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Subject: Re: Pitch shifting
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Date: Mon, 03 May 21 07:28:25 +0000
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From: pehache <pehache.7@gmail.com>
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Le 01/05/2021 à 17:55, François Guillet a écrit :
> 
> Les vocoders qui font ça existe.
> S'il y a peut-être du bricolage pour peaufiner le son, je pense qu'il y 
> a quand même une méthode pour traiter la phase, et c'est ce que je 
> cherche.

Voir ce code Matlab (qui tourne OK aussi dans Octave) que j'avais 
récupéré il y a pas mal d'années. J'avais testé, il faisait ce qu'il 
était censé faire, mais il me semble aussi que j'avais noté quelques 
problèmes (mais je ne me souviens plus lesquels). 


%**********************************************************************************

function y = pvoc(x, r, n)
% y = pvoc(x, r, n)  Time-scale a signal to r times faster with phase 
vocoder
%      x is an input sound. n is the FFT size, defaults to 1024.  
%      Calculate the 25%-overlapped STFT, squeeze it by a factor of r, 
%      inverse spegram.
% 2000-12-05, 2002-02-13 dpwe@ee.columbia.edu.  Uses pvsample, stft, istft
% $Header: /homes/dpwe/public_html/resources/matlab/RCS/pvoc.m,v 1.2 
2002/02/13 16:14:54 dpwe Exp $

if nargin < 3
  n = 1024;
end

% With hann windowing on both input and output, 
% we need 25% window overlap for smooth reconstruction
hop = n/4;
% Effect of hanns at both ends is a cumulated cos^2 window (for
% r = 1 anyway); need to scale magnitudes by 2/3 for
% identity input/output
scf = 2/3;

% Calculate the basic STFT, magnitude scaled
X = scf * stft(x', n, n, hop);

% Calculate the new timebase samples
[rows, cols] = size(X);
t = 0:r:(cols-2);
% Have to stay two cols off end because (a) counting from zero, and 
% (b) need col n AND col n+1 to interpolate

% Generate the new spectrogram
X2 = pvsample(X, t, hop);

% Invert to a waveform
y = istft(X2, n, n, hop)';


%**********************************************************************************

function c = pvsample(b, t, hop)
% c = pvsample(b, t, hop)   Interpolate an STFT array according to the 
'phase vocoder'
%     b is an STFT array, of the form generated by 'specgram'.
%     t is a vector of (real) time-samples, which specifies a path through 

%     the time-base defined by the columns of b.  For each value of t, 
%     the spectral magnitudes in the columns of b are interpolated, and 
%     the phase difference between the successive columns of b is 
%     calculated; a new column is created in the output array c that 
%     preserves this per-step phase advance in each bin.
%     hop is the STFT hop size, defaults to N/2, where N is the FFT size
%     and b has N/2+1 rows.  hop is needed to calculate the 'null' phase 
%     advance expected in each bin.
%     Note: t is defined relative to a zero origin, so 0.1 is 90% of 
%     the first column of b, plus 10% of the second.
% 2000-12-05 dpwe@ee.columbia.edu
% $Header: 
/homes/dpwe/public_html/resources/matlab/dtw/../RCS/pvsample.m,v 1.3 
2003/04/09 03:17:10 dpwe Exp $

if nargin < 3
  hop = 0;
end

[rows,cols] = size(b);

N = 2*(rows-1);

if hop == 0
  % default value
  hop = N/2;
end

% Empty output array
c = zeros(rows, length(t));

% Expected phase advance in each bin
dphi = zeros(1,N/2+1);
dphi(2:(1 + N/2)) = (2*pi*hop)./(N./(1:(N/2)));

% Phase accumulator
% Preset to phase of first frame for perfect reconstruction
% in case of 1:1 time scaling
ph = angle(b(:,1));

% Append a 'safety' column on to the end of b to avoid problems 
% taking *exactly* the last frame (i.e. 1*b(:,cols)+0*b(:,cols+1))
b = [b,zeros(rows,1)];

ocol = 1;
for tt = t
  % Grab the two columns of b
  bcols = b(:,floor(tt)+[1 2]);
  tf = tt - floor(tt);
  bmag = (1-tf)*abs(bcols(:,1)) + tf*(abs(bcols(:,2)));
  % calculate phase advance
  dp = angle(bcols(:,2)) - angle(bcols(:,1)) - dphi';
  % Reduce to -pi:pi range
  dp = dp - 2 * pi * round(dp/(2*pi));
  % Save the column
  c(:,ocol) = bmag .* exp(j*ph);
  % Cumulate phase, ready for next frame
  ph = ph + dphi' + dp;
  ocol = ocol+1;
end


%**********************************************************************************


function d = stft(x, f, w, h)
% D = stft(X, F, W, H)                            Short-time Fourier 
transform.
%	Returns some frames of short-term Fourier transform of x.  Each 
%	column of the result is one F-point fft; each successive frame is 
%	offset by H points until X is exhausted.  Data is hamm-windowed 
%	at W pts..
%	See also 'istft.m'.
% dpwe 1994may05.  Uses built-in 'fft'
% $Header: /homes/dpwe/public_html/resources/matlab/RCS/stft.m,v 1.1 
2002/02/13 16:15:55 dpwe Exp $

s = length(x);

if rem(w, 2) == 0   % force window to be odd-len
  w = w + 1;
end

halflen = (w-1)/2;
halff = f/2;   % midpoint of win
acthalflen = min(halff, halflen);

halfwin = 0.5 * ( 1 + cos( pi * (0:halflen)/halflen));
win = zeros(1, f);
win((halff+1):(halff+acthalflen)) = halfwin(1:acthalflen);
win((halff+1):-1:(halff-acthalflen+2)) = halfwin(1:acthalflen);

c = 1;

% pre-allocate output array
d = zeros((1+f/2),1+fix((s-f)/h));

for b = 0:h:(s-f)
  u = win.*x((b+1):(b+f));
  t = fft(u);
  d(:,c) = t(1:(1+f/2))';
  c = c+1;
end;


%**********************************************************************************


function x = istft(d, ftsize, w, h)
% X = istft(D, F, W, H)                   Inverse short-time Fourier 
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