RealNC wrote:Also, don't confuse adaptive sync with VRR. They've got nothing to do with each other.
Well, there is some confusion, so let me clear it up:
Well, according to VESA.org,
VESA Adaptive-Sync is a VRR technology. The people who created Adaptive-Sync!
In fact, at the signal level, FreeSync and VESA Adaptive-Sync is nearly identical. In fact, many VESA Adaptive-Sync displays successfully function with an AMD Radeon graphics card as an unofficial, uncertified FreeSync display!
That said, VESA Adaptive-Sync do also focus on additional purposes, like throttling back refresh rates to save power, etc. So not all VESA Adaptive-Sync monitors are used for VRR gaming.
Functionally, and fundamentally,
FreeSync, VESA Adaptive-Sync, and HDMI 2.1 VRR has some interoperable features.. Their technology (protocol) venn diagram have overlap! They all use the same VRR technique of variable-sized blanking intervals and that subset of functionality is so similar that they are mostly interoperable with each other in the subset of supported VRR functionality. There are feature differences, but the core features actually work with each other.
e.g. ToastyX CRU forcing FreeSync out of a HDMI port, successfully worked on an experimental HDMI 2.1 Game Mode VRR display (works). Or running FreeSync to a display that's only been made VESA Adaptive-Sync compatible but not officially certified by AMD (works).
The standard is (fairly) open and they're all remarkably similar at the technical video signal level.
All of the standards (VESA Adaptive-Sync, HDMI 2.1 VRR,
and FreeSync) use exactly the same technique of variable-height black spacers (VBI - VSYNC interval) between refresh cycles to vary the time between refresh cycles.
A display's horizontal scanrate is constant (e.g. a 135KHz horizontal scan rate means 135,000 pixels rows per second). That's always fixed speed even for variable refresh rates. So to vary the amount of time between refresh cycles, you add/remove those offscreen dummy scanlines (blanking interval lines) between refresh cycles.
This is still true in the digital era (DVI, HDMI, DIsplayPort) as the digital equivalent of the analog signal. Graphics are still transmitted one pixel at a time, left to right, top to bottom. Sure, they might be packetized (e.g. DisplayPort micropacket) but the pixels are still packed in a calendar-style-scanout order (left-to-right, then top-to-bottom) with dummy offscreen spacers at top/bottom (
vertical front/back porches/sync intervals) as well as left/right (
horizontal front/back porches/sync intervals).
That's the numbers in a Custom Resolution Utility....such as ToastyX or NVIDIA or AMD
Vertical Back Porch -- dummy rows of offscreen pixels above top edge of screen
Vertical Front Porch -- dummy rows of offscreen pixels below bottom edge of screen
Vertical Sync -- dummy rows of offscreen pixels between Vertical Back Porch & Vertical Front Porch
Horizontal Back Porch -- dummy rows of offscreen pixels beyond left edge of screen
Horizontal Front Porch -- dummy rows of offscreen pixels beyond right edge of screen
Horizontal Sync -- dummy rows of offscreen pixels between Horizontal Back Porch & Horizontal Front Porch
Why?
They're simply pauses in the video signal.
To give electronics time to begin the next row of pixel, or to prepare to begin the next refresh cycle.
Cable transmission sequence for one scanline:
Horizontal Back Porch - Horizontal Visible (Active) - Horizontal Front Porch - Horizontal Sync
Cable transmission sequence for successive scanlines:
Vertical Back Porch - Vertical Visible (Active) - Vertical Front Porch - Vertical Sync
It's always in this ordering: BackPorch-Visible-FrontPorch-Sync-BackPorch-Visible-FrontPorch-Sync-BackPorch-Visible-FrontPorch-Sync-etc-etc-etc BackPorch goes before Visible. FrontPorch goes after Visible. Sync between FrontPorch and BackPorch.
Left to right, top to bottom, "calendar-style" scanout sequence. Rinse and repeat per refresh cycle.
Back in the 1930s and 1940s, for analog TVs, it was to give time for the electron gun to move to the correct position.
Today, it's still there even in year 2018 in your digital cable as dummy pauses to your 4K or 8K OLED display.
Not all displays need them, but many do. So numbers become lowest common denominators.
Asides: Reduced Blanking Intervals are used on some displays to increase Hz. Large Vertical Totals are used on other displays for things like QFD (Quick Frame Delivery -- padded Front Porch -- faster scanout + longer VBI = less VSYNC ON lag) or to reduce strobe crosstalk (more time between refresh cycles for LCD GtG to finish completing -- GtG "lags" behind the scanout. So you want the bottom edge to finish refreshing, before strobe backlight flash, before beginning to refresh the top edge)
And...this is how variable refresh rate piggybacks on this. Variable refresh rate varies the size of the Back Porch in realtime (on the fly). The number you entered in ToastyX or whatever, will always only be the number used at the highest Hz. To get lower Hz, the Vertical Back Porch varies (to much bigger numbers) to achieve the lower Hz's.
Yes, VESA Adaptive-Sync.
Yes, FreeSync.
Yes, HDMI 2.1 VRR.
Albiet a bit complex to wrap head around, it actually becomes really simple (for someone numerically inclined, or a programmer) once somebody understands how pixels are transmitted from GPU to display in a calendar-style scanout sequence, left-to-right, top-to-bottom. And then understanding that variable refresh rate is simply the concept of varying the size of padding between refresh cycles. Like metaphorically adding invisible "weeks" between calendar months (aka refresh cycles).
The clever way they piggybacked VRR onto a "1930s video signal concept", means it's possible to force VRR out of HDMI, then use an HDMI-to-VGA adaptor.... Analog VRR! Some
old multisync CRTs successfully works with VRR signals.
All of these standards are reasonably open enough that an electronics engineer (who understands the VRR specs) can simply write a custom signal generator that intentionally outputs a varying-refresh-rate signal -- and it'd automatically work on any of those supported VRR displays as long as within the specs (including refresh rate range).