50 fps on 60 Hz screens: the smear that is not the codec
The case arrived the way most do: “HEVC channels smear on my LG; the normal ones are fine”. And it was true. What was not true was the cause. After measuring it on the actual TV, the codec turned out to be innocent.
The measurement that changed the diagnosis
We compared two channels from the same provider, same codec (HEVC), same resolution (1920×1080), same TV and same network:
| Channel | Codec | Resolution | Frames/s | Result |
|---|---|---|---|---|
| A | HEVC | 1920×1080 | 50 | Visible smear in motion |
| B | HEVC | 1920×1080 | 25 | Looks perfect |
The only variable that changes is the cadence: 50 fps versus 25 fps. The channels the user called “HEVC” happened to be the ones broadcasting at 50 frames per second; the codec was just the label they carried in the playlist.
What 6:5 cadence is and why you see it
A panel refreshing at 60 Hz draws 60 images per second. If the video brings 50, they do not fit: every second there are 10 refreshes to spare, so the TV has to repeat one frame in every five. That ratio (6 refreshes for every 5 frames) is the 6:5 cadence.
In a static scene nothing happens. In a pan or with an object crossing the screen, the repeated frame produces a periodic hitch, twelve times a second, which the eye reads as smearing, dragging or “ghosting”. A 25 fps channel does not show it because 25 fits into 60 more evenly (each frame lasts 2.4 refreshes, but the error is spread out and the result is far smoother), and a 30 or 60 fps channel fits exactly.
The fix already lives inside the TV
Almost every Samsung and LG panel can refresh at 50 Hz; they do it without you noticing when you watch terrestrial TV, which in Europe broadcasts at 50 fps. The problem is not that the TV cannot, but when it decides to.
Why the app was to blame
When we reviewed our webOS app we discovered that its “fullscreen” was just a style change: the video was enlarged with CSS until it filled the screen, but the browser's native fullscreen function was never called. Visually identical; to the TV, completely different.
Samsung and LG firmware only switches the panel frequency when it detects a video in native fullscreen. A web page with a big video is still a web page, and web pages run at 60 Hz. Hence the 50 fps channel smeared: the TV never received the signal that it should switch to 50 Hz.
The detail that matters: the container, not the video
The first version of the fix requested fullscreen on the video element itself. It worked for the cadence, but it made the controls vanish: in native fullscreen the browser shows only that element, and the info bar (channel, programme, time) is a sibling element, so it was left out and there was no way back with the remote.
The correct solution is to request fullscreen on the container that wraps both the video and the controls. The video still fills it entirely (which is what the TV checks) and the interface is drawn on top. Kraid also listens for the fullscreen-change event to resync its state if the TV leaves fullscreen on its own, for instance when you press the home button on the remote.
On Android TV: AFR
On Android the mechanism is different and more direct. The app can ask the system to match the HDMI output's refresh rate to the video it is about to play: this is AFR (Auto Frame Rate). Kraid applies it on Android TV and Fire TV when the device allows it: when you open a 50 fps channel the output goes to 50 Hz; when you return to a 25 or 30 fps one it goes back to 60 (or 50, which also divides 25 evenly). You can see the current mode in the player's info overlay and turn it off in Settings if your TV flickers when switching, which happens on some older models.
How to check whether your smear is a cadence issue
- Look at the channel info in your player: if it says 50 fps (or 50/1), it is a candidate.
- Compare it with a 25 fps channel from the same provider and codec: if that one looks clean, the codec is not the culprit.
- On Smart TV, make sure the app enters native fullscreen (the TV's own interface disappears entirely). On Android TV, turn AFR on.
- If the smear appears on every channel, including 25 fps ones, it is something else: usually the TV's motion smoothing (TruMotion, Motion Plus…). Turn it off.
And if on those same channels, besides the smear, you sometimes only hear the audio with a black screen, that is a different problem we have also documented: HEVC on Smart TV: sound but no picture.
Frequently asked questions
Why do only some HEVC channels smear on my Smart TV?
Because it is not the codec: it is the cadence. When we measured it, the smearing channels were broadcast at 50 fps and the ones that looked fine, with the same codec and resolution, at 25 fps. Showing 50 frames per second on a panel that refreshes at 60 Hz forces one in every five frames to be repeated, and in motion that is perceived as smearing or ghosting.
What is 6:5 cadence?
It is the ratio between the panel's 60 refreshes and the video's 50 frames: every 5 frames occupy 6 refreshes, so one of them is shown twice. That repeated frame, twelve times a second, is what the eye notices in pans and on moving objects.
Why does my TV not switch to 50 Hz by itself?
It can, but Samsung and LG firmware only switches the panel frequency when it detects a video in native fullscreen. If the app merely enlarges the video with CSS until it fills the screen, to the TV it is still a web page and it stays at 60 Hz.
What about Android TV or Fire TV?
Android lets the app ask the system for the refresh rate that matches the video; this is known as AFR (Auto Frame Rate). Kraid does it on Android TV when the device and the HDMI output allow it, so a 50 fps channel is shown at 50 Hz with no intervention.
The right cadence on every platform
Kraid enters native fullscreen on Smart TV so the panel switches to 50 Hz, and on Android TV matches the refresh rate automatically (AFR). Hardware 4K HEVC, EPG, recording and multi-view. Free for 7 days.
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