PsychoPy and GSYNC

[Stewart Heitmann]

Hi all,

Does anybody know how well psychopy works with the new GSYNC technology provided by NVidia?

GSYNC does away with traditional fixed refresh rates we all know and love. Instead, it allows the monitor to refresh the display whenever the graphics card is ready with a new frame. Consequently, refresh rates vary on a frame by frame basis.

Presumably psychopy's window "flip" command is meaningless with GSYNC. So I would like to know whether other means (such as user-programmed clocks for example) could be used to attain accurate flip times within a GSYNC environment? Does anybody know?

I am aware that the obvious answer is not to use GSYNC, however I wish to generate a flicker stimulus with an arbitrary duty cycle (up to 60 Hz). Conventional fixed frame rates restrict the duty cycles that I can achieve.  In principle, the variable frame rates of GSYNC should permit me to generate arbitrary duty cycles. My concern is timing accuracy. I still need my duty cycle to be accurate which is why the timing of frame updates is crucial.

Does anybody have any experience with GSYNC?

Stewart Heitmann
University of Pittsburgh


 

[Jon Peirce]

I haven't tried it. I imagine that the flip() command will have its effect but the flip will occur sooner than waiting for the next 16.6ms period. I imagine that if there is a delay caused by screen not being ready then the filp() command will still wait.

But the point is I haven't had a device (or time) to play yet.

Let us know what happens - will be great to know

cheers
Jon

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[Sol Simpson]

I've been wondering about this as well. When reading about this, and other adaptive variable refresh rate technologies, coming too market it makes me worried. Hopefully there will be a disable option when you want the monitor to 'vsync' at a constant interval.

[Stewart Heitmann]

Just to follow up, 

I have since acquired some GSYNC hardware (NVIDIA GTX 970 graphic card + BENQ XL240G monitor) and have it running under 64 bit Linux (Ubuntu 14.04LTS, kernel version 3.13.0-44). The proprietary NVIDIA driver seems to be working well although it was rather tricky to install.

You may be relieved to know that the NVIDIA driver does allow GSYNC to be disabled at will. So does the BENQ monitor for that matter.

The psychopy flip command works as expected when GSYNC is disabled. It also seems to work when GSYNC is enabled. In the latter case the frame interval timings are more variable (as expected). However, the frame intervals (reported by win.FrameIntervals) appear to be longer (slower) under GSYNC. That surprised me. However it is only an early indication. I haven't had a chance to investigate properly.

Stewart Heitmann

[Genís Prat Ortega]

Hi Stewart,

I am running an experiment where I have to update the contrast of two gabor patches every 50 ms while a eyetracker is testing if the subjects is fixation its gaze.
Now I have a monitor with a refresh rate of 60Hz and a gforce  210. Thus I update the stimulus every 3 frames. But I drop some frames. I think that If I can fix the refresh rate to 20 Hz then my computer will have enough time to update stimulus and check fixation without dropping frames. Do you think GSYNC can fix a refresh rate at 20 Hz?

Thank you very much in advance

Genís

El dijous, 5 febrer de 2015 17:47:56 UTC+1, Stewart Heitmann va escriure:

[Stewart Heitmann]

Hi Genis,

You can disable GSYNC altogether to get very reliable frame timing. I do that on my machine (linux) via the Nvidia X-Server Settings software. You can then set your graphics card to 20Hz fixed frame rate. You can use PsychoPy to verify the actual frame rates you get.

Stewart Heitmann