mpeg2video vs. libx264: Why the huge difference in filesize?

[yetanotherlogin]

Hello

Out of curiosity, I converted the following video recorded by a camcorder...

Input #0, mpeg, from 'M2U00022.MPG':
Duration: 00:07:50.07, start: 7222.478100, bitrate: 9456 kb/s
Stream #0:0[0x1e0]: Video: mpeg2video (Main), yuv420p, 720x576 [SAR 64:45 DAR 16:9], 25 fps, 25 tbr, 90k tbn, 50 tbc
Stream #0:1[0x80]: Audio: ac3, 48000 Hz, 5.1(side), s16, 448 kb/sAt least one output file must be specified

... from mpeg2video to libx264:

ffmpeg -i M2U00022.MPG -c:v libx264 -c:a copy M2U00022.converted.MPG

The original file is 555.636.736 bytes while the output is down to 72.609.792 bytes. Why such a big difference? What does libx264 do to the video to achieve such big compression?

Besides using a smaller screen size (eg. 720x576 -> 360x288) and recompressing sound from ac3/5.1 to MP3/stereo, what other tricks could I use to further reduce the filesize?

This is for viewing on computers, so it should sound good but video doesn't need to be of the highest quality (which is why I reconfigured the camcorder from HD to STD).

Thank you.

[Baldrick]

h264 compresses better than the older mpeg2.

And a camcorder that must use realtime compression uses higher bitrate(=bigger file size) instead of complex compression encoding.

You can also adjust the quality or bitrate settings in ffmpeg and you can maybe make it even smaller.(..or bigger if the quality isn't good enough).

[sanlyn]

Well, let's see: cut the encoded frame size size in half (can't possibly be interlaced at that size, of course, so there goes some more data into the ether), made garbage of the audio, used a compressor much newer and more advanced than ye ole MPEG, lost more data with lossy re-encoding.....

gosh, I can't imagine why you'd have a smaller file size ! ! ! !

This is for viewing on computers, so it should sound good but video doesn't need to be of the highest quality

Well, not all of us have the funds lying around for an extra computer just for low-quality a/v.
Interesting exercise, but people actually do this sort of thing all the time. A search thru this forum quickly demonstrates that counter-restoration is a current trend. Many make even greater reductions. I'm sure they'll chime in with suggestions.

[Selur]

cut the encoded frame size size in half

720x576 -> 360x288 produces a quarter and not half the frame size,...

what other tricks could I use to further reduce the filesize?

use a lower crf value
smooth and denoise the material (there are a few filters that ffmpeg support,... http://ffmpeg.org/ffmpeg.html)

What does libx264 do to the video to achieve such big compression?

-> What exactly is the difference between MPEG-4 AVC and MPEG-2

[yetanotherlogin]

Thanks much for the infos. I'll check the archives of the video/audio conversion forums for tips.