The params (FPS is 25): 

 params->rc.b_cbr = TRUE; 

 params->rc.i_bitrate = 1500; 

 params->rc.i_vbv_max_bitrate = 1500; 

 params->rc.i_vbv_buffer_size = 1500; 

 params->rc.f_rate_tolerance = 0.05;

Additional Information & Tips

ABR (Average Bit Rate)

ffmpeg -i input -c:v libx264 -b:v 1000k output.mp4

This provides something of a "running average" target, with the end goal that the final file match this number "overall on average" (so basically, if it gets a lot of black frames, which cost very little, it will encode them with less than the requested bitrate, but then the next few seconds of (non-black) frames it will encode at very high quality, to bring the average back in line). Using 2-pass can help this method to be more effective. You can also use this in combination with a "max bit rate" setting in order to prevent some of the swings.

CBR (Constant Bit Rate)

There is no native CBR mode, but you can "simulate" a constant bit rate setting by tuning the parameters of ABR:

ffmpeg -i input -c:v libx264 -b:v 4000k -minrate 4000k -maxrate 4000k -bufsize 1835k out.m2v

In the above example, -bufsize is the "rate control buffer" so it will enforce your requested "average" (4000k in this case) across each 1835k worth of video. So basically it is assumed that the receiver/end player will buffer that much data so it's ok to fluctuate within that much.

Of course, if it's all just empty/black frames then it will still serve less than that many bits/s but it will raise the quality level as much as it can, up to the crf level.

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 (as shown in the documentation):

ffmpeg -i input.avi -b:v 64k -bufsize 64k output.avi

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 64k). If we don't specify the -bufsize option, ffmpeg will still calculate and correct the average bit rate produced, but more lazy, at some intervals which can 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. 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, 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.

There are also -minrate and -maxrate options which control the minimum and maximum bit rate tolerance (in bits/s). With these options you can give ffmpeg a hint of how much of a bit rate tolerance (a difference from the specified average bit rate) you are willing to accept.

Creating the output file of specific size

For this scenario it is by far the best to use a 2-pass encoding, using the -pass option, like it is explained in the example of targeting a specific output file size.

CBR simply means that vbv-maxrate is equal to bitrate and that bufsize is set to the appropriate value for the device.

http://qtoue.com/3Jrwd8m3/encoding-h264-cbr-videos-with-ffmpeg.html

Here is what I have for low-latency CBR video to an MPEG tranport stream:

ffmpeg -i sintel_trailer-720p.mp4 -an -tune zerolatency -x264opts bitrate=4000:vbv-maxrate=4000:vbv-bufsize=166 -vcodec libx264 -f mpegts -muxrate 4000K -y trailer.ts 

According the x264 developer's blog you set:

• vbv-maxrate = bitrate = B = target bitrate
• vbv-bufsize = B / fps (in this video's case that's 24 fps)

Finally, set the ffmpeg switch for x264 of -tune zerolatency.

下面是一个在项目中使用的配置：

-s 1920*1080 -pix_fmt yuv420p -i xxx.yuv -vcodec libx264 -r 30 -nal-hrd cbr -bufsize 1835k -b:v 4M -bt 4M  -minrate 4M -maxrate 4M

注意到设置不同的bitrate需要去调整bufsize来满足CBR的要求。实际得到的码率还是有一点波动，但是已经是非常的接近CBR了。

http://blog.yikuyiku.com/?p=2029