What corresponds to talk now is about the quality of these paintings. Let’s take a frame and see its quality and weight in kb … This is called bitrate or kbps which means KiloBytes per Second or Kilobytes per second, and is the measure used to determine the image quality, the higher the bitrate, the image quality will be better, but the weight of the file will be greater too. There are standards and limits for bitrate, this for the MPEG format. For a DVD, which is an MPEG2 video format, the limitation is more than 1500 and less than 8000 kbps, for VCD it is exactly 1150 kbps, and for example a MiniDV tape of a handycam uses a bitrate of over 30000, this is a lot of quality and when you go to hard disk you see the large file size. For the other formats it depends on the codec and its configuration, being able to compare in quality an MPEG2 format with 3000 kbps with an AVI with Xvid codec at 1500 kbps, this difference is given by the way in which the video is compressed, the Xvid codec It does better than the MPEG2 format.
Now, bitrate can be constant throughout the video or it can be adjusted to changing needs, this is Constant Bitrate (CBR) and Variable Bitrate (VBR), when we use Constant Bitrate all video always uses the same amount of information to show Each frame, whether you need it or not, this can make the video heavier than it could be or that it does not have the right quality in certain parts or has plenty of information where it is not needed.
Let’s see the following example, where a small video clip is shown where it requires a lot of bitrate or kbps and another part where it does not need so much, encoded in MPEG CBR and VBR. We see the difference in the weight of the files and their comparison with an Avi with DivX codec with a lower bitrate. In minute 2 another video is shown where very little bitrate is needed and as when coding in VBR or CBR it does not greatly affect the final result.
When we use Bitrate Variable we can define the maximum and minimum to use and let the coding program scan the video and adjust the amount of information as necessary, for this you can use one or two passes (usually no more than two are used), that is, the program goes through the video once, calculates and in the next pass applies the bitrate correctly. It can also be done in a single pass, but the calculation can be wrong, this because to make the necessary bitrate changes makes use of something called keyframes or keyframes. In addition there are programs that detect scene changes and there apply keyframes to change the amount of bitrate occupied, this may be in one pass, but it may also not be as accurate as two passes.
To prevent the file from being excessively complex and heavy to process, when encoding an amount of information is allocated for video blocks, these blocks are defined by the keyframes, for example, there may be 90 frames between each keyframe, and if the video run at 30 frames per second, then the bitrate can change only every 3 seconds of video (3 seconds x
30 frames = 90 frames), and if there is a change of scene, for example, from a dark and slow shot to a scene with light and moving, if we are in the middle of a block between keyframes we will notice that the scene in motion is seen bad, with large video blocks and little definition, this can happen when the coding is done in a single pass using Bitrate Variable, so it is preferred to use 2 passes, in this way the program is responsible for detecting the change of scenes where there is need to adjust the bitrate, and accommodate the keyframes of
correct way, so that in each change the quality of the video is adjusted and seen
all right. And what happens when we have a whole video with movement, well, the bitrate remains high for most of the time, this will make a video that is heavier than another of the same duration but where the content is more static while maintaining good quality . If we use Constant Bitrate for both videos, one with a lot of movement, and another with more static content, the result will be very similar, it will only change the quality of the content, unless we use a very high bitrate to give quality to the video with movement, which it would be excessive for a video of more static scenes, which would generate a heavy file without any need. Therefore, we have videos that can be heavier than others, and videos with different qualities. This is where we need to know what should be adjusted when encoding a video and how automatic adjustments do not always give the same results, this because each video may have different coding needs.
