What is Bit Rate?
Bit rate is the amount of video and audio data transferred per second, and the unit called bps (bits per second) is used.
It is expressed as “video file capacity = bitrate value x time”. The higher the bit rate, the better the picture and sound quality, but the larger the final file size. Adjust the bit rate when you want to reduce the file size when encoding a video file or when you want to improve the image quality.
The higher the bit rate, the higher the image quality, but if the original video has a low bit rate, no matter how high the bit rate is, the image quality cannot be expected to improve. . Also, if you are using a CPU with a slow processing speed or a hard disk with a slow rotational speed, if you play a video created at a high bit rate, the processing may not be in time and they will be lost. frames. Also, when posting a video on the home page, it cannot be played properly if the internet line speed is slow.
The DVD-Video standard requires a total video and audio bit rate of 9848 kbps or less, and the Blu-ray standard (BDMV) requires a total video and audio bit rate of 40,000 kbps or less. Therefore, it is necessary to adjust the bit rate according to the purpose and environment. There are two kinds of
encoding methods: constant bit rate (CBR) and variable bit rate (VBR).
Select the encoding method that suits your purpose, such as image quality at encoding time and final size balance.
Constant Bit Rate (CBR)
All constant bit rates are assigned the same bit rate at all times. Set a high bit rate when you want all video files to be high quality, and set a low bit rate when you want to reduce the file size (a low bit rate can make noise noticeable in a scene with a lot of movement).
It always assigns the same bit rate, so you can easily predict the size of the finished file. Therefore, it is recommended to use it when there is an upper limit for the file size after encoding, or when you want to keep the data transfer rate constant.
Also, in WMV (Windows Media Video), etc., 2-pass CBR can be set. With 2-pass CBR, the amount of data distributed to each video is strictly adjusted and is a mechanism to target higher image quality.
Variable Bit Rate (VBR)
Variable bit rate automatically assigns a high bit rate to fast moving scenes and a low bit rate to slow moving scenes. Since the bit rate is assigned according to the scene, the file size can be reduced while the image quality is relatively high, but the final file size is difficult to predict.
VBR can be divided into two types. 1-step encoding (fixed quality) and 2-step encoding (average bit rate).
1-pass MPEG-2 encoding can shorten the processing time for export by analyzing the video and encoding while maintaining the specified constant quality. However, it is difficult to predict the size of the finished file.
In 2-pass encoding, after analyzing the information from all video data in the 1st pass, the bit rate is assigned and encoded in the 2nd pass based on that information. Although the processing time is long because it is processed twice, it is possible to assign the bit rate efficiently, making it possible to create high-quality video. By specifying the average bitrate, you can roughly predict the size of the file.
