Understanding the difference between 720x480 and 640x480

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Level 1

4 points

Understanding the difference between 720x480 and 640x480

Hello,

I have a puzzling question in regards to exporting and aspect ratio that I can't seem to figure out.

Starting from the beginning. When I shoot footage with my Canon GL2 and capture it to my computer via Firewire cable, it captures the footage in DV file format. If I open that file before even starting Final Cut, the file opens in Quicktime, and when I click 'Get Info', it tells me that this file is 720 by 480. I understand that... what I don't understand is if I measure the actual size of the clip when it opens on my computer screen, it measures out to 640 by 480 pixels. That part I don't get.

1) I know that 720 by 480 is the NTSC standard, but if 720 by 480 is 4:3, then why does 4/3 = 1.3333 and 720/480 = 1.5?

2) And if 'Get Info' is telling me that the file is 720 by 480, why is it that when I open it, the video only measures out to be 640 by 480 pixels?

3) Interestingly, while watching the DV clip in Quicktime, if I open 'Movie Inspector' the format reads "DV, 720 by 480 (640 by 480)". What does that mean?

4) Next, if I put this file into Final Cut (or iMovie, either or) and then export it using Quicktime Conversion and adjusting the size to "NTSC 720x480 4:3", it still exports a file that measures out to be 640 by 480 pixels. Why is FC exporting a video that is measuring out to be 640 by 480, when in the settings it specifically lists it as 720x480?

5) Finally, after bringing this exported file into my third party converter, I am able to convert the file to 720 by 480, and the converted file actually measures out to an actual size of 720 by 480 pixels. However now, the video looks stretched. Why is this so?

I've been told that I need to deliver the videos I export in several different formats/codecs, however, I was told that they all need to be "the NTSC standard, 720 by 480, 4:3". But am I delivering the file that measures out to 640 by 480 with a crisp picture? Or the converted file that measures to an actual 720 by 480 with a stretched picture?

Thank you so much to anyone that can explain this to me.

JB

Posted on Aug 1, 2010 6:35 PM

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Answer 1

Studio X

Level 7

27,229 points

Aug 1, 2010 7:05 PM in response to J.B. Biunno

The opening chapters of the Final Cut Pro manuals have a very coherent explanation of video formats and do a good job explaining the difference between square pixel formats (ie computer graphics) and non-square formats (ie video).

Once you understand the whole concept that pixels do not have to be square, the rest will begin to make sense.

Good luck.

x

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Answer 2

Aug 1, 2010 7:12 PM in response to J.B. Biunno

It's the mystifying Pixel Aspect Ratio: Square vs. Non-square pixels.

SD video uses non-square pixels. They are skinnier, by .9. (than a square pixel)

You computer monitor, and others, uses square pixels.

So to get a more accurate representation, in QuickTime, it will display 720x480 movies as 640x480. This is to compensate for the non-square to square translation.

720 x .9 = 648. So it's close to 640.

You can adjust QT's "aperture", or playback characteristics. Here's a link.

This might be a bit brief, but this Pixel Aspect Ratio is something that makes some people's head swim. They get SO confused over it. If this doesn't answer all your questions I'd do some generalized research into the Pixel Aspect Ratio concept.

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