Why VRR Monitors Need Variable Overdrive (Dynamic Overdrive)

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Why VRR Monitors Need Variable Overdrive (Dynamic Overdrive)

Post by Chief Blur Buster » 28 Jan 2019, 01:51

Crossposted here, because it's a monitor-engineering question that even technical users ask about.

Many cheaper/older FreeSync monitors do not even have dynamic overdrive during FreeSync operation, which can cause worsened variable ghosting effects. Some newer FreeSync monitors now has dynamic overdrive! Read on to find out why variable overdrive can be important for variable refresh rates...

<Blur Busters Technology Pandora Box>

Why VRR Displays (GSYNC, FreeSync) Need Dynamic Overdrive
pegnose wrote:Could you perhaps spare a few words as to why dynamic overdrive tuning is a thing with VRR? Could you not just tune overdrive to the fastest frame transition needed on a given display with a set max G-Sync frame rate and leave it at that?
That reduces motion quality especially if there's different overdrive needs at different refresh rates.

LCD pixels aren't static. Due to LCD being physical molecules that have momentum, there's a GtG pixel response (Grey to Grey). They're in continuous momentum (fades from one color to the next). Look at high speed videos of LCD Refresh Cycles

If you interrupt that momentum early or late, the average color of a pixel may be different. (photons emitted from that pixel for one frametime)

Now, when we intentionally change overdrive settings, you can see various ghosting artifacts:
  • Too excessive overdrive -- the trail is overshooting (bright corona)
  • Too little overdrive -- the trail is undershooting (big blurry trail)
  • Just right overdrive -- both the leading/trailing edges are symmetric motion blur.

Imagine getting coronas at high framerate and ghosting at low framerates, only getting the neutral look at mid framerates. Now, the most common situation is that fixed overdrive is calibrated for the highest framerates in VRR, so what happens is that you get perfect looking motion at high Hz but you get coronas/ghosting a low framerates in VRR.

Now imagine that varying in realtime depending on framerate!
That's what happen on a very terrible poorly-tuned VRR display; the varying overshoot/undershoot behavior.

- Imagine a pixel still in transition during a variable frametime.
- We're transitioning from a darkgrey to lightgrey pixel.
- For simplicity, let's assume GtG100% for this particular color combo is one refresh cycle.
- Imagine 1st 33% of max-Hz refresh cycle, a particular specific pixel is dark-grey
- Imagine 2nd 33% of max-Hz refresh cycle, a particular specific pixel is mid-grey
- Imagine 3rd 33% of max-Hz refresh cycle, a particular specific pixel is light-grey
- Now, colormix 33% darkgrey, 33% grey, 33% lightgrey (temporal blend) = averages out to mid-grey pixel
- Now you shorten frametimes to only 80% of original
- You're temporally colormixing 33% darkgrey, 33% grey, and 13% lightgrey = averages out to a darker-grey pixel.
- Now the blurtrail looks different (ghostier or coronia-er)
- So thus... now you understand why we need dynamic overdrive for VRR!!!

(To monitor engineers: I know this is simplistic, but this is the easiest way to explain to laypersons not aware why variable overdrive is needed for variable refresh rate monitors).

You need dynamic overdrive to make sure that the average pixel color remains consistent, dynamically intelligently speeding up/slowing down pixel response to make sure that the pixel transitions look consistent with consistent motion blur at all frame rates (refresh rates).

And complicating things more, you need to predict the NEXT frametime to properly do dynamic overdrive, because the overdrive is to also help the upcoming refresh cycle that will replace the current (still GtG-momentuming) refresh cycle on the screen.

Now, complicating this EVEN more, is that GtG pixel response speed varies depending on the original color and final color. That means you may to tune over 65,000 different GtG combinations for each shade of grey (per color channel -- red, green, blue). So now you may have massive overdrive lookup tables that are also run with a variable formula that tries to predict the next frametime. Thus requiring a custom powerful FPGA chip in the TCON in the first G-SYNC monitors to do all that realtime per-pixel overdrive processing.... Now you know one of the (multiple) reasons why G-SYNC monitors tended to have a fairly big premium, for high quality variable overdrive, it's a massive amount of realtime processing to get the best quality dynamic overdrive!

To watch pixel response in realtime -- GtG in momentum -- look at the high speed videos of LCD refresh cycles. And you'll understand why a pixel is in continuous-changing-color, especially on slower LCDs. It's always fading from one color to the next (as the Liquid Crystal Display molecules in an LCD spin to block/unblock polarized light). The manufacturer rating of GtG is simply from the GtG 10% point in curve to GtG 90% point in curve. So a GtG benchmark for black-to-white transition is actually the the stopwatch time from a very dark-grey pixel to a very light-grey pixel. But the full GtG 100% is much longer, often over several refresh cycles on many screens. The artifacts of this can still remain human visible.

Faster native pixel response (e.g. 0.5ms overdriven GtG with 3ms native GtG) can lessen the need to do a dynamic overdrive. But dynamic overdrive makes motion consistency look better at a wider range of VRR framerates.

Oh, another pandora box item: Panel temperature! Like forgetting a smartphone in a freezing car in the middle of winter, and the screen responding sloooooow. Cold LCDs are really slow. Don't forget to warm up your panel to room temp if playing on a cold panel. Like when you're saving money and set the thermostat really low when away, then you start using the panel. So turn on the panel and run it for 30 mins to warm it up to reduce ghosting. Freezing rooms in cold may have more of a ghosty-variance effect for fluctuating VRR framerates. This is more problematic on VA panels but has also affected TN/IPS panels (e.g. more overdrive variance in VRR, or increased strobe crosstalk, or monitor ghosts/coronas a bit more than usual). You want to play the monitor at full room temperature (or slightly warmer) to match its original overdrive tuning for minimum ghosting/coronas.

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