A few people, including Erik Flister and me, did it in a quick
and dirty way on different setups.
I did some quick and dirty tests a couple of months ago with
a photo-diode, a CRT and some trigger box on WinXP with
NVidia hardware.
I didn't take any notes or calculate any statistics, just a series
of repeated measurements under different loads to see if
it looks good. All tests showed that the VBLTimestamp/
Stimulusonsettimestamps are in agreement with the photo-
diode. Measured difference was less than 1 millisecond,
however that was also the resolution of my equipment, so
i can't make any claims as to how much more accurate than
1 msecs it was. In theory it should be accurate to a few dozen
microseconds, but one would need a better measurement
technique to resolve that.
We did something similar when testing the new low latency sound
driver on WindowsXP and OS/X MacBookPro, basically comparing
emitted sound pulses with emitted light pulses and found
similar results - better than 1 msec.
So i assume the timestamps are pretty reliable on a CRT on a
properly configured setup, but any kind of independent
measurements is very welcome. Theres a restriction on MS-Windows:
The high precision timestamping is disabled when PTB detects
any of these: a) broken systems, b) Intel graphics chips,
c) Dual display setups, so noise of the timestamps may
be much higher in those cases. In case c), one can manually
reenable the mechanism after checking that everything is
ok.
Regarding KbCheck: I'm not aware of anybody testing this
recently. I'd expect the latency between registration of a
keypress by the computers hardware and the reported keypress
by KbCheck to be lower than 1 millisecond, but the real problem
is the keyboards and USB connection: If you use a standard
keyboard connected to your computer with a standard USB
connection you can get latencies or timing jitter in
the range anywhere between 20 - 50 msecs (worst case),
depending on keyboard, experiment and typing behaviour
of your subject.
This is an inherent limitation of keyboard technology, nothing
solvable by software. There are companies that sell keyboards
specially tailored to the needs of psychophysics, these should
do much better (for a much higher price tag) - they are basically
response boxes that look (to you) and act (wrt. to the operating
system) mostly like normal keyboards, but they are response boxes.
best,
-mario
--- In psychtoolbox@yahoogroups.com, "rogierland" <rogierland@...> wrote:
>
> Has anybody measured the relation between "Screen"'s
> "FlipTimestamp"/"VBLTimestamp" and actual changes in luminance on a
> CRT screen using a photodiode?
>
> Same question for "KbCheck"'s "secs" relative to the actual keypress.
>