G-Sync is a proprietary adaptive sync technology developed by Nvidia aimed primarily to eliminate screen tearing and the need for software deterrents such as Vsync. G-Sync eliminates screen tearing by forcing a video display to adapt to the framerate of the outputting device rather than the other way around, which could traditionally be refreshed halfway through the process of a frame being output by the device, resulting in two or more frames being shown at once. In order for a device to use G-Sync, it must contain a proprietary G-Sync module sold by Nvidia.
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G-Sync faces some criticism due to its proprietary nature, it still being pushed when free alternatives such as the VESA standard Adaptive-Sync, a specific brought from eDP, are now optional features of DisplayPort version 1.2a. While AMD's FreeSync relies on the above-mentioned optional component of DisplayPort 1.2a, G-Sync requires an Nvidia-made module in place of the usual scaler in the display in order for it to function properly with select Nvidia GeForce graphics cards.
However, not all features offered by G-Sync can be achieved by relying on the VESA display standard.
NVIDIA built a special collision avoidance feature to avoid the eventuality of a new frame being ready while a duplicate is painting on the screen (something that could generate lag and/or stutter) in which case they anticipate the refresh and wait for the next frame to be completed. Overdriving pixels also becomes tricky in a non-fixed refresh scenario and solutions predicting when the next refresh is going to happen and accordingly adjusting the overdrive value must be implemented and tuned for each panel in order to avoid ghosting.
Hardware
The module carry all the functional parts, it is based around an Altera's Arria V GX family FPGA produced on the TSMC 28LP process, paired with three DDR3L DRAM chip to attain a certain bandwidth, for an aggregate 768MB capacity. The employed FPGA also feature a LVDS interface to drive the monitor panel. It's meant to replace common scalers and be easily integrated by monitor manufacturers which only have to care of the power delivery circuit board and input connections.
GPU and system requirements
G-Sync Notebook
Nvidia announced that G-sync will be available to notebook manufacturers and that in this case, it would not require a special module since the GPU is directly connected to the display without a scaler in between. According to Nvidia, fine tuning is still possible given the fact that all notebooks of the same model will have the same LCD panel, variable overdrive will be calculated by a shader running on the GPU, and a form of frame collision avoidance will also be implemented.