Dear group,
I am using the last beta version of PsychToolbox with Matlab R2006a and
a Turion 64x2 processor running on windows XP. I am posting this message
because I detected some timing precision problems and I would like to
ask your opinion about what may be causing them and if there is any way
to improve timing precision. The simple script I used to detect the
problems is the following:
[w rect]=Screen('OpenWindow', 0);
Priority(7);
gray=GrayIndex(0);
Screen('DrawText', w, 'A' , 200, 100, [0, 0, 0, 0]);
flipini=GetSecs;
[VBLTimestamp StimulusOnsetTime FlipTimestamp]=Screen('Flip', w);
WaitSecs(2);
endwait=GetSecs;
[VBLTimestamp2 StimulusOnsetTime2 FlipTimestamp2]=Screen('Flip', w);
Screen('CloseAll');
I attach a txt file with the messages I receive in my command window
after executing the script. The one that worries me most is:
WARNING: PTB's Screen('Flip') command missed the requested stimulus
presentation deadline
WARNING: a total of 2 times during this session!
The timimng problems become obvious by calculating some of the variables:
StimulusOnsetTime-flipini=0.00672851696254, which means a delay of 6ms
and
StimulusOnsetTime2-endwait=0.01884155754124, a delay of 18ms!
Another script I use to prompt timings is:
Priority(7);
times=zeros(1,10000);
for i=1:10000;times(i)=Getsecs;end
d=diff(times);
max(d)
Here, max(d) fluctuates between 0.0001 (which is good, but appears
rarely) and 0.005. mean(d), however is always 0.
I would appreciate any opinion about these timings. Are they normal? Do
they have to do with the monitor refresh rate? If the monitor refresh
rate is 60Hz how come I have a 18ms delay? Are there ways to improve
precision? Can I at least be certain that timing errors are constant?
Thank you all for your attention,
Pandelis Perakakis
PhD researcher in psychophysiology,
University of Granada,
Spain
I am using the last beta version of PsychToolbox with Matlab R2006a and
a Turion 64x2 processor running on windows XP. I am posting this message
because I detected some timing precision problems and I would like to
ask your opinion about what may be causing them and if there is any way
to improve timing precision. The simple script I used to detect the
problems is the following:
[w rect]=Screen('OpenWindow', 0);
Priority(7);
gray=GrayIndex(0);
Screen('DrawText', w, 'A' , 200, 100, [0, 0, 0, 0]);
flipini=GetSecs;
[VBLTimestamp StimulusOnsetTime FlipTimestamp]=Screen('Flip', w);
WaitSecs(2);
endwait=GetSecs;
[VBLTimestamp2 StimulusOnsetTime2 FlipTimestamp2]=Screen('Flip', w);
Screen('CloseAll');
I attach a txt file with the messages I receive in my command window
after executing the script. The one that worries me most is:
WARNING: PTB's Screen('Flip') command missed the requested stimulus
presentation deadline
WARNING: a total of 2 times during this session!
The timimng problems become obvious by calculating some of the variables:
StimulusOnsetTime-flipini=0.00672851696254, which means a delay of 6ms
and
StimulusOnsetTime2-endwait=0.01884155754124, a delay of 18ms!
Another script I use to prompt timings is:
Priority(7);
times=zeros(1,10000);
for i=1:10000;times(i)=Getsecs;end
d=diff(times);
max(d)
Here, max(d) fluctuates between 0.0001 (which is good, but appears
rarely) and 0.005. mean(d), however is always 0.
I would appreciate any opinion about these timings. Are they normal? Do
they have to do with the monitor refresh rate? If the monitor refresh
rate is 60Hz how come I have a 18ms delay? Are there ways to improve
precision? Can I at least be certain that timing errors are constant?
Thank you all for your attention,
Pandelis Perakakis
PhD researcher in psychophysiology,
University of Granada,
Spain