# Re: DualHead ScreenTest report **URL:** \ **Category:** Uncategorized\ **Created:** [March 30, 2002, 4:16pm UTC](https://psychtoolbox.discourse.group/t/re-dualhead-screentest-report/914 "2002-03-30T16:16:16Z")\ **Posts on this page:** 1\ **Page:** 1
**Author:** ![denispelli](https://avatars.discourse-cdn.com/v4/letter/d/cdc98d/32.png) [@denispelli](https://psychtoolbox.discourse.group/u/denispelli)\ **Post date:** [March 30, 2002, 4:16pm UTC](https://psychtoolbox.discourse.group/t/re-dualhead-screentest-report/914/1 "2002-03-30T16:16:16Z")
> \>Hello, > \> > \>My name is Dave, and I am working with Rhea Eskew. > \> > \>I have attached a copy of a screentest report. And I was hoping you would be > \>able to help me interpret it. > \> > \>Thank you very much. > \> > \> > \>-- > \>David Richters > \>Northeastern University > \>(617) 373-5057 dear dave thanks for sending in your reports. they reveal a surprising and possibly important anomaly. We routinely ask users of the PsychToolbox to attach a ScreenTest report to their bug reports. You've also included a TimeVideo report. TimeVideo, for those who don't know, is a much older free-standing application that ScreenTest was originally based on. I wrote both. It's still available: \<[http://vision.nyu.edu/VideoToolbox/Download.html](http://vision.nyu.edu/VideoToolbox/Download.html)\> The surprise is that TimeVideo indicates that your DualHead card is working perfectly in every way, but that ScreenTest reports that it lacks the desirable but optional feature of waiting for blanking when doing SetClut. The two reports contradict each other on this point. The simplest explanation is that the DualHead card supports the relatively new optional Apple driver call that turns on and off waiting for blanking by cscSetEntries (i.e. SetClut), but with a nonstandard polarity. TimeVideo is older and doesn't know about this call, so it simply leaves your card in the default state, which apparently has this feature on. SCREEN or ScreenTest, to be helpful, tries to turn the feature on, but apparently in fact turns it off. i'm guessing here, but this would explain your symptoms. I've enclosed an enhanced version of WaitBlankingTest that explicitly tests this issue. Could you run it and email the results to the forum? best denis ps you can test my diagnosis in your programs by setting these three preferences: SCREEN(which,'Preference','AskSetClutDriverToWaitForBlanking',0); % nonstandard polarity SCREEN(which,'Preference','SetClutCallsWaitBlanking',0); % rely on driver, not interrupt SCREEN(which,'Preference','SetClutDriverWaitsForBlanking',1); % tell psychtoolbox that driver waits the preference settings are lost when you close the last window on a device, so to do this test you may need to paste the three lines above into your program at some point AFTER the first window is opened. NOTE TO ALL: [denis.pelli@...](mailto:denis.pelli@...) is my preferred email address. Please use that address. My "verizon.net" address will go away soon, partly because Verizon forces me to use that as my "From:" address. My current email program won't allow me to designate a separate "Reply-to:" address.