Introduction

Limiting the output bit rate to a certain range makes sense if you are live streaming or need to be constrained to a certain specification (such as Blu-ray encoding), where the decoder might not be able to handle large bitrate spikes.

There are several options you can use to steer the encoder's bitrate:

• -b:v: specifies the target (average) bit rate for the encoder to use
• -minrate specifies a minimum tolerance to be used
• -maxrate specifies a maximum tolerance. this is only used in conjunction with bufsize
• -bufsize specifies the decoder buffer size, which determines the variability of the output bitrate

Note: Constraining the bitrate might result in low quality output if the video is hard to encode. In most cases (such as storing a file for archival), letting the encoder choose the proper bitrate is the constant quality or CRF-based encoding.

Example: Creating a live stream with limited bit rate

In this case, you want to have a live stream with more/less constant bit rate (using the option -b:v), to be able to control the bandwidth used.

We will instruct the libx264 encoder to simulate a stream transmission with a virtual buffer (just like the real buffer at the decoding side). The technique will constrain the bit rate in order not to exceed a certain threshold value which would take more time to transmit and would cause the decoding buffer to underflow waiting for the new data to arrive.

The typical example would be something like this:

ffmpeg -i input -c:v libx264 -b:v 2M -maxrate 2M -bufsize 1M output.mp4

The key option here is -bufsize which tells the encoder how often to calculate the average bit rate and check to see if it conforms to the average bit rate specified on the command line (-b:v 2M).

What does -bufsize do?

Based on the -bufsize option, ffmpeg will calculate and correct the average bit rate produced. If we didn't specify -bufsize, these intervals could be significantly longer than we would want. This would cause the current bit rate to frequently jump a lot over and below the specified average bit rate and would cause an unsteady output bit rate.

If we specify a smaller -bufsize, ffmpeg will more frequently check for the output bit rate and constrain it to the specified average bit rate from the command line. Hence, lowering -bufsize lowers the bitrate variation that the encoder can produce.

Specifying too small -bufsize would cause ffmpeg to degrade the output image quality, because it would have to (frequently) conform to the limitations and would not have enough of a free space to use some optimizations (for example, optimizations based on the frame repetitions and similar), because the buffer would not contain enough frames for the optimizations to be effective.

The suggestion is to play around with the combinations of -bufsize, starting from the same value like the one specified for the -b:v option (or even half of it) and increasing it until your output bit rate starts jumping too much above/below the specified average bit rate. Then you know you've reached the limit and should lower the -bufsize value a little bit in order to get back on the safe side.