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What causes parts of the pic on some DVDs to move around?

post #1 of 6
12/5/08 at 5:46pm
Thread Starter 
Just put in the newest anamorphic version of "Phenomena" from the Anchor Bay Collection Tin of 5 Films by Dario Argento.

My question is, "What causes parts of the picture to move independently of the static parts?" It makes it look 3-D.

Is it poor authoring, using sub-par equipment or what?

I tried the search button but nothing came up. I'm not even sure I'm correctly describing what I'm seeing.

I've read about it before, but it's so bad at the beginning of this film, I want to read about it again.
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post #2 of 6
12/5/08 at 7:00pm

Re: What causes parts of the pic on some DVDs to move around?

I think that's the MPEG-2 compression. It sort of 'remembers' which parts of a frame are redundant from frame-to-frame (the static parts) and repeats those instead of wasting bits on them and saves the bits for the parts of the frame that are moving and changing from frame-to-frame. That's why you're seeing the moving areas 'move' and the static areas stay still.

Not sure if I'm explaining that right (LOL)- there was a great article several years ago in an issue of Stereo Review that explained the process very well.

The fact that it's so obvious on the DVD you're watching- I don't know if that means the disc was compressed poorly or if your DVD player isn't handling the compression well. Someone else around here surely knows, though!
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post #3 of 6
12/6/08 at 7:44am

Re: What causes parts of the pic on some DVDs to move around?

I noticed the same thing on the Saturday Night Live Season-1 DVDs. I called it "smearing". I believe it's caused by putting too much info on to a DVD (to save money I'm sure). Someone with technical expertise can explain it better than me but I think it has something to do with the "bit rate" of the DVD.
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post #4 of 6
12/6/08 at 9:24am
Thread Starter 

Re: What causes parts of the pic on some DVDs to move around?

Thanks guys!
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post #5 of 6
12/7/08 at 6:45pm

Re: What causes parts of the pic on some DVDs to move around?

I remember seeing this digital artifact as far back as the 1980s, when occasionally you'd see the pattern of wallpaper wobble behind somebody in a movie or tv series. As I understand it, it's not just a problem with DVDs, but all kinds of digital video sources and is all to do with the compression of the data. Cable channels with low bitrates (like QVC) suffer from all kinds of weird effects like mosquito haloes - mosquito noise around moving figures. I believe the wobbly wallpaper sometimes happens when you get frame weave on flim (worn sprocket holes cause the film to oscillate from side to side), but the compression algorithm misses the movement as it doesn't reach the threshold needed to be recorded as movement within the frame. Therefore the pattern stays where it is and the parts of the frame that are picked up as moving do so in relation to the pattern.

I think.
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post #6 of 6
12/7/08 at 6:54pm

Re: What causes parts of the pic on some DVDs to move around?

This was particularly problematic with MPEG-1 compression and shows up a lot in low bitrate MPEG-2 product. It's really quite disconcerting to see, for example, a person's facial features appear to float around on his/her face.
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