[MPlayer-users] How to find "technical differences" in MPEG?

[Loren Merritt]

On Mon, 19 Feb 2007, karl-heinz-1950 at web.de wrote:

> I have two MPEG-2 files. The first was recorded with a settop DVD
> recorder (we call it here the large one), the second was created out
> of this file using mandvd (which uses mencoder) (we call it here the
> small one). The last file is in size about 60% of the first one. I
> want to find out why. What I did so far is I started "mplayer
> <filename>" to see the technical details in the console, you can find
> the full output at the end of the email.
[snip]
>
> As you can see the smaller file has a (slightly) higher resolution and
> a higher bitrate (this is due to the settings I know that it won't
> increase quality). So I would expect a file which is slightly larger.
> Why did I get a smaller file? The only reason I can think of is that
> the larger file uses CBR for the video and the smaller one VBR. But
> how can I verify this?

Don't trust the bitrate value printed by MPlayer. It's just read from
an informative header in the file, and there's no guarantee that the
muxer put the correct bitrate value there.
Since 9800 kbps is the highest rate allowed on dvd, a muxer that
actually says 9800 probably didn't know the bitrate and just played it
safe by writing that number.
Any mpg I create with mencoder or ffmpeg has a nominal bitrate of 0 kbps, 
which is just as obviously wrong as 9800.

--Loren Merritt