This test is meant for Microsoft Windows only!
Performs a reliability test of your systems timing hardware. This script
tries to find out if your systems clock works correctly, ie., if
GetSecs(), WaitSecs(), Screen(‘Flip’) and the PsychPortAudio functions
for timed stimulus onset and clock queries will work correctly.
The optional parameter ‘n’ specifies how many samples to use for each
test run. A larger number means longer runtime, but higher accuracy of
the results. Default is 100000 samples.
The test shows a lot of different data plots, but you should focus on the
results printed to the Matlab command window. If they sound ok then you
can probably ignore the plots.
The test exercises your timing hardware: It takes ‘n’ time samples in a
loop which repeatedly asks GetSecs for the time. GetSecs is used in a
special debug-mode which also provides time readings from an alternative
hardware clock. After sampling, the script compares timestamps from the
normally used high precision timer against samples taken from the low
precision reference timer. In an ideal world on a well working system,
both timers should deliver roughly the same time readings. A fixed offset
between the two clocks is normal - they both count time from system
bootup or reset, but they are started with a slight offset at system
start. A slowly drifting offset is not uncommon on lower quality
hardware, this is considered ok as long as the drift is small. Important
is that time is monotonically increasing on both clocks and that the
ratio of elapsed time in both clocks is nearly the same: Dividing elapsed
time as reported by the high-res timer by the elapsed time reported by
the low-res timer should yield a ratio of very close to 1.0. The test
also checks for regular low-res timer ticks, if pause()’ing Matlabs
execution for multiple seconds affects the clocks in some worrying
manner (e.g., time slowing down) or if quick switching between multiple
cpu cores of a multi-core machine causes time inconsistencies, e.g., time
The test first runs a block of trials with PTBs timing in “normal mode”,
where PTB applies a couple of fixes for broken hardware.
Then it disables those fixes and retests, to assess how earlier PTB
versions worked on your system - when no such special workarounds were
Please note that this test is not 100% water-proof, it could
theoretically have ‘false negatives’, as many problems with the clocks
are associated with the behaviour of system power management, which
itself is influenced by all kind of factors, e.g., what other
applications are running in parallel to Matlab, if there is any network
traffic, what kind of hardware is connected to the system…
We will try to improve the test as we learn more about possible causes of
Starting with PTB releases as of ‘beta’ from 26th November 2007, PTB also
performs a couple of runtime checks to spot more timer problems. These
checks are performed all the time while your scripts are executing, so if
you see some “CRITICAL-WARNING” messages about timer problems at the
Matlab prompt while your scripts are executing, better take them serious!
For more (up to date) information about system configurations that might
suffer from clock problems, background information and troubleshooting
tips, visit the Psychtoolbox Wiki’s FAQ section, specifically:
Btw. Currently there are no known problems with timers and clocks on PC
hardware running recent Linux distributions or on any Apple Macintosh
computers running MacOS/X ;-) – That’s why first versions of this test
are only targeted at MS-Windows.