V-Sync, Free-Sync, G-Sync, Adaptive-Sync and Fast-Sync

Let's talk about V-Sync, Free-Sync, G-Sync, Adaptive-Sync and Fast-Sync

There is a lot of misinformation out there and I wrote this to clear up any confusion.

Screen tearing

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  Screen tearing is a visual artifact in video display where a monitor shows information from multiple frames in a single screen draw. The artifact occurs when the GPU output is not in sync with the display's refresh rate. Here is an example of tearing.

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  While playing a game your FPS rarely stays the same all the time. It moves around as the complexity of the image the video card has to draw changes based on what you are seeing. This is where tearing comes in.

V-sync

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  V-sync is the solution that most people are aware of. It solves tearing but at the cost of both latency and fixed refresh cycles (30,60,90,120, etc) V-sync uses double or triple buffering. V-sync introduces a large amount of input lag.
  

VESA Adaptive-Sync

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  Adaptive-Sync allows your monitor to sync its refresh rate to your GPU while inside the VRR(variable refresh rate) window of your monitor.
  Adaptive sync is a VESA standard and is the foundation of Free-Sync.
  Adaptive sync can be used with select AMD(r260 and newer GCN1.1 or later architecture, full list here) or intel (depending on the iGPU) chips.
  Currently Nvidia does not support Adpative-Sync but Tom Peterson said that if enough people wanted it, Nvidia is willing to reconsider. It is rumored that Nvidia's mobile G-Sync solution is a derivative of Adpaptive-Sync since they do not use an FPGA in their laptops that are G-Sync certified.

Adaptive-Sync Whitepaper


AMD Free-Sync

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  FreeSync allows your monitor to sync its refresh rate to your GPU while inside the VRR window of your monitor. VRR window is very important for spec that often times is not listed in marketing materials. An example VRR window is 48hz-75 hz. It varies widely between monitors and some are as good as 30-144hz.
  FreeSync falls back onto V-sync when the FPS goes above the refresh rate of the monitor or anything above the VRR window.
  Free-Sync can work through both DisplayPort and HDMI
  AMD has implemented several improvements to Adaptive-Sync most notably Low Framerate Compensation(LFC) which improves smoothness and reduces Judder at low frame rates. LCS works on monitors where the max VRR window is at least 2.5x the minimum VRR window.
  AMD Free-Sync has a qualification process that was talked in another thread by /u/AMD_Robert , here is the quote
  "Adaptive-Sync is the underlying standard, but makes no qualitative demands of adopters. As far as the spec is concerned, a 2Hz range is Adaptive-Sync, for example. Obviously that is useless for gaming. We have specific tests for backlight bleed, DRR range, motion blur, backlight flicker, pixel persistence, etc. We want monitors that bear our brand and logo to clear a certain quality threshold for the users that might buy them. To put it bluntly: the rubric is not available because this is a competitive industry and we're not interested in having our rubric needlessly and pettily nitpicked."

Link to AMD Free-Sync FAQ
More information of LFC
How are DisplayPort Adaptive-Sync and AMD FreeSync™ technology different?
Is DisplayPort Adaptive-Sync the industry-standard version of AMD FreeSync™ technology

Nvidia G-sync

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  G-sync is a hardware and software solution to allow tear free gaming through the entire refresh rate range of the monitor regardless of the VRR window(example 0-165hz). It uses a custom FPGA to store frame buffers (independently of the GPU) and uses logic to re-display some frames in order to fix tearing. PCper has a good article where they use an oscilloscope to show what is happening behind the scenes.
  G-sync falls back onto either Fast-sync or V-sync when the FPS goes above the refresh rate of the monitor.
  G-sync works in both windowless and windowed modes.
  G-sync is not only the chip that goes into monitors, OEMs have to go to Nvidia to qualify a monitor based on many factors including ghosting performance to become G-sync certified.
  G-Sync can work through both DisplayPort and can support HDMI inputs(Gen 2 only).

Link to PcPerspective oscilloscope article
Link to Nvidia G-Sync FAQ page

Nvidia Fast-Sync

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  Fast-Sync focuses on low latency high frame rate tear free gaming for games like CSGO and Overwatch. The main point of Fast-sync is that is allows the game engine to run its physics engine at a high frame rate (example 300fps) while the game only displays on your display at a lower frame rate(example 120Hz). FastSync is Different than Triple Buffering, although it does work somewhat similarly. It works much like Open GL triple buffering but at a driver level. It works very differently than the way Windows DX11 triple buffering works.

I highly encourage you to listen to this youtube video that PCperpective did with Tom Peterson from Nvidia. It goes into detail about how fastsync works.
Link to PCPER youtube interview with Tom Peterson
Slides from interview
Fast-Sync latency comparison

FAQ's

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  Q. I have an AMD card or APU what can I use?
  A. Free-Sync, Adaptive-Sync, V-Sync.
  
  Q. I have a Nvidia card what can I use?
  A. G-Sync, V-Sync, Fast-Sync(if you have a card that supports it), and potentially Adaptive-Sync in the future(read Adaptive-Sync section).
  
  Q. I have an intel processor with iGPU what can I use?
  A. V-Sync and Adaptive-Sync if your CPU supports it.
  
  Q. Aren't AMD Free-Sync and Nvidia G-sync the same, just from different companies?
  A. No. All of these technologies attempt to resolve the same issue, tearing, but work very differently. G-Sync is tear free through the entire refresh rate range on the monitor, Free-Sync is tear free only through it VRR window(this is different than refresh rate!).
  
  Q. I read somewhere that you can hack the VRR window on a monitor just by changing some settings, Is that legit?
  A. No, forcing a VRR window onto a monitor through software is just going to leave you with a juddery, stuttery, are teary mess. AMD's LFC can effect a monitors VRR window if the max refresh is ≥2.5X min. refresh