reaching constant video bitrate by using ffmpeg

Question

We use ffmpeg and a transcoding script for transcoding and want to make some batch files which we can use for transcoding.

For example I use a parameter called video_kbit and if I am writing in 30000 it should reach 30 Mbit. Of course if I use 6000 as parameter it should reach 6 MBit as well, so I have one script which reaches every video bitrate I want.

As my settings are now, I only reach 18.1 Mbit. Only when I use 15000 as a parameter I am reaching my goal for a constant video bitrate of 15 MBit. If I use 8000 as parameter I get 10.1 MBit as a result. So under 15000, I get a higher bitrate and over 15000, I get a lower bitrate than I want.

My presettings are:

ffmpeg -threads "4" -i "$2"  -f mp4 -c:v libx264 -crf 1 \
-bufsize 30000k -maxrate ${FC_PARAM_video_kbit}k \
-acodec libfaac -ac 2 -ab ${FC_PARAM_audio_kbit}k -ar 44100 \
-pix_fmt yuv420p -vf scale=${FC_PARAM_width}:${FC_PARAM_height} -y "$3"

And I am using these parameters:

FC_PARAM_video_kbit = 30000
FC_PARAM_audio_kbit = 192
FC_PARAM_width = 1920
FC_PARAM_height = 1080

I have tried using a higher bufsize and using profile:v and level settings, but nothing got me near the constant video bitrate of 30000 Mbit.

Do you guys have any ideas or suggestions for a better way to reach my goal?

Answer 1

Setting the CRF (a constant quality mode) and a video bit rate at the same time does not make any sense. You want either of those, exclusively.

To achieve fixed bit rate, you must set minrate, maxrate and b:v to the same value. Your script would look something like this (but ideally you'd change bufsize as well):

ffmpeg -threads "4" -i "$2" -c:v libx264 \
-bufsize 30000k -maxrate ${FC_PARAM_video_kbit}k \
-minrate ${FC_PARAM_video_kbit}k -b:v ${FC_PARAM_video_kbit}k \
-c:a libfaac -ac 2 -b:a ${FC_PARAM_audio_kbit}k -ar 44100 \
-pix_fmt yuv420p -filter:v scale=${FC_PARAM_width}:${FC_PARAM_height} -y "$3"

But then again, does this make sense? The question is what you really need to achieve. There's rarely a point in forcing a high bit rate when it does not deliver any better visual quality (and only bigger file sizes). Wanting 30 MBit/s for x264 video – even if it's 1080p HD content – seems a bit like overkill to me.*

x264 is great at delivering really good quality if you just set a CRF low enough. Anything below 17 or 18 should be unnoticeable in terms of visual distortion, and a CRF of 0 means lossless encoding. Rather set the CRF to a value that looks good enough for you and don't worry about the bit rate. It's really meant as a set-and-forget type of option that you can apply to batches of video to achieve a similar quality output.

Lastly, don't forget that if, say, a second of video can be compressed losslessly (i.e. with CRF 0) into only 10 MBits, then setting the bit rate to 30 MBit/s will not force this part of the video to actually consume 30 MBit. x264 does not "create" information just to bloat the output to a requested bit rate. It will stay at 10 MBit.

_{* If you want to archive, or distribute the video for further processing, choose an intermediate codec like ProRes or DNxHD.}