LFC on a 75Hz monitor?

Talk about AMD's FreeSync and VESA AdaptiveSync, which are variable refresh rate technologies. They also eliminate stutters, and eliminate tearing. List of FreeSync Monitors.
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Man
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Joined: 05 Aug 2021, 10:14

LFC on a 75Hz monitor?

Post by Man » 10 May 2022, 02:37

Looking at a 1080p FreeSync monitor with a sync range of 40 - 75Hz. If I overclock that monitor to 80Hz via CRU, does it mean I'll also get LFC?

I mean, if the frame rate drop to 39FPS, the monitor can run at 78Hz to compensate for it and so on, correct?

A few videos on YouTube claim that LFC is handled by the GPU and isn't exactly a hardware level feature i.e the monitors are ordinary VRR panels so I think it should work, at least in theory.

Has anyone tested it?

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Chief Blur Buster
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Re: LFC on a 75Hz monitor?

Post by Chief Blur Buster » 18 May 2022, 16:57

Man wrote:
10 May 2022, 02:37
Looking at a 1080p FreeSync monitor with a sync range of 40 - 75Hz. If I overclock that monitor to 80Hz via CRU, does it mean I'll also get LFC?
For reliable LFC, you need about a 2.4x differential between min:max Hz.

There needs to be an overlap due to the variable frametimes that may collide with a monitor-still-busy refreshing.

With only 2x ratio, there's no error margin and you get latency out of the wazoo as LFC repeatedly fails and frames are forced to wait for the monitor to finish refreshing, and sometimes LFC mistimes do happen, which further increases latency and stutter. So you need enough monitor-not-busy-refreshing time between refresh cycles, for more LFC flexibility.

Yes, LFC is controlled by the graphics driver for VESA Adaptive Sync / FreeSync / G-SYNC "Compatible" / HDMI VRR / and other generic VRR implementations.

But the GPU output is busy for (1/MaxHz) when doing either an original refresh cycle or a repeat refresh cycle. Reliable stutter-free lag-free LFC depends on the GPU output not being busy currently outputting a refresh cycle. Framepacing is never perfect, so trying to do LFC in stuttery 30-40fps situations

Wide VRR ranges produce much better LFC, which is why 48Hz-240Hz range can be tweaked to have invisible LFC stutter even in the worst LFC cases. The GPU output is only busy for 1/240sec even at 30fps, where LFC can reliably repeat-refresh without colliding with a previous refresh cycle still in progress on the GPU output.

Easiest range extension is lowering the min Hz, since some LCD panels will refresh as low as the territory of 10-20Hz, but not all of them do. Also, many graphics drivers will often refuse to do LFC anyway, but you can also monitor the monitor's framerate counter (actually a refresh cycle counter) to see if it suddenly doubles or triples when framerates get low -- this can be used for testing whether LFC is active or not.
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