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Canon VIXIA Series AVCHD and HDV Camcorders
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Old April 7th, 2007, 05:54 AM   #1
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25p - how does it work?!

Hi guys

Sorry for being slow here but can anyone explain how the HV20 records 25p compared with usual 50i. I understand how 50i is recorded to tape with alternate fields but am still a little confused how 25p sits inside a 50i shell? If the cam is recording 25 full frames per second how is each field made up? Is it literally 2 identical fields following each other or is it that each field is still recording only half the information? If its the latter wont there be interlacing artifacts in VLC etc?

sorry for confused ramble!
Cheers
Fergus
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Old April 7th, 2007, 07:17 AM   #2
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Even though the camcorder is shooting progressive frames, the way the video is stored on tape has to be compatible with normal interlaced TVs so people without progressive TVs can play it back. So, the progressive frame is divided into two fields just like if you were shooting interlaced. The difference is there is no temporal displacement between the two fields to introduce motion blur and soften the image.
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Old April 7th, 2007, 11:04 AM   #3
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ok thanks guy so that being the case there should be no interlacing artifacts since although each frame is made up of two fields there is not 1/50th of a sec difference between them?
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Old April 7th, 2007, 11:55 AM   #4
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Exactly.

Except that, if you're displaying on an interlaced TV, you may still see interlaced artifacts -- not in your footage, but from the TV converting your progressive footage to interlaced. If displaying on a progressive TV you should never see interlaced artifacts from 25p footage.

And as for what you said earlier, about it being "identical fields" -- no. The fields are not identical -- if they were, you'd have half resolution. The fields contain unique data, but the data for both fields were imaged at the same instant in time. There is no temporal difference between them, but there is a difference in what each field contains.
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Old April 7th, 2007, 12:20 PM   #5
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thanks chaps - much clearer now
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Old April 8th, 2007, 04:59 AM   #6
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ah sorry one more question:

If the one progressive frame is recorded as one instant in time but over two fields then when played back on my PC the raw file will play back one field following the other? In other words wont playing back the raw file introduce a temporal difference between the two fields so that field 2 is 1/50th sec later than field 1? Although this wont introduce motion interlace artifacts since there is no difference in motion, it still wont be correct? Unless blend fields deinterlace method is used in VLC to play both fields at the same moment in time?

Thanks
Fergus
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Old April 9th, 2007, 08:25 PM   #7
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Quote:
Originally Posted by Fergus Anderson View Post
ah sorry one more question:

If the one progressive frame is recorded as one instant in time but over two fields then when played back on my PC the raw file will play back one field following the other? In other words wont playing back the raw file introduce a temporal difference between the two fields so that field 2 is 1/50th sec later than field 1? Although this wont introduce motion interlace artifacts since there is no difference in motion, it still wont be correct? Unless blend fields deinterlace method is used in VLC to play both fields at the same moment in time?

Thanks
Fergus
if you play it back on a PC (i.e. a PC monitor) both fields will be displayed at the same time. hence it will be true progressive and you should not see any artefacts. what you describe is what you would get on a CRT television when playing back a progressive signal.
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Old April 10th, 2007, 03:45 AM   #8
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thanks for the clarification
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