ffmpeg oddity: AVCHD -> MPEG4, results in *smaller* file size
- From: Theo <theo@xxxxxxxxxxxxx>
- Date: Tue, 29 Jan 2008 17:17:54 -0800 (PST)
Hello, all. I have successfully set up my Ubuntu Linux 7.10 system to
decode AVCHD into MPEG-4 AVC (with MP3 audio) in an AVI container. I
am doing this through a series of steps through different programs
(xporthdmv to split the audio and video from the MTS file, then ldecod
to encode the .mpv video into .yuv, and then ffmpeg to encode the .yuv
into mpeg4 and put everything back together in an .avi container)
Everything seems to work, except that now the resulting .avi is of
significantly smaller file size than the original MTS (AVCHD) file,
which from what I understand, should be the opposite (AVCHD being
efficiently compressed, and the uncompressed data should be about x4
the file size). I am wondering if I am accidentally downgrading the
quality or other data in my process. Here is my ffmpeg command:
ffmpeg -r 29.97 -s 1440x1080 -i $videofifo -i
$audiofile \
-vcodec mpeg4 -aspect 16:9 -sameq
-b 15000k \
-acodec libmp3lame -ac 2 -
ab 256000 \
$outputfile
The original file is from a Canon HG10, outputting to 1440x1080
interlaced, supposedly 60 frames per second (1080/60i), and at 9 mb/s
bitrate (less than the "full" 15 mb/s bitrate of HD, but still good
quality).
The format I am wanting it to encode into is MPEG-4 AVC at the same
quality for everything possible, with no loss of quality. This is for
importing into a video editing system, so I'll downgrade the quality
for distribution later.
The file size is resulting in about 20-25% less than original (an 80MB
MTS file come out as a 60MB MPEG4/AVI file.)
Some things I think that might be causing the problem, but don't know
enough about encoding to verify:
* Since the original is supposedly in 60 frames per second, and I'm
currently in 30 frames per second, could that be the cause? I've tried
encoding in 60 fps in the ffmpeg stage, but the resulting video does
not play well: it is too fast, and out of sync with audio.
* The video is a Blue Screen production, so most of the background is
stationary and a solid color. Could the MPEG4 be compressing this,
resulting in the smaller file size?
Thanks for any help.
.
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