FFmpeg H.264 Constant Bitrate (CBR) Encoding

Example command using a single-frame VBV:

ffmpeg -i <input> -r 25 -c:v libx264 -x264opts nal-hrd=cbr:force-cfr=1 -b:v 2500k -minrate 2500k -maxrate 2500k -bufsize 100k output.ts

where:

nal-hrd=cbr	set the hypothetical reference decoder (HRD) to CBR and pack the bitstream to the specified bitrate
force-cfr=1	force constant framerate timestamp generation
-b:v	the video bitrate
-minrate -maxrate	same as the video bitrate for constant bitrate
-bufsize	the video buffering verifier (VBV) buffer size, set for capped frame-size encoding (bufsize = bitrate / framerate)

Additional notes:

• for even lower latency use -tune zerolatency and intra-refresh.
• single-frame VBV can be inaccurate with small frame-sizes and multi-threading.  If the encoder reports many VBV underflows try setting -threads 1