[FFmpeg-user] q values and bitrate

[Chris Vincze]

Hi there

<mailto:ffmpeg-user at ffmpeg.org>

I'm trying to encode MPEG2 files for broadcast. The spec is MPEG-2 @ 50Mbps with all frames as intra frames.


So I've done a few experiments and have come across a conundrum.


Setting the q flags like this:


ffmpeg -i input.%04d.dpx -vcodec mpeg2video -pix_fmt yuv422p -intra -qmin 1 -qscale 1 -output.mpg


gives a constant q=1.0 while transcoding and produces a file of approximately 25Mbps (although weirdly the file bitrate is given as: bitrate: 25551 kb/s while the actual stream is specified as 104857 kb/s. Is that the peak or just incorrect?)


Duration: 00:01:29.95, start: 1.000000, bitrate: 25551 kb/s


Stream #0.0[0x1e0]: Video: mpeg2video, yuv422p, 720x608 [PAR 608:405 DAR 16:9], 104857 kb/s, 25 fps, 25 tbr, 90k tbn, 50 tbc Stream #0.1[0x1c0]: Audio: mp2, 48000 Hz, 2 channels, s16, 384 kb/s



However, specifying a constant bit rate like this:


ffmpeg -i input.%04d.dpx -vcodec mpeg2video -pix_fmt yuv422p -intra -b 50000k -minrate 50000k -maxrate 50000k -bufsize 2000000 -output.mpg


gives a varying q whilst transcoding somewhere between 1 and 6. The resulting file is 50Mbps (in both the file and the video stream)



However, visually, the quality of this file is not as good as the 25Mbps (as suggested by the q factor), although it should be better I would have thought.


Unfortunately the files will be rejected by the broadcaster if they are not 50Mbps. So my question is how can I produce a file of 50Mbps AND q=1. It is my understanding that the q flags and the bitrate flags are mutually exclusive (I've tried running both).



Thanks very much for your help


Chris