Question

I have information (20,000 frames of data) about an audio track that I have auto-correlated using:

[r,lags] = xcorr(XX,XX,'biased');

And it looks like this:

Which hopefully is so far so good. Ideally I would like to be able to take the frame number that corresponds to the highest part of the second peak. I’ve read around and tried a load of different methods but I just can’t seem to get it to retrieve the information for me.

Would anybody be able to shed some light on what I have to do?

Many thanks!

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edit1:
I have tried using findpeaks, but it doesn’t seem to work for me. I’m not sure if that’s because I’m using the wrong data or not.

edit2: I’m currently testing a method to use on just this audio track, but soon I want to expand it so that I can perform this method on a whole directory of files, so I kind of need a script that can detect peaks rather than finding the information myself.

edit3: My .M file:

[y, fs, nb] = wavread('Three.wav');                 %# Load the signal into variable y

frameWidth = 441;                                   %# 10ms
numSamples = length(y);                             %# Number of samples in y
numFrames = floor(numSamples/frameWidth);           %# Number of full frames in y
energy = zeros(1,numFrames);                        %# Initialize energy
startSample = zeros(1,numFrames);                   %# Initialize start indices
endSample = zeros(1,numFrames);                     %# Initialize end indices

for frame = 1:numFrames                             %# Loop over frames
  startSample(frame) = (frame-1)*frameWidth+1;      %# Starting index of frame
  endSample(frame) = frame*frameWidth;              %# Ending index of frame
  frameIndex = startSample(frame):endSample(frame); %# Indices of frame samples
  energy(frame) = sum(y(frameIndex).^2);            %# Calculate frame energy
end                                                 %# End loop

XX = filtfilt(ones(1,10)/10, 1, energy);            %# Smooths signal

[r,lags] = xcorr(XX,XX,'biased');                   %# Auto-correlates the data
plot(lags,r), xlabel('lag'), ylabel('xcorr')        %# Plots data

Answer 1

EDIT:

%# load the signal
[y, fs, nb] = wavread('Three.wav');
y = mean(y,2);                               %# stereo, take avrg of 2 channels

%# Calculate frame energy
fWidth = round(fs*10e-3);                    %# 10ms
numFrames = floor(length(y)/fWidth);
energy = zeros(1,numFrames);
for f=1:numFrames
  energy(f) = sum( y((f-1)*fWidth+1:f*fWidth).^2 );
end

%# smooth the signal (moving average with window size = 1% * length of data)
WINDOW_SIZE = round(length(energy) * 0.01);  %# 200
XX = filtfilt(ones(1,WINDOW_SIZE)/WINDOW_SIZE, 1, energy);

%# auto-correlation
[r,lags] = xcorr(XX, 'biased');

%# find extrema points
dr = diff(r);
eIdx = find(dr(1:end-1) .* dr(2:end) <= 0) + 1;

[~,loc] = sort(r(eIdx), 'descend');
loc = loc(1:min(3,end));                     %# take the highest 3 values

%# plot
plot(lags,r), hold on
plot(lags(eIdx), r(eIdx), 'g*')
plot(lags(eIdx(loc)), r(eIdx(loc)), 'ro')
hold off, xlabel('lag'), ylabel('xcorr')

and the lag values corresponding to the marked peaks:

>> lags( eIdx(loc) )
ans =
           0       -6316        6316

Note that we smoothed the signal prior to computing the derivative of the autocorrelation function in order to find the extrema points