Movies to Video

After my DVD player died and I was able to bring it to life, I noticed a button on it I had never used before. It was the Progressive Scan button. I bought this DVD player partly because it was progressive scan meaning that instead of showing you every other line of the picture 60 times a second it would show you a fresh frame 60 times a second. But apparently it would only do that if you pressed this button and I had never done that.

So I pressed the button (and nothing happened because I had to enable the button first so eventually it did turn on) and the TV went from showing a full-screen wide image to a letterboxed 4:3 image (itself surrounded by gray bars on each side of the screen). This was confusing but I was able to enlarge the image so a widescreen image filled up the entire screen again. The question then was is this really a progressive scanned image? There’s no way to tell by looking at it and the Sony TV doesn’t have a way of letting you know.

Well, it does if you go into a secret Service Menu that you can only get to by pressing DISPLAY 5 VOL+ before you hit POWER to turn it on. Apparently you can really mess up the TV by changing settings so I got out quickly but this menu confirmed the resolution of 480 lines in progressive. Regular TV is 480 lines but is interlaced so that even-numbered lines refresh and then one sixtieth of a second later the odd-numbered lines refresh. This is important because if you didn’t interlace and just showed a frame every thirtieth of a second (which is essentially what you are seeing) the eye would detect a flicker. But at sixty frames per second the eye sees a continuous image.

Movies are filmed at 24 frames per second and avoid flicker by showing each frame twice for a perceived rate of 48 frames per second.

This brings up a problem: How do you convert a movie at 24 frames per second to a video at 30 or 60 frames per second? You can’t just make up frames that aren’t there and you don’t want to play the movie fast forward either. Well, apparently the easy fix to the problem is that instead of showing each movie frame twice they show even-numbered frames 3 times and odd-numbered frames 2 times. Now 24 frames per second becomes 24×2.5 frames per second which is 60 frames per second.

It gets more complicated than that because the video signal is still being interlaced so that you see odd-numbered lines from a frame then even-numbered lines from the next frame (which is the same frame). Eventually you see an interlaced “frame” of some lines from one frame and the rest of the lines from the other.

Anyway this is called 3:2 pulldown. Progressive scan DVD players are able to do reverse 3:2 pulldown which means they extract the original movie frames and avoid the interlaced mishmash that happens during the transition between two different frames.

I learned all of this from this site but he complicates things by talking about line doublers. Now you would have a hard time finding a DVD player that isn’t progressive scan, but it is interesting that unless you have a HDTV (or EDTV) that you can’t really see it.

FYI: HDTV comes in essentially two different types. One type shows 720 lines progressive. The other shows 1080 lines but is interlaced. In Atlanta, Channels 2 and 5 are 720p, but the rest are 1080i. My HDTV receiver converts 720p to 1080i because my TV can’t deal with 720p and actually would downgrade the picture to 480p.

What I think is interesting about this is that even with the progressive scan DVD player I am not getting a “fresh” frame of content 60 times a second. In fact I’m seeing some frames twice and some frames three times. So more than half of what you see is just re-runs!

One thought on “Movies to Video

Leave a Reply

Your email address will not be published. Required fields are marked *