Confused? What the hell is a "Vertical Total"? And why does ToastyX CRU look like an airplane cockpit? And what is this silly "VT1350" sorcery I see for BENQ Blur Reduction discussions? Is this black magic. And what does VSYNC have to do all of this confusing stuff?
Let this glossary help de-mystify this for you!
Both AMD and NVIDIA control panels have custom resolution tweaking utilities built in. Or you can use a third party tweaking utility such as ToastyX CRU. In these utilities, you see terms like "Vertical", "Horizontal", "Vertical Front Porch", "Back Porch", "Sync", etc. Let's simplify for you.
Not all pixels refresh simultaneously at the same time. Many cables have to deliver one pixel at a time. Many screens have to refresh one pixel at a time.
Video cables deliver pixel rows from top to bottom. Most screens also refresh from top to bottom. Watch a high speed video of a screen refreshing from top-to-bottom, seen from www.blurbusters.com/scanout ... The separator between refresh cycles is called VSYNC, a portion of the Vertical Blanking Interval.
First, let's introduce good old-fashioned VSYNC. Have you ever seen an old TV with a rolling picture, because of a mis-adjusted VHOLD?
Today, this is actually all adjustable. Here's an example of a Custom Resolution Utility screen, accessed via NVIDIA Control Panel, when you create a new Custom Resolution.
Some of these numbers metaphorically represents the thickness of that VSYNC black bar separating refresh cycles. Although not displayed onscreen, one can imagine that the vertical blanking interval is just offscreen the top/bottom edges of your screen. Essentially, your computer is outputting a bigger signal (to allow synchronization intervals and porches) and your screen is only showing the centre of that bigger signal.
Whatever is in the GPU's visible graphics buffer (aka "front buffer") is automaically embedded in a larger virtualized resolution (hidden offscreen pixels that are overscan/porch and synchronization markers) as the GPU transmits out of the video output, one scanline at a time:
The CRU (Custom Resolution Utility) terminology, is explained as:
To understand better, just imagine your computer monitor's pixel grid is a big calendar
An Earth calendar is like a low-resolution screen of 7x5 pixels, with each day as 1 pixel.
However, a 1920x1080 monitor is more like a massively giant alien calendar (Of TestUFO alien fame!) where each weeks is a huge 1920 days long and more than 1080 weeks tall! A very huge grid of millions of days in one "alien month".
Transmitted one pixel at a time left-to-right, top-to-bottom as a way to serialize 2D images over what is essentially a 1D signal (like VGA or HDMI or DisplayPort). Newer signal standards may be packetized (e.g. pixel row compressed all at once) but it's still fundamentally a sequential stream serialization of a series of images (refresh cycles). This can also be both the video cable point of view, as well as the panel point of view (CRT, LCD, OLED) for displays that have a top-to-bottom scanout, as seen in high speed videos.
- Horizontal Sync: There are hidden synchronization pixels beyond left-edge of screen (like a Sunday) & right-edge of screen (like a Saturday). The purpose is to signal a display to begin outputting the next row of pixels.
- Vertical Sync: Also known as VSYNC or Vertical Blanking or VBI. There are hidden synchronization pixels beyond top edge and after bottom edge (e.g. like 1st week and 5th week). The purpose is to signal a display to begin the next refresh cycle. Videogames use "VSYNC ON" / "VSYNC OFF" to tell the game whether or not to synchronize frames with the refresh cycles of the display, for better smoothness. Sometimes when saying VSYNC, it excludes the padding/overscan (Vertical Back Porch and Vertical Front Porch), but often VBI and VSYNC is used interchangeably for most signals because the porches usually are tiny.
- Vertical Blanking Interval -- Known as VBI. The entire interval between refresh cycles. The sum of (Vertical Back Porch + Vertical Sync + Vertical Front Porch).
- Visible resolution: Metaphorically this is like the weekdays of 2nd through the 4th weeks of a calendar. The "pixels" (days) at the top/bottom/left/right edges of the calendar is invisible (hidden off-screen). Like only being able to view the centre of a calendar through a rectangular stencil. That's the "active resolution"
- Horizontal & Vertical Totals: More complexly, within both Horizontal Sync and Vertical Sync there can be multiple hidden dotclock/pixels (Front Porch, Sync, Back Porch). Instead of just 1 hidden pixel at the edges. There can be hundreds of hidden pixels for display synchronization purposes.
- Front & Back Porch: This is the padding (often hidden black or gray pixels) between the visible pixels & the synchronization pixels (which can be darker than black, aka "below black). Porches can be viewed as padding between the visible signal & synchronization signal.
- Refresh Cycle: One full frame worth of pixels. For a 120Hz display, that's 1/120th of a second worth of pixels.
("one refresh cycle" for an Earth calendar is one complete month)
- Dotclock: Number of pixels per second.
(The "dotclock" of an Earth calendar is 1 "pixel" per day! While a computer monitor doing 1920x1080 144Hz is doing more than 300Mhz -- over 300 million pixels per second).
- Horizontal Scanrate: Number of pixel rows per second.
(The "horizontal scanrate" of an Earth calendar is 1 week.)
- Vertical Scanrate: Same as Refresh Rate. Number of refresh cycles per second.
(The "vertical scanrate" (also "refresh rate") of an Earth calendar is 1 month.)
- Vertical Total: is the grand total of Vertical Resolution + Vertical Sync + Vertical Back Porch + Vertical Front Porch
- Horizontal Total: is the grand total of Horizontal Resolution + Horizontal Sync + Horizontal Back Porch + Horizontal Front Porch
- CRU: Custom Resolution Utility
- VT: Vertical Total
- VT1350: Vertical Total of 1350 pixels. Often mentioned along with BENQ Blur Reduction (XL2411 / XL2420 / XL2720 series)
Pixels are transmitted over a cable (VGA, DVI, DP, HDMI) from the computer to the monitor essentially one pixel at a time sequentially (even if packetized on DisplayPort). At dotclock rate (e.g. often 300-350Mhz dotclock for 1920x1080 120-144Hz).
Like days on a calendar are transmitted, left to right, top to bottom, scanned-out one "week" at a time. The hidden "days" at the edge of this metaphorical calendar are used for synchronization purposes and not visible on the screen. Porches and sync are hidden off the edges of the screen.
The sequence is BackPorch-Visible-FrontPorch-Sync. Beginning horizontally left-to-right, and then vertically top-to-bottom, pixels are much like how "days" are output on a calendar sequentially. Pixels are transmitted as one continuous stream, in sequence -- BackPorch-Visible-FrontPorch-Sync-BackPorch-Visible-FrontPorch-Sync-BackPorch-Visible-FrontPorch. Porches is padding that always separate visible pixels & sync pixels. This is a longtime carryover from an old analog video signals, which is well explained here. On analog CRT displays, the porch padding often allowed time for the electron gun to move back to the left edge of the next scan line, and then begin accelerating into a scan line (analog equivalent of a new row of pixel) before displaying image data (visible data) -- amongst holding other signals (e.g. colorburst signal, when color TVs first got introduced). The (often black) porches are hidden by the overscan of a CRT display. While the meanings of porches and sync has changed with digital signals -- and become far more flexible on modern gaming monitors -- they still continue to serve a very important display-synchronization purpose today.
Nontheless, we've used the same raster structure for almost 100 years, from 1930s analog TV broadcasts, all the way through 2020s DisplayPort. It's amazing how the raster signal sequencing is fundamentally unchanged, even through this digital era.
Does VSYNC ON versus VSYNC OFF affect the display signal?
Generally, no. The GPU is still transmitting the vertical synchronization signal. The use of VSYNC OFF in games simply tells the game to avoid waiting for the next refresh cycle. Instead of waiting for the end of the refresh cycle (e.g. "metaphorically waiting for the end of the calendar month", creating lag) -- it simply tells the graphics card to immediately begin the next rendered frame at the current point in the display output.
If you imagine the display pixel grid like a calendar grid -- just imagine the first half current month (e.g. February, current frame) is attached to the second half of next month (e.g. March, next frame). The calendar keeps moving ahead at the same dotclock (1 "day" at a time) at the same position on screen. The day-of-month keeps merrily moving exactly where it is spatially in this grid (e.g. specific day on calendar) even while you've suddenly jumped exactly 1 month ahead (VSYNC OFF). The tearline occurs where the splice occurs skipping a full frame ahead (sudden one-month skip ahead). Often (for non-GSYNC monitors) the monitor doesn't even know what "happened" -- the graphics card (GPU) simply immediately switched to the next frame in real-time.
Remember the rolling picture of a 1970s TV?
Yep! When you mis-adjust the VHOLD knob of an old analog TV -- that black bar is the vertical synchronization (VSYNC) signal. The thickness of the black bar is the total resolution of the (Vertical Front Porch + Vertical Sync + Vertical Back Porch). Which means, the bigger the Vertical Total, the thicker the black bar will be, if your computer monitor was theoretically capable of losing vertical synchronization and rolled like a 1970s TV. Synchronization of modern monitors are extremely strong and nearly bulletproof, so you will never see a rolling picture on newer displays -- but if you are old enough to remember an old TV, this helps make the Custom Resolution Utility concepts easier to understand.
Closed captions were transmitted in the porch scanlines. The vertical porch scanlines are often grayer than the actual "Vertical Sync" scanlines. Special signals are sometimes hidden in these (e.g. closed captioning signals. If you misadjust the VHOLD setting of an old TV, and roll your rolling on a closed-captioned TV channel -- you will see flickering white pixels/rectangles at the edge of the black bar here! Vertical Back Porch was popular for hiding hidden digital data in an old analog TV signals.
Vertical Totals tweaking is popular with some monitors
Vertical Totals became popular on Blur Busters Forums as an enhancement to several blur-reduction modes on some monitors, most particularly BENQ/Zowie XL2411, XL2420, XL2720. Essentially, large vertical totals can be used to accelerate scanout to panel (active refreshing) and add longer pauses between refresh cycles (idle). For more detailed information, see Why Do Large Vertical Totals Help Blur Reduction?.
What about FreeSync and variable refresh rate? How do they work in CRU math?
FreeSync simply varies the size of the blanking intervals to space out the refresh cycles. (For that 1970s TV diagram if you're familiar with that VHOLD black bar, it is tantamount to changing the height of that VHOLD bar). The scan line rate (pixel rows per second = horizontal scan rate) is always permanently constant, so adding/removing scanlines in the blanking interval is the method of varying the refresh rate dynamically on the fly. Every single refresh cycle, the refresh rate changes to exactly match frame rate, even a fluctuating frame rate. When creating a CRU mode for FreeSync, you're creating the timings for the maximum refresh rate (top end of VRR range) -- essentially you're defining the smallest blanking interval that is planned to be used. The graphics card will only *add* to the blanking interval to slow down the refresh rate during variable refresh rate operation (FreeSync / VESA Adaptive-Sync / HDMI 2.1 VRR) down to the bottom end of the supported range. ToastyX CRU is also capable of letting you force FreeSync out of HDMI, which has been a trick to allow FreeSync to work on CRT monitors since some multisync CRTs are apparently tolerant of dynamically-changing blanking intervals.
New Article Coming On This
New Blur Busters article coming on main Blur Busters website within a few weeks (after the brand new site has launched), that expands CRU 101 further.