When using FFmpeg for video encoding, setting a constant bit rate (CBR, Constant Bit Rate) ensures that the bit rate of the video stream remains consistent. This is particularly useful in scenarios requiring a fixed data bandwidth, such as live streaming. Constant bit rate guarantees stable operation of players or decoders at a fixed data rate, preventing buffer overflow or data underflow.
To configure constant bit rate in FFmpeg, adjust the encoder parameters. Here, we demonstrate using the most common encoder, libx264, to set CBR. Below is a specific command-line example that converts the input video input.mp4 to the output video output.mp4 with a bit rate of 2000 kbit/s:
ffmpeg -i input.mp4 -c:v libx264 -b:v 2000k -minrate 2000k -maxrate 2000k -bufsize 4000k output.mp4
The parameter explanations are as follows:
-c:v libx264: Specifies the video encoder as libx264.-b:v 2000k: Sets the target bit rate to 2000 kbit/s.-minrate 2000kand-maxrate 2000k: These parameters define the minimum and maximum bit rates, both set to 2000 kbit/s, ensuring a constant bit rate.-bufsize 4000k: Sets the bitrate control buffer size, typically configured as twice the target bit rate.
With this configuration, FFmpeg will strive to maintain the output video's bit rate at 2000 kbit/s, even when the video content becomes more complex or simpler, keeping the bit rate as consistent as possible.
When applying these settings, note that constant bit rate may introduce trade-offs between video quality and file size. In scenarios with significant scene changes, such as dynamic video content, using CBR can result in lower video quality compared to variable bit rate (VBR). Therefore, when selecting the bit rate mode, weigh the specific requirements and constraints.