Re: [dvd::rip] incorrect bitrate calculation for divx4

[Joern Reder <joern@xxxxxxxxxxxxx>]

Christian Lambert wrote:

> Noticed that your movies are a little undersized?

Yes, but I don't think that dvdrip's or transcode's bitrate calculation
is respoinsible here ;)

> The Divx4 codec assume that 1kbit = 1000 bits (not 1024 bits)
> so if you take this into account, the bitrate that transcode
> or dvdrip propose is always a little lower than what it should
> be and therefore result in slightly undersized movies.
> 
> The divx3 codec assumed that 1kbit = 1024 bits.

As Thomas wrote, transcode uses the 1000 bit rule, and I can say: 
dvd::rip does calculate with 1000 bits, too. ;)

  $video_bitrate = int($video_size/$runtime/1000*1024*1024*8);

  with $video_size in MB and $runtime in seconds

  $size_in_bits       = $video_size*1024*1024*8
  $bits_per_second    = $size_in_bits / $runtime
  $video_bitrate_kbit = $bits_per_second / 1000   (Note: 1k = 1000)

  q.e.d. ;)

What I noticed is, that the difference between calculated and the 
effective size is regularly too big to be the result of a 1000/1024 
calculation difference. I think DivX4 has problems to hit the exact 
bitrate in general. Even with multipass encoding the result is not very
exact (but always smaller than calculated, so it's no major problem in 
my eyes, because it always fits on the given number of CDs, and you can
make at least an additional backup of your /etc folder, too ;).

Regards,

Joern

-- 
Joern Reder - joern@xxxxxx
Development Head ZYN! Coding Division - http://www.zyn.de/