FAQ: Understanding HDMI Quick Frame Transport (lower lag)

[bbdmsa]

It only counts as overclocking if you increase it past the display's rated specs. If you create a 60Hz QFT mode on a 240Hz display for example using the same pixel clock that the display uses in its 240Hz mode, then it's not overclocking. The display already supports that pixel clock.

Okay, so assume the display accepts the signal with the same (max) refresh rate and greater vblank, meaning higher pixel clock. Is there anything you can say about the resulting horizontal scan rate of the panel, or does it depend on the implementation? What factors come into play here?

[RealNC]

Okay, so assume the display accepts the signal with the same (max) refresh rate and greater vblank, meaning higher pixel clock. Is there anything you can say about the resulting horizontal scan rate of the panel, or does it depend on the implementation? What factors come into play here?

If you exceed the pixel clock, the firmware usually refuses the signal. There are some displays that allow higher clocks, but most don't.

[bbdmsa]

If you exceed the pixel clock, the firmware usually refuses the signal. There are some displays that allow higher clocks, but most don't.

I just want to know hypothetically what would happen. As I mentioned before, my monitor is "overclockable" to 200 Hz even if it skips frames, so presumably it accepts a standard 200 Hz pixel clock. Therefore, in terms of pixel clock, the monitor should theoretically be able to handle 144 Hz QFT with 200 Hz frame delivery speed.

[RealNC]

I just want to know hypothetically what would happen. As I mentioned before, my monitor is "overclockable" to 200 Hz even if it skips frames, so presumably it accepts a standard 200 Hz pixel clock. Therefore, in terms of pixel clock, the monitor should theoretically be able to handle 144 Hz QFT with 200 Hz frame delivery speed.

That's up to the firmware. It might skip frames in QFT mode or it might not. There's no way to tell until you try.

[bbdmsa]

That's up to the firmware. It might skip frames in QFT mode or it might not. There's no way to tell until you try.

Just tested this. I was able to raise vertical total by about 15% from CVT-RB until there were obvious consequences. Regular QFT works fine. How can I tell if the panel is actually scanning out faster?

[RealNC]

Just tested this. I was able to raise vertical total by about 15% from CVT-RB until there were obvious consequences. Regular QFT works fine. How can I tell if the panel is actually scanning out faster?

It has to scan out faster because that's what the GPU is sending. If it doesn't scan out faster, then that means you're gonna see something missing (like part of the image isn't visible, or frame skipping.)

[bbdmsa]

It has to scan out faster because that's what the GPU is sending. If it doesn't scan out faster, then that means you're gonna see something missing (like part of the image isn't visible, or frame skipping.)

The Chief has mentioned how cable scanout and panel scanout can be different, and how panels can be either fixed scanrate or flexible scanrate. How do we know the display isn't doing some kind of processing or buffering to remove/ignore extra vblank and end up scanning out at the same rate?

[RealNC]

The Chief has mentioned how cable scanout and panel scanout can be different, and how panels can be either fixed scanrate or flexible scanrate. How do we know the display isn't doing some kind of processing or buffering to remove/ignore extra vblank and end up scanning out at the same rate?

One way to find out would probably be testing it with scanline sync (RTSS) or latent sync (Special K). You can target a specific scanline to hide tearing in, so you can move that target up/down while the "visualize tearlines" option is enabled. If the tearing appears at the top of the screen while is should be still hidden inside the VT, then the display is not using the VT you configured.

So basically note the scanline number at which tearing is barely visible at the bottom of the screen, and the number at which it just appears at the top. Subtract the latter from the former to get an estimate of the used VT.