[mythtv-users] NuppelVideo container format
Michael T. Dean
mtdean at thirdcontact.com
Sun Nov 27 16:10:05 EST 2005
Felix Rubinstein wrote:
> Additionally, as I understood, if you take a file in Myth native
> format with resolution 704x480 which size is 1,756MB after decoding it
> to DVD (i.e. MPEG-2) format, the file will shrink to 899MB.
> Now is the one million dollar question, why the native file (original)
> is so large?
Because it uses a different bitrate.
A video recorded at 4Mbps video with 192Kbps audio will take almost
exactly 1.75GiB/hr *regardless* of format (the almost exactly is because
there will be some very small differences due to container format).
And, the differences due to container format will be negligible compared
to the size of the file.
In other words if I use MPEG-2 or MPEG-4 or MPEG-4 AVC (H.264) or any
other format with a given bitrate, the file will be the same size, since
the size is determined exclusively by the bitrate. Now, in fact, most
videos are encoded with variable bitrate, so it may not be obvious that
bitrate is the one and only factor that determines file size (with a
variable bitrate, we can't just multiply by time to get the file size,
we must sum the instantaneous bitrate over time).
It is true that certain encoding formats are more efficient at
video/audio compression than others. Therefore, people generally say
that MPEG-4 videos are "smaller" than MPEG-2 videos. What they're
actually saying is that the MPEG-2 compression algorithm requires a
greater bitrate than the MPEG-4 compression algorithm to achieve a given
level of quality. Note, however, that these differences are not due to
the container format, but instead, due to the compression algorithm
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