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Canon XH Series HDV Camcorders
Canon XH G1S / G1 (with SDI), Canon XH A1S / A1 (without SDI).


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Old May 7th, 2007, 06:19 AM   #1
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XH A1 compresses in HDV then writes in DV to tape?

Hi!
I have now shot my first project in SD DV (after shooting some stuff in HDV) and while capturing I noticed that the image looked somewhat MPEG2-compressed.

This should not be since I recorded in 4:3 DV 25f. Also I played the tape on an old DV-camcorder so the signal is definitely DV. I captured in Premiere Pro 2 with the usual DV-settings.

I suspect that the A1 internally uses the HDV signal as a basis and converts it to DV. The compression is not that obvious but it is there.


You may have to download the image to be able to zoom in and see what I mean.

http://img143.imageshack.us/img143/1...ession1bm7.png
Here is a portion of the image resized to 400%:
http://img516.imageshack.us/img516/6...ession2pc9.jpg

Look at the lower edge of the pit with high contrast and the edge of the wheelbarrow. There are some obvious compression artifacts there. And look at the blue trousers which show macroblocking.

This is not a huge problem for me since I care more about HDV but I find it disturbing that nobody seems to have noticed this yet. A DV-image should not look like this, right? Maybe it is only something the PAL-Version of the camera does? Maybe it´s the Frame-mode? Or are my eyes playing a trick on me?
Any thoughts?
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Old May 7th, 2007, 07:02 AM   #2
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Hard to say. DV-compression does use mjpeg and it also causes those jpeg-compression artifacts. Shoot something where there's A LOT of motion and it should be apparent if it's gone through the mpeg2 encoder.
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Old May 7th, 2007, 11:22 AM   #3
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re

I have the same experience, DV have some mpeg2 compression residue.

When you make SD mpeg2 (from this DV), it is more noticable.
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Old May 7th, 2007, 11:37 AM   #4
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From a RFC:

"The DV format only uses the DCT compression technique within each frame, contrasted with the interframe compression of the MPEG video standards. All video data, including audio and other system data, are managed within the picture frame unit of video."

"The DV video encoding is composed of a three-level hierarchical structure. A picture frame is divided into rectangle- or clipped-rectangle-shaped DCT super blocks. DCT super blocks are divided into 27 rectangle- or square-shaped DCT macro blocks."

So with the DV format, upon close examination, you can expect to see some macro block artifacts. However, because each frame stands alone compression wise, they may not be as apparent in video (as opposed to individual frame grabs).
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Old May 7th, 2007, 11:50 AM   #5
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My take on this. Blow ANY DV signal up to 400% & you'll see the same results.

The original pic you posted looks fine here.

Bill
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Old May 7th, 2007, 12:57 PM   #6
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Blow ANY DV signal up to 400% & you'll see the same results.


http://www.videoproduce.cz/images/DV_400pct_vx2000.jpg

it's not look like the same result (specially about edges)
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Old May 7th, 2007, 01:45 PM   #7
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How does it look when playing back, not from a freeze frame?
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Old May 7th, 2007, 01:47 PM   #8
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The sawtooth along high contrast edges in Pavel's picture look more like interlace artifact from a slowly panning camera than compression artifact.
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Old May 7th, 2007, 03:51 PM   #9
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Pete, you are right. Only interlacing, nothing else (like XH-A1 dv).

I mean that difference is too big, XH-A1 make worse DV. Best result for "sd video" is first shot HDV, than make downconversion (by a sequence of still).

But I can be wrong (hope .-) ).
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Old May 7th, 2007, 04:17 PM   #10
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Pavel, your example & Roland's aren't anywhere near the same contrast/color saturation & can't really be compared... at least that's the way I look at it.

Notice the blocking artifacts where indicated. It's quite similar to Roland's with the blue pants.

Bill
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Old May 7th, 2007, 10:57 PM   #11
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Make a custom preset with reduced sharpness, HDF and DHV values. This reduces some of the enhancement and will get rid of the stair-stepping on strong verticals and diagonals. Try different amounts of each until you get the amount of smoothness you like. The artifacts you're seeing will be less of a problem with less enhancement processing.
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Old May 8th, 2007, 01:46 PM   #12
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Compositing

Here's what I want to try one day not too long from now.

HD image is 1440x1080 at 1:33 aspect.
DV image is 720x480 at 0.9 aspect.

I want to try shooting in HDV, but to frame my shot such that the subject and the important part of the image I am capturing are occupying only a small portion of the HDV image. The rest of the HDV image will be wasted. I will then transfer the HDV stream to computer, and instead of resizing the image (which, inherently, introduces artifacts), I will crop the image to 720x480 discaring the part of the frame I don't need (I can do that, because I shot this way). So, there will be no resizing, hence, less artifacts.

Of course, this approach creates problems for wide-angle shooting situations.
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Old May 9th, 2007, 03:21 AM   #13
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Color subsampling?

That looks more like the result of red & blue channel color subsampling in DV (4:1:1) - that's why you only see it in very saturated blue or red areas and not in other parts of the picture. I see this all the time in DV - it's often more apparent when looking at the frames on a computer vs. on playback as the analog outputs on most DV cameras seem to smooth this out on playback.
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