Successive Variation in Ampltide/Pitch Period a.k.a (Shimmer & Jitter)

Hello

I have a simulated voice signal, from that signal I can extract thepeak
amplitudes and the time which it occurs using a peak findingalgorithm
called peakdet (peakdet.m). Now what I want to do isdetermine how much
successive peak amplitudes vary (this is calledshimmer) and how much
successive pitch periods vary (this is calledjitter). I implementedthis
using MATLAB (JitterShimmer.m). To test my code, I made a sinewave with
no additive phase or amplitude, thus I should expect a jitter and
shimmer of zero. I get avalue very close to zero for shimmer (about
10^-14), but for jitter Iget 0.08 which isnt quite zero. Can you see the
problem? Anysuggestions as to how this can be done differently?

Shimmer --This is the average absolute difference between the amplitudes
of consecutive periods, divided by the average amplitude
Jitter--This is the average absolute difference between consecutive
periods, divided by the average period.

%peak finding algorithm --->peakdet.m
% obtained from http://www.billauer.co.il/peakdet.html
function [maxtab, mintab]=peakdet(v, delta, x)
%PEAKDET Detect peaks in a vector
% [MAXTAB, MINTAB] = PEAKDET(V, DELTA) finds the local
% maxima and minima ("peaks") in the vector V.
% MAXTAB and MINTAB consists of two columns. Column 1
% contains indices in V, and column 2 the found values.
%
% With [MAXTAB, MINTAB] = PEAKDET(V, DELTA, X) the indices
% in MAXTAB and MINTAB are replaced with the corresponding
% X-values.
%
% A point is considered a maximum peak if it has the maximal
% value, and was preceded (to the left) by a value lower by
% DELTA.

% Eli Billauer, 3.4.05 (Explicitly not copyrighted).
% This function is released to the public domain; Any use is allowed.

maxtab = [];
mintab = [];

v = v(:); % Just in case this wasn't a proper vector

%conditions to check/report error
if nargin < 3
x = (1:length(v))';
else
x = x(:);
if length(v)~= length(x)
error('Input vectors v and x must have same length');
end
end

if (length(delta(:)))>1
error('Input argument DELTA must be a scalar');
end

if delta <= 0
error('Input argument DELTA must be positive');
end

%program
mn = Inf;
mx = -Inf;
mnpos = NaN;
mxpos = NaN;

lookformax = 1;

for i=1:length(v)
this = v(i);
if this > mx,
mx = this;
mxpos = x(i);
end

if this < mn,
mn = this;
mnpos = x(i);
end

if lookformax
if this < mx-delta
maxtab = [maxtab ; mxpos mx];
mn = this; mnpos = x(i);
lookformax = 0;
end
else
if this > mn+delta
mintab = [mintab ; mnpos mn];
mx = this; mxpos = x(i);
lookformax = 1;
end
end
end

% actual code -->JitterShimmer.m
clear all
close all

fs00;
dt=1/fs;
t=[0:dt:1];
y=sin(2*pi*100*t);
%find peaks in signal
[maxs,mins]=peakdet(y,0.6,t);

plot(t,y);
hold on;

%plot only maximums onto plot
plot(maxs(:,1),maxs(:,2),'g*'); hold off;

%maximum peak values time in column 1
peaktime=maxs(:,1);
%peak maximum values in column 2
peakval=maxs(:,2);

%pitch period
pitchperiodss(diff(peaktime));

pchdiff=zeros(length(pitchperiods)-1,1);

%difference b/w successive pitch periods
for k=1:length(pitchperiods)-1
pchdiff(k)s(pitchperiods(k)-pitchperiods(k+1));
end

avgpchdiff=mean(pchdiff);
avgpch=mean(pitchperiods);

jitt=(avgpchdiff/avgpch)

pkdiffs(diff(peakval));
avgpkdiff=mean(pkdiff);
avgpks=mean(peakval);

shim=avgpkdiff/avgpks Start a New Thread