Understanding Display Scan-Out Lag With High Speed Video

High Speed Video Makes Display Refresh Behaviour Easy To Understand

Blur Busters released the high speed video of motion blur reduction in year 2012, and we were the world’s first website to measure the input lag of G-SYNC via high speed camera in 2014.

New TestUFO Test For High Speed Cameras

We now have a new Test UFO test, called TestUFO Scan-Out which automatically rotates between 4 different brightly colored images every refresh cycles. This test pattern is intentionally extremely flickery, for the purpose of maximizing contrast between refresh cycles during super-slow-motion video analysis.

High Speed Video Of 60Hz IPS Desktop Monitor

The screen refreshes top-to-bottom, with a fuzzy zone wiping down the screen.  The fuzzy zone is the LCD pixel response lagging behind (GtG fade zone). The fuzzy zone appears to about one-quarter the height of the screen, which is what is expected for GtG pixel response (5ms) approximately 1/4th of a refresh cycle (16.7ms for 60Hz).

High Speed Video Of 60Hz MacBook Laptop

The refreshing behaviour is very similar to a typical laptop, top-to-bottom, with a fuzzy zone representing the LCD pixel response.

High Speed Video Of 60Hz iPad Mini 4

Here’s where this diverges interestingly; this iPad scans from top-to-bottom in portrait mode. When filmed in landscape mode, this becomes a sideways scan effect:

High Speed Video Of 60Hz Samsung Galaxy Tab S4 OLED

And for a really clean refresh cycle, here’s a high speed video of an OLED, provided by Edward on Twitter. The incredibly fast pixel response of an OLED means the fade zone is extremely tight in comparison to LCD:

Not All Pixels On Displays Refresh Simultaneously

As seen in these high speed videos, displays often refresh sequentially, in a top-to-bottom fashion.

There can be a latency differential for top edge and bottom edge, which can get rather significant in some sync modes (VSYNC ON) at lower refresh rates. The refreshing sequence is illustrated temporally in this scan graph:

It is worth noting that refresh cycles cannot be transmitted instantly all at once on a cable.

Cable Scan-Out Versus Display Panel Scan-Out

Pixels are transmitted one pixel row at a time over a cable. This is because GPUs have to serialize two-dimensional refresh cycles over a one-dimensional cable. Pixels are output one pixel at a time, left-to-right, top-to-bottom.

Panel refreshing may be a different pattern, different direction, or different velocity, and may be totally different (e.g. DLP) from the cable sequence. However, most high end eSports gaming monitors are now able to real-time synchronize panel refresh to cable delivery sequence with only line buffer processing rather than full frame buffer processing. This is much much like an old CRT tube, except done flickerfree in a sample-and-hold fashion, and this reduces input lag to the absolute minimum possible.

This is classic raster-based scanning on the cable, the refresh cycle direction of top-to-bottom has remained identical for nearly 100 years all the way from 1930s television broadcasts through 2020s DisplayPort and HDMI 2.1 cables, even with variable refresh rates!

Sub-Refresh-Cycle Latencies Are Possible With VSYNC OFF

At frame rates exceeding refresh rate using VSYNC OFF, latencies of less than a refresh cycle are possible even for screen bottom edge. Here is a scan graph at 180 frames per second at 60 Hz, generating 3 tear lines per refresh cycle:

Running at a frame rate three times the refresh rate, will result in 3 tear lines per refresh cycle, although some tearlines may end up hidden in the interval (VBI) between refresh cycles.

In this example, each frame slice is 1/3rd refresh cycle latency, and the updated frame (slice) is metaphorically spliced in realtime at the display output’s current scan-out position.

There are advantages of framerates above refresh rates including lower input lag — which can end up being less than a refresh cycle even for the screen bottom edge. This is why paid professional eSports players often prefer VSYNC OFF.

Variable Refresh Rate Uses Variable-Length Intervals Between Refresh Cycles

Variable refresh rate (VRR) actually varies the size of the vertical blanking interval (the pause between refresh cycles) in order to temporally space-apart refresh cycles.

The top-to-bottom raster scan still remains, however the intervals between refreshes can vary instead of being a fixed interval. Here is the scan diagram of 100fps at 144Hz:

In this asynchronous-refreshing situation, the monitor actually waits for the computer to begin transmitting the new refresh cycle over the cable.

When a video game is finished rendering a new frame and presents it to the GPU video output, the GPU output essentially immediately begins transmitting instead of waiting for the next fixed refresh interval. The monitor immediately begins the new refresh cycle right on the spot.

Another huge benefit of variable refresh rate is very low scan-out lag. A 240Hz G-SYNC monitor can display a 30Hz refresh cycle in 1/240sec, refreshed near real-time off the video cable. This is a “Quick Frame Transport” (QFT) behavior of VRR.

Conclusion

Display scan-out research has become much easier with the advent of inexpensive high speed cameras.  Reviewers and manufacturers are now strongly encouraged to purchase high speed cameras to analyze scan-out latency of their displays, and troubleshoot problems caused by:

  • Confirming Whether Display Scan-Out Is In Sync With Cable Scan-Out
  • Confirming Existence Of Sub-Refresh Latencies For Screen Bottom
    This is possible with VSYNC OFF at frame rates significantly exceeding refresh rates.
  • Abnormally high latency of lower refresh rates than higher refresh rates.
    Cause: Display buffering to scan-convert a slow-scanning 60Hz signal to a fast-scanning 240Hz panel. 
  • Benchmarking Strobe Backlight Quality & Strobe Crosstalk

Inexpensive High-Def 1000fps Cameras

There are now inexpensive high definition Super Slow Motion Cameras capable of 1000fps HD that works excellently for display scan behavior research with the new TestUFO Scanout Test.

Some of these only records 1000fps for only a fraction of a second, however this is plenty for refresh behavior analysis and debugging latency engineering problems in gaming monitors.