[Libav-user] video bitrate control far from accuracy

[谭艳梅]

Carl Eugen:
            hi,thanks for your help.  here are more details as follows:

>(The frame rate or the bit rate?)
I mean the video bit rate not frame rate. and I need low video bit rate like 22kbps\55kbps in my app 

> Is this reproducible with ffmpeg (the application)?
yes, it is reproducible with the same input source file。 and different deviation will different input souce file.
but the same is : video bit rate in output file is not what I wanna to get . there is always a big deviation between  what I want and what I get.
    
> for a given input file, you can use a high constant 
> quantizer to test:
>$ ffmpeg -i input -qscale 31 -vcodec h263 out.avi

I have try like this in ffmpeg.exe command line like this:
ffmpeg.exe -i cq-SD.wmv  -qscale 31 -vcodec h263 -r 5 -b:v 22k -ar 8000  -s 176*144 -acodec libopencore_amrnb -ac 1 -b:a 8k 44.3gp
but get 9kbps video bit rate of the output file, if I adjust "-qsale 31" to "-qsale 13" and keep other input parameter the same, can get 22kbps video bit rate.
what trouble me is that if input file be changed then the out put file's video bit rate will not be 22kbps. so it seems not take effect.

Any assist will be appreciated!

yanmei tan

> -----原始邮件-----
> 发件人: "Carl Eugen Hoyos" <cehoyos at ag.or.at>
> 发送时间: 2014-07-17 17:31:23 (星期四)
> 收件人: libav-user at ffmpeg.org
> 抄送: 
> 主题: Re: [Libav-user] video bitrate control  far from accuracy
> 
> 谭艳梅 <tanyanmei at ...> writes:
> 
> > I'm transcoding with ffmpeg API .  but the video frame 
> > rate of  the target file is far from accuracy .
> 
> (The frame rate or the bit rate?)
> 
> Is this reproducible with ffmpeg (the application)?
> 
> To test if it is at all possible to reach a given bitrate 
> for a given input file, you can use a high constant 
> quantizer to test:
> $ ffmpeg -i input -qscale 31 -vcodec h263 out.avi
> 
> I don't think 22k for qcif and h263 is realistic.
> 
> If you really meant frame rate than you can choose it 
> freely assuming the container allows it.
> 
> Carl Eugen
> 
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