FFMPEG An Intermediate Guide/General
This section will cover general video information and references.
Variable vs. Constant Bitrates in Video
- See the Variable bitrate article on Wikipedia for more details
A compressed video describes large chunks of pixels as they move across the screen. Having low bitrates means there is less information about these chunks that can be described, which results in a noticeable degradation of quality when we see it play back.
A video encoded in a constant bitrate spreads a given number of bits evenly over all the frames of a second. This is usually quicker to encode and gives the video a very predictable size, but it tends to use up more disk space and many sudden changes in the video may not adjust well.
A video encoded in a variable bitrate is closer to the workings of a zip file, where it may take an average amount of bits per second but analyze the input video for changes. Rapid motion tends to use a higher bitrate, while a steady shot with little motion will usually have a lower bitrate. This normally takes longer to encode and works best with two pass encoding but results in smaller files and smoother quality of video.
Two-pass encoding is used in conjunction with a variable bitrate to help the encoder learn how to optimize itself for potential changes in bitrate and improve itself.