## Finding local maxima

The following code snippet is something that I put together for a school project some time ago. In this project I was trying to apply some speech processing techniques to alter the pitch of a male speakers voice. At the time I was searching for the formants and trying to move them manually in the frequency domain.

It became clear at the time that I would have to take each FFT frame and find the local maxima (all of them) and check their spacing. Thus this function was born! If you give it a 1-dimensional vector it will return both the indices in which each local maximum is found as well as it's amplitude.

I hope this code can find its way to other fun and exciting applications!

```
function [time,amp] = maxima(signal, thresh, len)
% Finds the local maximum values of the given signal.
%
% Usage: [IND,AMP] = MAXIMA(X, THRESH, N);
%
% X is the data
% THRESH is the threshold that signal must be greater
% than in order to be counted. (Default = 0.0)
% N is the number of points that need to be evaluated
% (Defaults = LENGTH(X))
% IND contains the indices of X that contain the maxima data
% AMP contains the maxima amplitudes
%
% Author: sparafucile17 10/02/2003
if(exist('len') == 0)
len = length(signal);
end
if(exist('thresh') == 0)
thresh = 0.0;
end
%Initialize data
index = 1;
prev = signal(1);
local_time = 0;
local_amp = 0;
prev_slope = 1; %allow a maxima point at the second sample
%prevent length error
if(len > length(signal))
len = length(signal)
end
%Loop through data and find local maximums
for loop_i=2:len
cur = signal(loop_i);
slope = cur - prev;
if((cur < prev) & (prev_slope > 0) & (cur > thresh)) %Positive slope and amplitude dips
local_time(index) = loop_i-1;
local_amp(index) = prev;
index = index+1;
end
prev = cur;
prev_slope = slope;
end
%After loop assign data to output variables
time = local_time;
amp = local_amp;
```