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Compression on tape by XLH1

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steve siegel
Compression on tape by XLH1
on Nov 2, 2010 at 10:23:40 pm

I have had an explanation of this by Canon, but it sounds too good to be true, so I am looking for a confirmatory opinion.
The XLH1 compresses video to tape using an MPEG2 based codec in a 4:2:0 color space. It is capable of exporting in a 4:2:2 color space via HDSDI port, but not on tape.
I had assumed that when the camcorder compresses for tape using the lossy MPEG2 codec, that data was just thrown away. Canon says that is not true. The information lost in 4:2:0 is still on the tape, compressed and only available using certain software, but still there, and it is 4:2:2!
The real kicker is this. I can import an 80 MB file from tape into my editing software (Adobe Premiere Pro CS4), and export the same clip (no editing done) using an uncompressed codec to get a beautiful 3 GB file. Either Premiere is adding junk, or there really was a lot of compresssed, invisible information in the 80MB file as it came from the tape. Can anyone confirm or refute that there is 4:2:2 data on the tape than meets the eye.


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Michael Galvan
Re: Compression on tape by XLH1
on Nov 3, 2010 at 9:16:05 pm

What is recorded on HDV tape will always be 25mb 4:2:0 LONG-GOP.

What Canon is saying is that it is possible to bypass this quality by recording from the HD-SDI port, which outputs a fully uncompressed 1.485Gb 4:2:2 HD signal. I do it all the time with my XL H1s, recording to a Nanoflash. Image Quality is a great deal better!

Hope this helps.


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steve siegel
Re: Compression on tape by XLH1
on Nov 9, 2010 at 10:25:08 pm

Michael,

Thanks for your reply, but I think you misunderstood the question. I now have an answer that may interest you. Of course you can bypass tape and record vis HDSDI to a flash card in 4:2:2. That's a given.
What is generally unrecognised, however, that MPEG2 4:2:2 data is stored on the tape as well. It is also apparently saved when footage is captured (at least in Adobe Premiere CS4) It just isn't accessed by the commonly used codecs. It is accessed by exporting as "uncompressed". The file you get is perhaps 9 times the size of a standard QT or AVI file.
I had the opportunity to look at the same footage displayed on a large professional monitor. First we looked at "raw" footage imported from tape. Then we looked at the same clip imported and exported as uncompressed. The pixellation in the former was absolutely absent from the latter at the same magnification. There was evidently information hidden in that file that could be teased out to give a superior image, and in my limited experience, was similar to that obtained with a Nanoflash.


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Michael Galvan
Re: Compression on tape by XLH1
on Nov 13, 2010 at 3:46:27 pm

I find this hard to believe. If the data stream is taking up the full 25mb, where would this 4:2:2 information be stored on tape?

And even if this was true, Canon probably would've touted this as an advantage over other systems.


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Michael Galvan
Re: Compression on tape by XLH1
on Nov 19, 2010 at 8:39:49 pm

I'd like to see an example of what you are seeing though. You are not in the NYC area by chance, are you?


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steve siegel
Re: Compression on tape by XLH1
on Nov 21, 2010 at 10:54:31 pm

I have no idea what the particulars are. All I know is what the Canon tech told me, and the fact that people at 20th Century Fox are able to do things with my uncompressed tape footage that they are only able other wise to do with higher quality stuff.

The only experiment I have done is this. Take a clip from tape and import into Adobe Premiere Pro CS4. Export as MPEG 2 24 fps, 1920x1080. You get an 80 MB file. Export the same clip as an AVI file using the setting "uncompressed 4:2:2" and get a 2 GB file.
Blow each of these clips up 2 or 4 times and the pixellation on the first one is noticably worse than on the second. There has to be more information in the second clip, and it has to come from somewhere.


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