Does my FreeSync range support LFC?

Divide the top of your FreeSync range by the bottom. If the answer is 2.5 or more, your range supports LFC under every rule that has been published. If it is under 2, LFC cannot work, whatever the box says. Between 2 and 2.5, it depends on which rule you go by. AMD's open-source driver allows it, and AMD's own FreeSync page uses a 60-144 Hz range (2.4x) as its LFC example. A 48-144 Hz monitor scores 3.0 and is fine. A 48-75 Hz monitor scores 1.56 and gets no LFC.

What LFC is, and what the range means

A FreeSync monitor can only change its refresh rate inside a set window, its VRR range. A 48-144 Hz range means the panel can refresh as slowly as 48 times a second and as fast as 144. Inside that window, the monitor waits for each frame, so there is no tearing (our VRR guide covers how that works).

LFC stands for Low Framerate Compensation. AMD's FreeSync page says it lets FreeSync work "when the framerate falls below the minimum refresh rate of the display". Frames "are duplicated and displayed multiple times" so the refresh rate lands back inside the range. AMD's example: on a 60-144 Hz display, a game at 40 fps is doubled to 80 Hz. Without LFC, falling below the minimum brings back tearing or stutter.

That is why the ratio matters. Showing each frame twice doubles the refresh rate, and the doubled rate has to fit under the top of the range.

Three numbers, three sources

2.5x is AMD's original rule. At its December 2015 technology briefing, AMD set the condition that Guru3D reported word for word: "LFC will only work on FreeSync displays where the maximum refresh rate is greater than or equal to 2.5 times the minimum refresh rate."

2x is the hard floor. ComputerBase's test of the Crimson driver, which brought LFC, put it plainly. The maximum refresh must be at least double the minimum, and AMD recommends 2.5x for LFC to work best. Its example: 30-60 Hz works, 35-60 Hz does not, because 34 fps doubled is 68 Hz. AMD's Linux driver draws the same line in code. In freesync.c, LFC goes by the name BTR (Below The Range). The driver switches it off when the maximum refresh is less than twice the minimum.

2.4:1 is NVIDIA's, and it is not an LFC rule. In May 2019 NVIDIA reported on its G-SYNC Compatible testing. Of 503 monitors, 273 "failed for lacking a VRR range of at least 2.4:1 (e.g. 60Hz-144Hz)". That is a test for NVIDIA's label. NVIDIA's article does not mention LFC or frame doubling at all.

Common ranges against all three rules

Monitors with NVIDIA's own G-SYNC hardware module sit outside this table. NVIDIA says those displays run VRR "from 1Hz to the display's max refresh rate", so they never fall below the range.

Worked case: a 60-144 Hz monitor

Take a monitor whose FreeSync range reads 60-144 Hz. The ratio is 144 / 60 = 2.4. By AMD's 2015 rule, that is no LFC. By the 2x floor, it qualifies. NVIDIA's article uses this very range as its 2.4:1 example. AMD's current FreeSync page uses it too, as a display that doubles 40 fps to 80 Hz.

Now walk a game down through the range. At 70 fps, the panel just runs at 70 Hz. At 59 fps, the frame rate is under the floor. Doubled, it is 118 Hz, which fits under 144. At 40 fps, doubled is 80 Hz, as AMD says.

The Linux code shows why headroom above 2x still helps. It does not flip LFC on and off at exactly 60 fps. It sets a margin: one frame at the minimum (16.67 ms) minus two frames at the maximum (13.89 ms), capped at 2.5 ms. (Frame time is 1000 / fps; our fps and refresh calculator does the sum.) Worked through the code, that switches LFC on below about 64.9 fps and off again above about 77.4 fps. The gap stops it from toggling every frame. At exactly 2x, as on a 30-60 Hz range, the margin is zero and there is no gap. The comment in the code says the margin is there "to avoid frequent enter-exits at the lower limit".

2.5, 2 or 2.4: which one is right?

2x is the arithmetic. Below it, a frame rate just under the minimum has no multiple that fits. On a 48-75 Hz range, 47 fps doubled is 94 Hz, above the 75 Hz top. No driver can fix that.

2.5x is the safe figure, not the law. It is what AMD stated in 2015. But AMD's current FreeSync page shows LFC on a 2.4x range, and AMD's Linux driver needs only 2x. That page gives no ratio, and we found no published figure for AMD's Windows driver.

2.4 is a different question. It is the range NVIDIA wanted before giving a monitor its G-SYNC Compatible label.

So read your ratio this way. At 2.5 or more, LFC is covered by every rule. From 2 to 2.5, it can work, and AMD's own example sits there, but nothing promises it. Under 2, it cannot.

How to check your own monitor

Find the range first: it is on the spec sheet, and AMD says the driver sets the range from "the capabilities reported by the display". Then cap your frame rate a few fps under the bottom of the range and watch the refresh counter in the monitor's menu. If LFC is on, the refresh reads about double your frame rate. ComputerBase saw exactly this on an Asus MG279Q (35-90 Hz, 2.57x): below 35 fps, the menu showed a suddenly doubled refresh rate.

Current tiers help too. AMD's tier table ticks LFC for FreeSync, FreeSync Premium and Premium Pro. It asks for at least 144 Hz (FreeSync) or 200 Hz (Premium and Pro) below 3440 pixels wide. Premium and Pro ask for 120 Hz at 3440 or wider. AMD says every Premium and Premium Pro display is "certified to meet mandatory LFC requirements". The page states no ratio.

What this does not cover

The ratio uses the range the GPU actually gets, which can be smaller than the box says. The Linux code caps the top of the range at the refresh rate you are running. Set a 48-144 Hz monitor to 100 Hz on the desktop and the driver works with 48-100, which is 2.08. On a GeForce card, AMD's rules do not apply. NVIDIA's 2.4:1 test says nothing about how its driver handles low frame rates, and this page does not either. LFC also only keeps the picture in sync. ComputerBase found it smoother than without, but not as smooth as inside the range, and the switch into LFC was noticeable. The fix for the top of the range is a frame cap, covered in our G-Sync fps cap guide.

Sources

• Guru3D, Radeon Technologies Group, Tech update December 2015, Hilbert Hagedoorn, December 8, 2015, via the Internet Archive (AMD's 2.5x condition).
• ComputerBase, Radeon Crimson im Test, page 3, Wolfgang Andermahr, November 24, 2015 (2x condition, AMD's 2.5x recommendation, 30-60 vs 35-60 Hz, MG279Q).
• Linux kernel, drivers/gpu/drm/amd/display/modules/freesync/freesync.c (BTR off below 2x, margin capped at 2.5 ms, top of range capped at the current refresh).
• NVIDIA, G-SYNC Compatible Testing, Phase 1 Complete, Andrew Burnes, May 27, 2019 (2.4:1, 273 of 503, G-SYNC from 1 Hz).
• AMD, AMD FreeSync Technology (LFC definition, 60-144 Hz example, tier table).