Finding peaks of a signal.

Hi. I have this wav file, from which i need to find out the notes played. I got the time domain envelope of the signal, now i need to find the peaks. I was specifically told to use the 1st and 2nd derivatives to do so.

Say my enveloped signal is "Y".

According to some articles i read, when i take the 1st derivative of Y, all max and min points would become zero. Then when i differentiate again, negative values would indicate a maximum. But when i try to implement it in MATLAB I'm getting errors. This is what i did;

W = diff(Y);

Z = diff(W);

And then I try to plot these two graphs. As I'm new to MATLAB Im finding it hard at some points to write codes... and unfortunately my project needs to be done in MATLAB codes. Would really appreciate if someone could write a proper code for this.

[Muthu Annamalai]

You may want to see a similar thread, http://www.mathworks.com/matlabcentral/answers/79539-pls-i-need-a-code-that-can-be-used-to-detect-the-peak-of-signal-peak-detector and the comment by Jan Simon.

[Ravi]

http://billauer.co.il/peakdet.html % This function is released to the public domain; Any use is allowed.