Have you noticed that motion isn’t always smooth on your TV when watching movies? This phenomenon is known as judder, and it happens when the frame rate of the content you are watching doesn’t divide evenly into the refresh rate of your television.
What Is Judder and Why Does It Happen?
Judder is caused by inconsistencies in the time a frame is displayed on the screen. This often appears when displaying cinematic 24p content on a panel that uses a refresh rate of 60 Hz, which means the display refreshes 60 times every second.
Since 24p content doesn’t divide evenly into 60, the TV must perform what is known as a 3:2 pulldown. This is where every other frame is displayed slightly longer than the one that came before it. So for example, frame 1 is displayed for two refreshes (at 1/60 second each) while frame 2 is displayed for 3 refreshes (also at 1/60 second each).
This pattern repeats, which causes the motion on screen to have uneven pacing, causing judder. This is the main cause of this phenomenon since the vast majority of TVs still use 60 Hz panels. Newer 120 Hz panels can eliminate this problem by performing a 5:5 pulldown which involves adjusting the refresh rate to better suit the content.
There are other causes of judder, including the fact that the cinematic 24p frame rate is not well suited to fast-moving objects on the screen. This is often visible in quick panning shots and is as much of a problem on a projector in a theatre as it is on the latest televisions.
Lastly, frame drops can also introduce judder and these are often caused by hardware limitations, like a frame rate dip in a game or the playback of a high bitrate video on an older device that simply can’t keep up, causing some frames to be ignored. This introduces inconsistencies in frame time which causes judder.
Some TVs Can Remove Judder
It’s not unusual for televisions to remove the type of judder associated with 24p content on a 60 Hz display. Many will do this automatically for all sources by reducing the frame rate to 48 Hz or increasing it to 72 Hz. Panels that refresh at 120Hz simply display each frame five times (the 5:5 pulldown), since
5 x 24 = 120 .
This includes movies you watch using internal apps, signals sent from DVD or Blu-Ray players, or set-top boxes like the Apple TV and Google Chromecast. You’ll need to do your research to find out which models remove judder from 24p contents before you buy your TV.
Don’t Confuse Judder With Stutter
Judder and stutter are similar in that they affect how motion is displayed on your screen, but the two issues have distinct characteristics. While judder is the result of inconsistent frame times, stutter is usually caused by low frame rates.
Even though the video on screen isn’t perfectly smooth, if the time between frames (and the number of times a particular frame is displayed) remains constant, then this qualifies as stutter rather than judder.
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