Re: ffmpeg oddity: AVCHD -> MPEG4, results in *smaller* file size

On Thu, 31 Jan 2008 04:29:02 -0800 (PST), in 'rec.video.desktop',
in article <Re: ffmpeg oddity: AVCHD -> MPEG4, results in *smaller*
file size>,
Theo <theo@xxxxxxxxxxxxx> wrote:

Personally, I would use the 15 Mbps recording mode. Even at that rate,
you get over five hours of recording time.

Yes, I certainly will be doing this in the future. The 9 MB/s was a
bit of an accident, unfortunately.

You should see a picture quality improvement at the higher data rate,
especially in fast-action scenes.


* Since the original is supposedly in 60 frames per second,

Theo, the recording format is 59.94 fields per second, not 60 frames
per second. The video is interlaced and there are two fields, an upper
and a lower, per frame, so the frame rate is 29.97 frames per second.

Ah, OK, I was thinking this might be the case, but thought it was 60
frames because every technical spec I'd read mentioned "frames"
instead of fields.

Well, we do often talk about frames per second, but in NTSC SDTV, it's
29.97 frames per second, not 60 frames per second. There are 59.94
fields per second in NTSC SDTV, which is sometimes (inaccurately)
referred to as 60 fields per second, just as the frame rate is
sometimes (inaccurately) referred to as 30 frames per second instead
of 29.97 frames per second.

Note that in the ATSC HDTV world, depending upon frame size, both full
(30, 60) and fractional (29.97, 59.94) rates are supported, so it's
more important there to accurately describe frame and field rates.

Just for the record, the so-called 24 frames per second progressive
mode on your HG10 is actually 23.976 frames per second because it's
embedded (with a 2:3 pulldown) within a 59.94i datastream. 23.976 is
sometimes referred to as 23.98, but that's okay because it really does
mean the same thing.


You would want to maintain, on the output side, the same 59.94 field
per second (29.97 frame per second) rate as the source.

The chosen datarates will determine the size of the output file. If
your source is 9 Mbps and you re-encode to 18 Mbps, the output file
will be about twice the size of the input file, for example, changes
to the audio notwithstanding.

Here the result seems to be opposite, though. I have 9 MB/s files, re-
encoding to 15 MB/s (can be seen in the ffmpeg command in last post),
and the resulting file is 25% smaller.

Then you're changing something else, such as the audio data rate, the
video frame rate, the video frame size, etc., to cause the decreased
file size.

Side note: MB means megabyte and Mb means megabit. There's a rather
large (8-to-1 ratio) difference between the two. :)

By the way, I think that you mentioned that you're changing the audio
(which may not be important in your particular application) to MP3. Be
aware that there's a drop in sound quality when transcoding from a
lossy compressed format such as Dolby Digital AC-3 (which is what your
HG10 records) to another lossy compressed format such as MPEG Layer
III.

Thanks both of you. I'll be playing around some more. I am also
attempting some lossless codecs.

Good lossless video codecs would include Huffyuv and Lagarith, the
latter of which is my favorite. While not necessarily a bad thing,
Huffyuv hasn't been updated in ages, whereas Lagarith is still
actively supported.

Huffyuv 2.1.1
http://neuron2.net/www.math.berkeley.edu/benrg/huffyuv.html

Lagarith 1.3.14 (released July 20, 2007)
http://lags.leetcode.net/codec.html

Good luck!

--
Frank, Independent Consultant, New York, NY
[Please remove 'nojunkmail.' from address to reply via e-mail.]
Read Frank's thoughts on HDV at http://www.humanvalues.net/hdv/
(also covers AVCHD and XDCAM EX).
.

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