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Understanding Bitrate, Resolution and Quality

Started ‎12-04-2012 by
Modified ‎07-20-2016 by

Understanding Bitrate, Resolution and Quality

by Ooyalan ‎12-04-2012 07:33 AM - edited ‎07-20-2016 10:30 AM (70,798 Views)




Understanding Bitrate, Resolution and Quality:


There are many folks asking how bitrate and resolution impact video quality. This short article should help shed some light on the matter.


Fundamentally this all boils down to a "bits per pixel" calculation.  

First calculate how many pixels your video has per second:
pixels per second = Width x Height x Frames per second

Then, divide the bitrate of your video by the pixels per second to get the bits per pixel (bpp):
Bits per pixel (bpp) = video bitrate / pixels per second

Any bpp values around 0.1 have very good quality (higher bitrates won't produce visually significant improvement). Any bpp values around 0.03 have poor quality (lower bitrates are usually unwatchable).

As an example for a standard web sized embed of 640x480 with a framerate of 30 that gives 9M pixels per second.  Thus bitrate range of 300k-900k would be reasonable.  For a larger embed of 1280x720 again with a framerate of 30 you’d have a watchable bitrate range of 1100k-3600k.

Of course it generally makes sense to have more than one bitrate and/or resolution for the same movie so it can adjust to the user’s bandwidth constraints.  The ratio between consecutive bitrates should be at least 30-40% so that the difference is noticeable.  It should also be less than 80-100%, as that would cause very large quality jumps between streams. On average, customers will be stuck with a lower quality stream than they could be watching if there are more bitrate gradations.

Where this starts to break down is if videos of different resolutions are played back on the same size screen.  This comes up the most with connected TVs.  For example when going from 1280x720 to 1920x1080 (2x the number of pixels) increasing the bitrate by only 30-50% instead of the expected 200% is acceptable because the reason to include the higher resolution stream in this case is just to help make some small details sharper, which is a subtle improvement and the actual viewer screen size isn’t changing.