Confused about rewrapping/transcoding AVCHD, 50i to 25p

[dtr]

Hi there,

Just got my GH2. Having loads of fun with the little beast!

There's one thing I'm confused about. Could someone clear this up for me?

I'm shooting in HBR 25fps mode (standard 1.1 firmware). The file is 50i. I use RewrapAVCHD to make a more compatible m4v file. This is still 50i.

Now if I wanted to downscale and compress this unedited file for example for quick uploading on the net, do I set the compressor to deinterlace (upper field first)? Or does that introduce unneeded quality degradation which is to be prevented with another rewrapping step before transcoding?
In other words, do I let the compressor deinterlace to 25p or should that be done earlier? If so, with what OSX tool (preferably freeware)?


Tanx for your attention, Dieter

[Axel]

Hi,
the HBR is "editing friendly" with 25p, flagged "as" interlaced. I don't know about NLEs not being able to edit 1080 25p, but there [i]are[/i] limitations. For example, to this day there is no general support for this format on bluray, whereas 25i [i]is[/i] supported.

To export for web, you shouldn't deinterlace, because there is no field dominance. Just put the settings manually to "25p", and everything will be fine.

The best software-encoders for mpeg4 have been tested to be Adobe Media Encoder, and x264. Go to this site:
[url=http://www003.upp.so-net.ne.jp/mycometg3/]http://www003.upp.so-net.ne.jp/mycometg3/[/url]
... and download the Quicktime-component.

Now, if you have Quicktime Pro, you can export as mp4 with x264 instead of H.264 (indeed faster and, i think, better). But the best transcoding tool for OSX, and freeware too, is MpegStreamclip. You chose "export as MPEG4" with x264 encoder, limit the bitrate in the GUI of MSC (enable "Multipass), then you go to "Options", and the very complicated looking x264 interface opens. For quality you actually only need to change two things (in the tab "Values"):
1. Set "Faster First Pass" from "Turbo2" to "disabled"
2. set "b_frame_strategy" from "fast" to "Optimal: Slow"

To open AVCHD directly with Quicktime, you need to download the Panasonic import plugin from this site:
[url=https://eww.pass.panasonic.co.jp/pro-av/support/dload/avccam_impt/agree_e.htm]https://eww.pass.panasonic.co.jp/pro-av/support/dload/avccam_impt/agree_e.htm[/url]

Nice side-effect: You now have the useful Quicktime movie-thumbnails for your mts-clips, even Coverflow will work.

[dtr]

Axel, thanks for the exhaustive answer!

So to recapitulate, the HBR mts file is actually encoded as 25p but reports to be 50i in software? (What's the logic of that?!)
In my vague understanding I thought the GH2's sensor/processor grabs 25p but writes it to a 50i file, each p frame to 2 i frames (i guessed it has to do with writing speed). To get the original 25p the 50i file should then be deinterlaced. This is not correct? 

[Axel]

[quote author=dtr link=topic=443.msg2793#msg2793 date=1332423304]So to recapitulate, the HBR mts file is actually encoded as 25p but reports to be 50i in software? (What's the logic of that?!)[/quote]

This I can't tell you. Panasonic says the HBR is "edit friendly", and I was wondering why. I had a Canon XH A1 before, and it had a mode called "25f", meaning something similar. For the sake of the Firewire-protocol and HDV meaning the same in every NLE, it was recognized as standard interlaced stuff. If you deinterlaced it, you didn't get a weird pushme-pullyu-effect, but considerable quality loss, because then, with the CCD sensor, the clips were [i]already[/i] deinterlaced. But the GH2s CMOS reads the image progressively, and 50 fields should in fact be more difficult to process. But I am no engineer.

[quote author=dtr link=topic=443.msg2793#msg2793 date=1332423304]In my vague understanding I thought the GH2's sensor/processor grabs 25p but writes it to a 50i file, each p frame to 2 i frames (i guessed it has to do with writing speed).[/quote]

It captures one frame within a certain time span (I think 40 ms, rolling shutter unfortunately as long as exposure time chosen), but it writes it as two seperate fields to the card. Can't have to do with writing speed, the framerate is nothing compared to the degree of compression that is processed in that time.

What you get with deinterlacing is a quality loss of up to 50 % (depending on the method). What you get by leaving the mts as is, some players like VLC will show fine comb patterns, as if the footage was indeed interlaced. I don't know why this is so, I can't explain it, I just made the experience that by exporting as 25p everything behaves like native progressive footage.

Perhaps someone else knows better.