720 x 480 vs. 640 x 480, which one to use?

where the 720 x 480 came from

For historical resons there is some "extra room" around the video frame. This is the root of all kinds of errors when people calculate aspect ratios:

Standard definition NTSC DV's sampling matrix is 720x480 (that includes the "extra room" around the image although often there is content in all pixels). DV uses rectangular pixels -- the pixel aspect ratio is 4320:4739 = ~0.912. So the pixels aren't square and you have to scale the image to see the correct proportions. iMovie and a TV set do this automatically but sometimes the poor user might see an unscaled image and think something is seriously wrong...

Notice that inside (and outside!) of that 720x480 sampling matrix the actual active picture size is 710.85 x 486. Weird but true.

Now, to see the correct proportions you have to scale rectangular pixels to square pixels. You usually do that by multiplying the horizontal pixel count by the pixel aspect ratio:

So the square pixel horizontal sampling matrix becomes 720 x (4320:4739) = ~656. So you should scale 720x480 to 656x480 to see the correct proportions on a computer monitor.

The same calculation applies also to the actual active picture size: 710.85 x (4320:4739) = 648. So the square pixel actual active picture size is 648x486 which is EXACTLY 4:3!! THAT is the 4:3 ratio we talk about -- not the 640x480 which is just a cropped value of the former weird size.

The same principles apply also to 16:9 ratios.

http://www.iki.fi/znark/video/conversion/

Hello Chris,

I'm no expert on working with stills/photos, but you might want to have a look at Karl Petersen's chart of photo sizes for iMovie 6
http://discussions.apple.com/thread.jspa?messageID=2090899&#2090899

I think Karl and Matti Haveri explain the issue quite well.

hope this helps

mish

Computers use square pixels. DV editors (like iMovie) convert the square to rectangular pixels.
TV sets use "lines" not pixels.
Some of your 640X480 image will not display on a TV. This is known as the TV "safe" area and you can see it in action when you use the iDVD app.
When you're working in iMovie you would import at 640X480 but it would display that image at 720X480. It wouldn't appear "stretched" as the intended playback (TV set or a QuickTime .mov file) would display the final project at 4:3 aspect ratio.

So it really doesn't matter which dimensions you use

erm... sometimes it doesn't and sometimes it does...

Usually the apps do the correct (or almost correct) rectangular <-> square pixel conversion automatically so the user can be blissfully ignorant (hmm... was that a proper english term 😉 about these things.

But sometimes these things raise their ugly head so the user may need to be aware of the principles.

See also:

http://www.sjoki.uta.fi/~shmhav/iMovieHD_6bugs.html#stills

switch to LCD, does this effect aspect ratio in any way?

No, flat panel TVs (LCD, plasma, SED) behave the same as old CRT TVs and thus also use rectangular pixels.

On the other hand, the new high definition TV uses square pixels so it behaves the same as a computer display in this respect.

Do you see a truer representation of an image on a LCD rather than a CRT?

IMO the CRT has better image quality than a LCD. YMMV.

Hi Chris, I have an LCD and a HD Plasma and personally, I think the LCD is clearer (perhaps because it is 40" and the plasma is 50"). By the way - I always thought these bigger tv's were widescreen - they arent!

Hey Matti... are you sure you are speaking english??? LOL