Bartiflier
lvl.2
United Kingdom
Offline
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Not entirely accurate. Watch something like a waterfall (or fast moving aerial video, but waterfalls have always been obvious problems on TV) at a low bitrate and you'll see lots of artifacts and blockiness as it is not possible to change every single pixel on the screen in every frame and show it clearly in the available bitrate used, so it approximates it badly and you see squares.
Things that don't change much, such as a talking head on TV needs only enough data per frame to show the face changing; the background can be copied from previous frames. TV channels actually change the amount of digital data being transmitted depending on the type of show being shown; a chatshow = low bitrate, action movie= higher bitrate.
An alternative to blockiness is blurring on fast motion, where, depending on aperture and shutter settings, instead of trying to show each frame clearly - and failing - it actually takes less data to compress a blurred image, so the data per frame can remain within the bitrate allowed without being too noticeable.
tl;dr: if each frame was an uncompressed photo, everything would always be clear all the time (if the shutter/aperture is set to do that). But, as bitrate lowers, something has to give and that applies to faster movements, when more changes per frame more quickly.
So bitrate is very much related to movement. If you're moving slowly, you can have a low average bitrate, where there will be the initial 'start frame' then a slow trickle of changes over the next frames, then another 'spike' for the next whole frame.
This was a problem on previous aircraft with the 'trickle' not being large enough and it was possible to see 'breathing' in the image every half second or so even when hovering, if videoing a detailed image, like a treescape as the image reset itself, then gradually got worse, then reset again. |
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