# Flip command time stamps - clarification from Mario **URL:** \ **Category:** Uncategorized\ **Created:** [November 1, 2006, 5:15pm UTC](https://psychtoolbox.discourse.group/t/flip-command-time-stamps-clarification-from-mario/1785 "2006-11-01T17:15:16Z")\ **Posts on this page:** 1\ **Page:** 1
**Author:** ![Allen\_Ingling](https://avatars.discourse-cdn.com/v4/letter/a/a183cd/32.png) [@Allen\_Ingling](https://psychtoolbox.discourse.group/u/Allen_Ingling)\ **Post date:** [November 1, 2006, 5:15pm UTC](https://psychtoolbox.discourse.group/t/flip-command-time-stamps-clarification-from-mario/1785/1 "2006-11-01T17:15:16Z")
Hello all, I emailed Mario to clarify 'Flip' command timestamps. Here is the edited version of that email. I thought of posting it here in case you had similar questions. "Hi Mario, Have a quick question. I went through your web document on [http://www.kyb.tuebingen.mpg.de/bu/people/kleinerm/ptbosx/ptbdocu-](http://www.kyb.tuebingen.mpg.de/bu/people/kleinerm/ptbosx/ptbdocu-) .0.5MK4R1.html. And I gathered the following: vbl - time stamp of the beginning of the vertical retrace - i.e., the timestamp when the electron gun 'just' finished the last horizontal trace and is ready to move back to the top left corner of the screen. stimOnsetTime - timestamp of the end of vertical retrace. fliptime - timestamp when the flipping process is over. If I understood these correctly, the stimOnsetTime must always be greater than vbl since vertical retracing takes finite time. Fliptime could be after stimOnsetTime. When I ran my code and looked at the time stamps, I invariably noted the following 1. The vbl and stimOnsetTime were equal. 2. The Fliptime was always after the stimOnsetTime even for a 100 X 100 window with 100 random dots. I have copy-pasted the results from the MATLAB window after running the code \*Mani\_drawdots.m\* (please see the attachments). EDU\>\>vbl = 9.172154133039482e+004 sot = 9.172154133039482e+004 flipt = 9.172154137748956e+004 miss = -0.01674934297625 beam = -1 EDU\>\> \*isequal(vbl,sot)\* ans = 1 EDU\>\> \*flipt - sot\* ans = 4.709474160335958e-005 Where am I doing the mistake? Thank you Cheers Mani The results are correct for a machine that does not support beamposition queries. On machines that do support beamposition queries (All OS-X machines with PowerPC processor afaik, don't know about the OS-X IntelMacs), we use the queried display rasterbeam position to compute a highly robust timestamp. See "help Beampositionqueries" at the Matlab prompt. In that case, vbl is the timestamp of the beginning of the vertical retrace == time of buffer swap. stimOnsetTime is the time of end of vertical retrace == real stimulus onset == when the display starts drawing the first scanline. fliptime is the time when the Flip command itself finished executing, that can be anytime after vbl and will be before stimonsettime on a fast machine. vbl and stimOnsetTime are unaffected by operating system scheduling jitter and other timing noise in the machine, whereas the fliptime is an indication on how "quick" the Matlab and C code is executed. On machines that don't support beamposition queries (all M$-Windows PC's, all Linux PC's, maybe some IntelMacs?), we cannot compute these precise timestamps, so internally we simply call GetSecs --\> vbl = stimOnsetTime and fliptime \> vbl and beam == -1. This method of timing is less robust but its the best we can do in that case. Its basically the same as calling GetSecs() in Matlab after the 'Flip' call, just with slightly less jitter because its executed in the C code instead of Matlabs interpreter. So your machine doesn't support beamposition queries :( -- What machine did you test on and with which graphics card? ciao, -mario"