temporal envelope (up to 60 
Hz) of the initial sound are roughly preserved by this code. Be aware 
that neither the spectrum nor the temporal envelope of the output are 
exactly the same than the input !
Below is the MATLAB code (the MATLAB signal toolbox is required). This 
code is not guaranteed !

Hope this help.

Nicolas Grimault


*****************
function new=SigToNoise(N);%N is the filename (including path) of the 
wav file (the input signal) to be transformed
   [sig fe bit]=wavread(N);%read the input signal (from a wav file)
   rmssig=sqrt(sum(sig(:,1).^2)/length(sig(:,1))); %RMS of the input signal
   [Fparam1 Fparam2]=butter(1, 60/(0.5*fe)); %low-pass filter Fc=60 Hz
   LPsig=filter(Fparam1,Fparam2,abs(sig(:,1)));%rectified temporal 
envelope of the input signal (Fc=60 Hz)
   phasefftnoise=2*pi*rand(length(sig),1);%a random phase vector
   fftsig=abs(fft(sig(:,1)));%FFT magnitude of the input signal
   fftnew=fftsig.*exp(-j*phasefftnoise);%the output signal is generated 
here in the frequency domain. The output has the same magnitude than the 
input signal but random phases
   new=real(ifft(fftnew));%output in the time domain (by  FFT-1)
   new=new.*LPsig;%the temporal envelope of the input is applied to the 
output
   rmsnewsig=sqrt(sum(new.^2)/length(new));%RMS of the output signal
   new=rmssig*new/rmsnewsig;%the RMS of the output is adjusted to be 
the same than the RMS of the input  
   if max(abs(new))>1 | max(abs(sig(:,1)))>1 %check for saturation
       disp('Error: saturation...');
       disp(['Maximum = ' int2str(max(abs(new)))]);
   end
   wavwrite(new,fe,bit,['new_' N]);   %write the output in a wav 
file      
end
********************


> Dear list members,
>
> I'm looking for a possibility to change/edit a environmental or at 
> least natural sound in a way  that it is not any longer recognizable 
> as what it is, but, however, for doing this with changing as less 
> acoustic parameters as possible. ;-)  The intention is to use these 
> sounds in a MMN paradigm and it is important that  both "versions" of 
> the sound are acoustically as similar as possible.
> The main point is that the sound (that can also be a more complex 
> thing as the sound of a barking dog or a something like that) should 
> be not possible to be recognized as what it is.
>
> I would be very happy about ideas or would like know if maybe somebody 
> has already tried something like this?  I would also be very happy 
> about recommations for software or matlab scripts that could be 
> helpful in respect to this. For example I'm looking for a way to 
> transfer the amplitude envelope from one sound to another (I already 
> wrote a matlab routine for this, but its not yet functioning perfectly 
> fine and the result is not completely convincing).
>
> So if anybody has a an idea I would be grateful.
>
> best regards and thanks a lot,
> Ursula Kirmse


--
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Nicolas Grimault
Cognition Auditive et Psychoacoustique
UMR CNRS 5020 Neurosciences et Systèmes Sensoriels
50 Av. Tony Garnier
69366 Lyon Cedex 07
France
Vocal: 33 (0)4 37 28 74 89
Fax: 33 (0)4 37 28 76 01
Email: nicolas.grimault@xxxxxxxxxxxxxxxxxxx
Web: http://olfac.univ-lyon1.fr/unite/equipe-02/grimault/grimault-f.html
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