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Dec 22, 2009, 8:50am

Battered Hat Productions :: Technical Information :: Digital Filmmaking Technical Discussion :: Understanding DV

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Topic: Understanding DV (Read 546 times)

BrokenToy
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Understanding DV
« Thread Started on Jan 18, 2005, 5:29pm »

I'm writing this article to help people understand exactly what DV is and how to think about editing for short films. DV is obviously short for Digital Video. But it has a much more specific meaning. DV is a compression format used by videos cameras to squeeze down video from around 35MB/s to under 5MB/s. It does this by a fairly simple spatial compression, which means it compresses each frame separately unlike mpeg 2 compression which is a temporal compressor and so compresses blocks of frames storing only the difference between them. The individual frames are compressed in a method not to dissimilar from your average jpeg, and the audio is not compressed in any way.

When you "capture" video from your DV camera, you are receiving the video and audio data as it is written on tape, and it's being written directly to hard disk as a file. This means that as long as you do it correctly, you could print to tape capture and repeat this indefinitely with no loss in quality of your image. This is a big deal, before DV, analogue formats lost data every time they were captured by a PC. Also there's no worry about what format to store your data while you edit it. DV is idea for editing, not too big, not too small. It requires little CPU time to uncompress/compress. Of course a classic one is this. You’ll notice there are many different cameras on the market the larger production cameras are DVCam and DVCPro, whereas most home users go for miniDV. So what's the difference?... the short answer, none. That's right, in terms of video and audio data stored, there's far more difference than whether you've got a PAL or NTSC camera. Of course now's there's HDDV but that's a whole other ball game.

Now a word on the specifics of DV. Because it's fucking confusing sometimes. In a PAL camera the data is stored as an image 720 pixels wide by 576 lines with 25 frames per second. But when shown on a television this image is stretched to a 4/3 ratio making something like 768x576. Why? Fuck knows. But it means if you want to add an image to footage, make your image in 768x576 then let your editing program or FX program squeeze it so that when show on television it's the right aspect ratio.

Interlacing and why you should give a shit:
Most DV cameras (with the exception of the Cannon XL2) record interlaced footage. An interlaced frame is actually two images called fields which both are half the height of the overall. These fields are displayed one after another on a television (on PAL at 50Hz and NTSC 60Hz), so even though the footage is at 25fps, 50 different images are shown in one second. Of course this can cause problems when trying to view a file on a pc. The differences in motion between the two fields shows up as horrible scan line differences. Don't panic, when show on a television this will disappear, also if you plan on making a DVD or VCD as your finished output, these incorporate interlacing so no worries there. But there are ways of working these fields together to form a progressive image (the name for video that is not interlaced):

Smart Deinterlace Filter by Donald Graft for VirtualDub
TomsMoComp (Toms Motion Compensated Deinterlacer) Filter for aviSynth_info _download

For both of these programs to work you should install the panasonicDV codec.
And a quick way of capturing your footage to avi from your DV camera is to use DVIO or WinDV.

here's what SmartDeinterlacer will do
before:
[image]

after:

These arn't 100% solutions, they do occationly make mistakes, but these mistakes can normally be removed through tweaking.

Another way to get rid of this interlacing problem is to switch your DV camera to 1/25 shutter. This creates a lot of motion blur but the interlaced fields seem to be of the same time frame and so mesh together perfectly (well they do the the xl1).

« Last Edit: Jan 19, 2005, 7:04pm by BrokenToy »

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BrokenToy
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Re: Understanding DV
« Reply #1 on Feb 9, 2005, 11:13pm »

what??

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