soundtouch变速主要采用WSOLA算法来进行变速。

http://www.surina.net/soundtouch/

https://blog.csdn.net/suhetao/article/details/5863477

The principle of WSOLA refer to following figure：

There are three important parameter: SequenceMs, overlapMs, seekWindowMs.

These parameters affect to the time-stretch algorithm as follows:

• DEFAULT_SEQUENCE_MS: This is the default length of a single processing sequence in milliseconds which determines the how the original sound is chopped in the time-stretch algorithm. Larger values mean fewer sequences are used in processing. In principle a larger value sounds better when slowing down the tempo, but worse when increasing the tempo and vice versa.
  
  By default, this setting value is calculated automatically according to tempo value.
• DEFAULT_SEEKWINDOW_MS: The seeking window default length in milliseconds is for the algorithm that seeks the best possible overlapping location. This determines from how wide a sample "window" the algorithm can use to find an optimal mixing location when the sound sequences are to be linked back together.
  
  The bigger this window setting is, the higher the possibility to find a better mixing position becomes, but at the same time large values may cause a "drifting" sound artifact because neighboring sequences can be chosen at more uneven intervals. If there's a disturbing artifact that sounds as if a constant frequency was drifting around, try reducing this setting.
  
  By default, this setting value is calculated automatically according to tempo value.
• DEFAULT_OVERLAP_MS: Overlap length in milliseconds. When the sound sequences are mixed back together to form again a continuous sound stream, this parameter defines how much the ends of the consecutive sequences will overlap with each other.
  
  This shouldn't be that critical parameter. If you reduce the DEFAULT_SEQUENCE_MS setting by a large amount, you might wish to try a smaller value on this.

function out = check_limits(in, min, max)

if in < min

　　out = min;

else if in > max

　　out = max;

else

　　out = in;

end

end

function [seekWindowLength, seekLength, overlapLength] = calcSeqParams(fs, tempo)

overlapMs = 8;

autoseq_tempo_low = 0.5;

autoseq_tempo_top = 2.0;

autoseq_at_min = 90;

autoseq_at_max = 40;

autoseq_k =(autoseq_at_max - autoseq_at_min) / (autoseq_temp_top - auto_temp_low);

autoseq_c = autoseq_at_min -autoseq_k * autoseq_temp_low;

autoseek_at_min = 20;

autoseek_at_max = 15;

autoseek_k =(autoseek_at_max - autoseek_at_min) / (autoseq_temp_top - auto_temp_low);

autoseek_c = autoseek_at_min -autoseek_k * autoseq_temp_low;

%calc sequenceMs

seq = autoseq_c + autoseq_k * tempo;

seq = check_limits(seq, autoseq_at_max, autoseq_at_min);

sequenceMs = round(seq);

seek= autoseek_c + autoseek_k * tempo;

seek= check_limits(seek, autoseek_at_max, autoseek_at_min);

seekMs = round(seek)

seekWindowLength = sequenceMs * fs / 1000;

seekLength = seekMs * fs /1000;

overlapLength = overlapMs * fs / 1000;

overlapLength  = overlapLength - mod(overlapLength, 8);

end

function corr = calcCrossCorr(mixingSeg, compareSeg)

len = length(compareSeg(:,1));

corr = 0;

norm = 0;

for i = 1: 1 : len

　　corr = corr + mixingSeg(i) * compareSeg(i);

　　norm = norm + mixingSeg(i) * mixingSeg(i);

end

corr = corr / sqrt(norm);

end

function offset = seekBestOverlapPosition(seekWindow, compareSeg, overlapLength, seekLength)

bestCorr = calcCrossCorr(seekWindow(1:overlapLength, 1), compareSeg);

offset = 1;

for i = 2 : 1 ： seekLength

　　corr = calcCrossCorr(seekWindow(i:i + overlapLength, 1), compareSeg);

　　if corr > bestCorr

　　　　bestCorr = corr;

　　　　offset = i;

　　end

end

end

function output = overlap(rampUp, rampDown)

len=length(rampDown);

for i = 1:1:len

　　output(i,1) = rampUp(i) * i / len + rampDown(i) * (len - i) / len;

end

end

function [output, outpos, lastCompare, inpos] = processSamples(input, inputLen, expectOutputLen, compareSeg, overlapLength, seekLength, seekWindowLength, tempo, isBeginning)

nominalSkip = tempo * (seekWindowLength - overlapLength);

sampleReq  = max(round(nominalSkip) + overlapLength, seekWindow);

inpos = 1;

outpos = 1;

offset = 0;

skipFract = 0;

while inputLen - inpos >= sampleReq

　　if isBeginning == 0

　　　　offset = seekBestOverlapPosition(input(inpos : inpos + overlapLength + seekLength - 1, 1), compareSeg, overlapLength, seekLength);

　　　　output(outpos:outpos + overlapLength - 1, 1) = overlap(input(inpos + offset : inpos + offset + overlapLength - 1, 1), compareseg);

　　　　ouputpos = outpos + overlapLength;

　　　　offset = offset  + overlapLength;

　　else

　　　　isBeginning = 0;

　　　　skip = round(tempo * overlapLength);

　　　　skipFract = skipFract - skip;

　　end

　　temp = (seekWindowLength - 2 * overlapLength);

　　if outpos + tmep < expectOutputLen

　　　　output(outpos : outpos + temp - 1, 1) = input (inpos + offset : inpos + offset + temp - 1, 1);

　　　　outpos = outpos + temp;

　　else　　　

　　　　output(outpos : expectOutputLen, 1) = input (inpos + offset : inpos + offset + expectOutputLen- outpos, 1);

　　　　outpos = expectOutputLen;

　　　　beak;

　　end

　　compareSeg = input (inpos + offset + temp: inpos + offset + temp +overlapLength - 1, 1);

　　skipFract = skipFract + nominalSkip;

　　ovlSkip = floor(skipFract);

　　skipFract = skipFract - ovlSkip;

　　inpos = inpos  + ovlSkip;

 end

lastCompare = compareSeg;

end

function output = changeTempo(input, fs, tempo)

inputLen = length(input(:,1));

outputLen = round(inputLen / tempo);

output = zeros(outputLen, 1);

[seekWindowLength, seekLength, overlapLength] = calcSeqParams(fs, tempo);

isBeginning = 1;

compareBuf = zeros(overlapLength, 1);

expectOutLen = outputLen;

[output, outpos, compareBuf, inpos] = processSamples(input, inputLen, expectOutLen, compareBuf, overlapLength, seekLength, seekWindowLength, tempo, isBeginning);

remainningSamples = inputLen - inpos;

%append zeros to the remainning data

remainningLen = remainningSamples + 200 * 128;

remainningInput = zeros(remainningLen, 1);

remainningInput(1:remainningSamples, 1) = input(inpos:inpos + remainningSamples - 1, 1);

if outputLen > outpos

　　expectOutLen = outputLen - outpos + 1;

　　isBeginning = 0;

　　[tempOutput, tempOutpos, compareBuf, inpos] = processSamples(remainingInput, remainingInputLen, expectOutLen, compareBuf, overlapLength, seekLength, seekWindowLength, tempo, isBeginning);

　　output(outpos:outputLen, 1) = tempOutput(1: tempOutpos);

 end

end

 main.m:

clc;

clear all;

[input fs] = wavread('test.wav');

tempo = 2;

 output = changeTempo(input, fs, tempo);

wavwrite(output, fs, 'output.wav');