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.
I used this in conjunction with my local maxima code to find the position and the width (using local minima) of the speakers formants. If you give this function a 1-dimensional vector it will return both the indices in which each local minimum 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] = minima(signal, len) % % Finds the local minimum values of the given signal. % % Usage: [IND,AMP] = MINIMA(X, N); % % N is the number of points that need to be evaluated % (Normally equal to LENGTH(X)) % X is the data % IND contains the indices of X that contain the minima data % AMP contains the minima amplitudes % % Author: sparafucile17 10/02/2003 %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 minimums for loop_i=2:len cur = signal(loop_i); slope = cur - prev; if((cur < prev) & (prev_slope > 0)) %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;