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Old December 11th, 2010, 04:56 PM   #1
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Coping with progressive material in a 50i frame

As many of you may be aware, there are a number of consumer cameras that now record a 25/30p picture, but then place it into a 50i/60i frame in order to meet the AVCHD specification.

As FCP cannot handle AVCHD natively, I use the built in capture facility to convert it to Prores.

But here's my concern. The native AVCHD footage looks fine in VLC, but after FCP has transcoded it to Prores the footage is flagged as 50i and exhibits very faint interlacing artefacts that are not present on the original footage.

These artefacts are very faint, but sure enough if if you look close enough on a moving object they are there. For example here is a section of an image (not resized) that I took on a Canon Legria that has been transcoded in FCP to Prores:

http://www.media2u.co.uk/images/stor...-interlace.jpg

As you can see from this section of an image of a moving train there are very faint lines there that aren't present on the master footage. Indeed the master footage looks exactly as you'd expect progressive footage to look.

So how can this be avoided? To my mind one possible explanation is that FCP sees the master footage as interlaced (it's 25p in a 50i frame) and sets up the encoder accordingly to expect interlaced material. As a result there are these faint artefacts all over the image.

This is only a theory and if it is indeed the case then I can't see a way around it.

Anyone else experienced this and is there a way around it? Any progressive footage shot on our 2 domestic AVCHD cameras seems likes its being degraded at the moment.

Thanks.
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Old December 12th, 2010, 05:07 AM   #2
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Hi Marcus and hope this helps.

I have a canon HF11 that has the capability of recording 50i or 25p (which I suspect is the 25p over a 50i that you are talking about)

I find that the 25p mode on these camera's is not very good at all, some of the sony HDV camera's also do this 25p mode and it is not true progressive and can cause the interlacing problems that you are talking about.

I find that it is best to shoot 50i with these AVCHD camera's, import as pro res LT and then do an progressive export from FCP as 25p.

Shooting 1920x1080i 50i on my canon HF11 also gives better less smeary pictures and once loaded into FCP at pro res LT and outputted as 1920x1080 25p or to 1280x 720p (apple TV) the pictures look very good as you can see on this example: YouTube - British Beef Jerky Shoot 7-8-09

For most of my videos they go to you tube as 720p so a lot of the time I also just export the master fils as the same format as the timeline (1920x1080 50i) and let quick time make it progressive when it goes to 1280x720p ( Apple TV)

Doing progressive through software is normally not the best way to do it but in the case of AVCHD it seems to work better this way if you shoot 1920x1080 50i and let FCP or QT do the progressive bit.
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Old December 12th, 2010, 06:45 AM   #3
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Quote:
Originally Posted by Gary Nattrass View Post
Hi Marcus and hope this helps.

I have a canon HF11 that has the capability of recording 50i or 25p (which I suspect is the 25p over a 50i that you are talking about)

I find that the 25p mode on these camera's is not very good at all, some of the sony HDV camera's also do this 25p mode and it is not true progressive and can cause the interlacing problems that you are talking about.
I have an HFS11 as well (as well as a Sony VG10).

The thing is though, if you review the raw footage (pre conversion to Pro-Res) it looks fine. It looks exactly as I'd expect progressive to look.

So it's my theory that its not the cameras that are the problem, but the AVCHD to Prores conversion process. That seems to be where the artefacts are introduced.

If there was another way to convert the AVCHD to Prores, that would be a good way of testing it. If you forced Prores to encode the footage as progressive rather than interlaced I suspect the problem would be solved.

But how? Compressor isn't interested in importing the raw AVCHD files.
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Old December 12th, 2010, 10:47 AM   #4
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It may just be that the RT pro res in the viewer is not playing it back at full bandwidth, I sometimes see playback problems with my AVC intra re-wrapped as pro res Hq on the macbook pro as it needs a lot of grunt to play it back.

My footage looks fine on the camera too but being domestic camcorders I don't trust that they are recording 25p at all and just something that looks ok for the average viewer, as said far better to keep it 50i and then do the progressive as a FCP output.
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Old December 12th, 2010, 11:51 AM   #5
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My only knowledge of this camcorder comes from what I've read -- but Adam Wilt found interlaced problems on his progressive material as described here:

ProVideo Coalition.com: Camera Log by Adam Wilt | Founder | Pro Cameras, HDV Camera, HD Camera, Sony, Panasonic, JVC, RED, Video Camera Reviews

Note in particular his comment:
Part of the problem with AVCHD, at least as recorded on the HF11, is that itís always recorded as interlaced, and thus shows interlaced 4:2:0 chroma
which he supports with a captured still that reveals precisely the problem you're describing.

HTH
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