A coalition of film directors and actors have been encouraging people to turn off the frame interpolation feature on their TVs, and Tom Cruise recently released a video doing just that.
Frame interpolation is also known as motion smoothing, though TV manufacturers have different names for the feature. Samsung calls it “Auto Motion Plus” while LG refers to it as “TruMotion”.
What Cruise does not explain is what motion smoothing is, and why it was developed.
Motion smoothing technology was developed thanks to a mismatch in the frame rates video was recorded in versus what TVs were capable of showing.
Frame rates are given Hz – a measure of frequency which may also be translated as shorthand for “per second”. A frame rate of 50Hz and 50 frames per second are the same thing.
When movie cameras used film to record and play video, the industry standardised 24 frames per second. This emerged in the 1920s.
However, when the television was developed decades later it was built with refresh rates of 50Hz and 60Hz.
Televisions operate at a different frame rate from movies because of the standards which ultimately emerged for the generation and transmission of electricity around the world.
Most places in the world use alternating current (AC) for mains electricity, which runs at a frequency of either 50Hz or 60Hz. North America standardised on 60Hz, while Europe and much of Asia adopted 50Hz. South Africa followed the European standard.
Japan is a notable exception, where one part of the country uses 60Hz for mains power, and the other uses 50Hz.
Early televisions relied on the frequency from mains power as a timing mechanism when drawing frames on the screen. In 50Hz regions, TVs operated at a refresh rate of 25 frames per second. Where mains power ran at 60Hz, TVs had a refresh rate of close to 30 frames per second.
South Africa adopted the European PAL standard for TVs, which used a 25Hz refresh rate, while North America used the 29.97Hz NTSC standard.
To compensate for the mismatched frame rates between movies and TV, the NTSC standard uses a technique called 2:3 pulldown to distribute four frames of video over five frames.
In PAL countries, video that runs at 24 frames per second is usually sped up slightly to 25Hz. This has the effect of shortening the film and, unless compensated for, marginally increasing the pitch of the audio track.
While NTSC’s 2:3 pulldown method ensures that the film’s runtime and audio are unaltered, it may cause other artefacts such as judder when the camera pans across the scene.
Motion smoothing compensates for these glitches that are introduced.
Disabling it may therefore cause the side effects of frame rate conversions to appear. It all depends on what the frame rate is of the video and what technologies your TV supports.
If your TV supports refresh rates of 120Hz, or has support for 1080p24 playback, then disabling motion smoothing when viewing video encoded at 24 frames per second should be exactly “as the filmmakers intended”.
However, if you are watching video from a broadcaster such as the SABC or DStv, disabling motion smoothing will not cause movies to playback at 24 frames per second, but more likely at 25 frames per second.
Fixing the root of the problem
Towards the end of his anti-interpolation video, Cruise said that filmmakers are working with TV manufacturers to change the way motion smoothing is activated to give viewers easier access and greater choice over when to use the feature.
In the meantime, when you are watching a movie, it is worthwhile to disable your TV’s video interpolation feature.
This advice only applies to movies, though, and users may also opt to reduce the amount of motion smoothing rather than disabling it entirely.
If you are watching video that was made for TV, leaving video interpolation enabled may actually be beneficial. This is especially true with sports, where it will result in much smoother motion as cameras track the action on the screen.