# timing on G4's

**URL:** <https://psychtoolbox.discourse.group/t/timing-on-g4-39-s/1255>\
**Category:** Uncategorized\
**Created:** [November 14, 2000, 11:25pm UTC](https://psychtoolbox.discourse.group/t/timing-on-g4-39-s/1255 "2000-11-14T23:25:38Z")\
**Posts on this page:** 1\
**Page:** 1

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**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 14, 2000, 11:25pm UTC](https://psychtoolbox.discourse.group/t/timing-on-g4-39-s/1255/1 "2000-11-14T23:25:38Z")

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> \>\> \>\>Hi Jeremy,  
> \>\> \>\>  
> \>\> \>\>This is just something you might want to look into, concerning timing  
> \>\> \>\>with video cards in the new G4s. The software we use (Vision Shell),  
> \>\> \>\>does not give accurate timing with the video cards that are built  
> \>\> \>\>into the new G4s. I have also had difficulty getting my timing to be  
> \>\> \>\>accurate with MATLAB and the psychophysics toolbox. I know the  
> \>\> \>\>website says everything is compatible, blah blah. But Just checking  
> \>\> \>\>my timing with this simple timing test I attached, I find that my G4  
> \>\> \>\>is much less accurate with timing than Helga's G3. Maybe its related  
> \>\> \>\>to the same video card problem. For example, waiting for 25 ticks, my  
> \>\> \>\>computer was off by anywhere from 1 frame to 3 or 4 frames, whereas  
> \>\> \>\>Helga's had absolutely perfect timing (i.e., it waited as long as I  
> \>\> \>\>told it to, down to the millisecond). I think you are using newer  
> \>\> \>\>computers for testing, so it might be worth checking the timing on  
> \>\> \>\>those.  
> \>\> \>\>  
> \>\> \>\>George  
> \>\> \>\>Attachment converted: Geldard:simple\_timing\_test.m (TEXT/MSIE) (0004BFF1)  
> \>\> \>  
> \>\> \>  
> \>\> \>dear jeremy & george  
> \>\> \>  
> \>\> \>We pay a lot of attention to timing issues, and in fact have identified  
> \>\> \>and solved timing problems in the new ATI video drivers that accompany  
> \>\> \>the G4 processors. At this time i'm not aware of any timing problems, and  
> \>\> \>our tests are quite rigorous. But we always welcome user reports that can  
> \>\> \>document a problem, as we all use different configurations that can  
> \>\> \>exhibit unexpected differences.  
> \>\> \>  
> \>\> \>by the way, it is very unlikely that the processor difference itself (G3  
> \>\> \>vs G4) matters. We have yet to find any circumstance where that mattered  
> \>\> \>(except, of course, for the welcome doubling of floating point speed in  
> \>\> \>the G4). What does matter, always, is the video driver, and they keep  
> \>\> \>changing, model to model, as apple and ATI fiddle. However, despite  
> \>\> \>problems we had to work around, I'm happy with the ATI drivers. They're  
> \>\> \>fast, and they're getting better.  
> \>\> \>  
> \>\> \>I didn't receive the attachment, but if George is simply measuring   
> \>\> \>how long it takes for 25 ticks to elapse,  
> \>\> \>and finding that it's slow, then the answer probably has nothing to do  
> \>\> \>with his video card. Macs aren't great at servicing interrrupts. They  
> \>\> \>turn off interrupts within interrupt service routines. So if your machine  
> \>\> \>is really busy servicing interrupts, it may miss a few. Thus if George's  
> \>\> \>machine has a really busy internet connection, or an empty zip drive (yes,  
> \>\> \>ironically, it issues more interrupts when empty), his machine may miss  
> \>\> \>a few ticks. He can test this assertion by turning off  
> \>\> \>interrupt-demanding services and noticing whether his timing becomes  
> \>\> \>accurate again.  
> \>\> \>  
> \>\> \>In the PsychToolbox, we offer a timing service based on an internal  
> \>\> \>counter in the PPC processor that's immune to interrupts (unlike all the  
> \>\> \>other timers offered by the Mac OS). So GetSecs is always accurate.  
> \>\> \>  
> \>\> \>best  
> \>\> \>  
> \>\> \>denis  
> \>\> \>  
> \>  
> \>  
> \>Hi Professor Pelli,  
> \>  
> \>Thank you for your thorough response. You were right on the nose by   
> \>the way. I was getting interruptions from our server. It was a silly   
> \>mistake, but I just overlooked this possibility. Thanks again for   
> \>helping us figure this out.  
> \>  
> \>George

dear george  
  
oh good! I could only guess, based on my experiences. Glad we got that   
under control.  
  
I've cc-ed to the psychtoolbox forum, so that others might learn from   
this experience.  
  
best  
  
denis