AVC H.264 with Video and Audio Constant Bit Rate (CBR)

[needvidhelp]

Greetings all,

I have to create an H.264 .mp4 with CBR. Why? Because a customer requires it. Have tried Digital Rapids, Clipster, Adobe Media Encoder, Telestream Vantage. All give the option to select CBR, but when you check the media info on the newly created file, it shows VBR for video bit rate. Any guidance would be greatly appreciated.

[LigH.de]

It is more or less impossible to create CBR video in a compressed format, especially efficiently encoded. The dependencies used in "inter" frame types P and B in a GOP are the techniques which save the most bitrate. A video stream containing only I frames is technically possible but would require a magnitude higher bitrates. And even then, every frame in the video would have a slightly different size, so you could only enforce a really constant bitrate by stuffing smaller frames with useless junk bytes until they all get the same size by wasting space.

The usual implementation of CBR video is: Try to minimize the bitrate differences between GOPs, and let it respect video buffer characteristics (VBV size and fill rate). In x264 you can control the response range to changes in the video complexity (how quickly x264 alters the quality to match the target bitrate) and specify VBV values according to e.g. characteristics of streaming networks or slow media like optical disks (here you may need to know the desired usage).

Whether an encoder uses "CBR" flags in the headers of the resulting video stream or container is another story...