26.1323GPPRelease 18Speech and video telephony terminal acoustic test specificationTS
function Skew = EstimatePPM(DelayVsTime, StepSize, DiffMaxPPM=500)
% Difference threshold from which on a difference is assumed to be a packet
% loss or insertion. With larger step sizes, the normal clock skew
% results in higher differences between steps and therefore requires a
% larger threshold.
DelayDiffThreshold = min( max( DiffMaxPPM*1e-6*StepSize, 1e-3 ), 4e-3 );
% Calculate median filter of length 3 to smooth away small spikes without
% soften the steps.
DelayMedian = DelayVsTime(:);
for i = 2:numel(DelayVsTime)-1
DelayMedian(i) = median(DelayVsTime(i-1:i+1));
% Calculate delay difference per time step.
DelayDiff = diff( DelayMedian );
% Exclude all indices with high delay differences as well as some
% transition range before and after.
Mask = true(size(DelayDiff));
Jumps = find( abs(DelayDiff) > DelayDiffThreshold );
for i = 1:length(Jumps)
Mask(Jumps(i)-1:Jumps(i)+1) = false;
% Generate "continuous" delay vs. time series
Delay = [ 0; cumsum( DelayDiff(Mask) ) ];
Time = ( 0:numel(Delay)-1)’;
% Estimate clock skew in PPM as slope of linear regression line
P = polyfit( Time, Delay, 1 );
Skew = P(1) / StepSize * 1e6;
Annex E (normative):
Packet delay and loss profiles for UE delay testing of MTSI-based speech with LTE access