Questions regarding vbi, vrr and display-source commication.

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Öcü
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Questions regarding vbi, vrr and display-source commication.

Post by Öcü » 08 Feb 2024, 02:02

Hi. Ive been reading about vrr and how displays change their vbi timing in combination with qft to match source (GPU) render (frame) timing but i got more and more confused so here my questions.

1-) How does display knows about the frame time of said frame. Does gpu send this information before or after pixel color information.
2-) How about hdr or backlight information is it sent before or after pixel color information
3-) What is the theoric limit for qft like hdmi or display port. I mean is it possible to transport ( cable scanout) 4k image in 2 ms ( like 500 hz on a 120 hz monitor ) or something like extremely short time.

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RealNC
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Re: Questions regarding vbi, vrr and display-source commication.

Post by RealNC » 08 Feb 2024, 10:27

Öcü wrote:
08 Feb 2024, 02:02
1-) How does display knows about the frame time of said frame. Does gpu send this information before or after pixel color information.
2-) How about hdr or backlight information is it sent before or after pixel color information
3-) What is the theoric limit for qft like hdmi or display port. I mean is it possible to transport ( cable scanout) 4k image in 2 ms ( like 500 hz on a 120 hz monitor ) or something like extremely short time.
After the pixels are transmitted, the vertical blanking interval begins. Normally this is short. For QFT it's longer, because the pixel data transmission takes less time. With a fixed refresh rate (so no VRR,) the overall time needs to be constant and thus the VBI is longer with QFT.

With VRR, the VBI is variable. The pixel data is always transmitted at a fixed speed (at the speed of the current video mode.) So you get QFT for free if you use the highest Hz mode the display supports and the game runs at a lower frame rate. But since the frame times are variable, the VBI needs to be variable. The end of the VBI is signaled through a data packet that the display recognizes.

The limit for QFT is the maximum pixel clock supported by the display. Usually this is simply the pixel clock of the highest Hz mode supported by the display. A 120Hz TV for example can use the 120Hz pixel clock for 60Hz sources. That's QFT. But for 120Hz sources, there is no QFT. But displays can support higher pixel clocks than that specifically for the use of QFT. For example a 120Hz TV, even though it doesn't support 240Hz, could have a pixel clock that matches 240Hz scanout speed so that it does QFT at 120Hz. Not sure if such TVs exist.

You can always try and overclock the pixel clock by using larger vertical totals, but usually you can't gain much there. At some point the display will refuse to operate.

No idea about HDR.
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