Soap opera effect is consumer lingo for a visual effect caused by motion interpolation, a process that high definition televisions use to display content at a higher refresh rate than the original source.
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The goal of motion interpolation is to give the viewer a more life-like picture. Some viewers, however, think the picture is too lifelike and that motion interpolation makes films on TV look as if they were raw video feeds. (Soap operas have traditionally been recorded on video, not film.)
Here is how motion interpolation works:
If a television screen has a refresh rate of 120Hz (120 frames per second) but the television is going to display film that was recorded at the standard 24 frames per second, the vendor must figure out a way to fill in an extra 94 frames each second.
One way to do this is to have the television repeat each film frame five times(5x24=120). Another way is to have a computer program in the television digitally analyze concurrent frames and use the data to create intermediary frames. The insertion of these frames is called interpolation and they are what cause the soap opera effect.
Many vendors allow viewers to turn off interpolation and force the television to repeat the same frame 5 times or use a more traditional 3:2 pulldown. This creates a more cinematic effect.
The commercial name given to motion interpolation depends on the vendor. Sony calls it MotionFlow, LG calls it TruMotion, Toshiba calls it ClearFrame, Mitsubishi calls it Smooth120Hz and JVC calls it Clear Motion Drive. Some vendors also refer to it as anti-judder.
Learn more about the soap opera effect:
Crunchgear says you can simply turn interpolation off and get the old, juddery picture you know and love.
James Mowery's blog explains anti-judder and motion smoothing technology.