Comments

Coursedesign wrote on 10/30/2004, 4:42 PM
NTSC as an analog broadcast format has no pixels at all.

NTSC DV in 4:3 is 720x480 with a 0.9091 pixel aspect ratio (ie. horizontally compressed pixels)

NTSC Widescreen DV in 16:9 is still 720x480, but with about a 1.2021 pixel aspect ratio (ie. horizontally elongated pixels)

The above is 640x480 if you are using square pixels. Square pixels is the default for computer graphics.

Then there is NTSC D-1 and D-5 720x486 (six extra lines), this is used by broadcasters primarily....

And then there is PAL...

Chienworks wrote on 10/30/2004, 8:28 PM
And more confusing still, if you use square pixels it should be 654.545454...x480, not 640.
rmack350 wrote on 10/30/2004, 11:06 PM
And that is because that 4:3 ratio of a video image is really meant to apply to the visible area of the frame-not the entire frame. A standard definition TV is supposed to show you a 4:3 area of the frame-it is not required to show the entire frame.

Consider that the technology is 50-60 years old and most older technology needed time to douse the signal at the end of a line before ramping it up again at the next line. The short of it is that there's the 4:3 area of the frame and then a little more at each side for "slop".

This was espcially true of tube technology but even a modern VHS deck shows a soft edge at the right and left bounds. In fact, even our DSR500 DV camera leaves a soft edge on the left side. It's inconvenient for PIPs.

Think of the digitization of a video signal as sampling. Each line of the NTSC or PAL frame lasts for a specific length of time. For DV, each line of video is sampled 720 times, but it could just as well be sampled 10 times, 100, times, or 1000 times.

If you displayed a frame from each of these samplingson a computer screen, one would be ten pixels wide, another 100, another 720, another 1000. But to make any of them look right they'd have to be reinterpreted for the computer screen. For NTSC, 655.545454x480 would look just right-for any of those examples. (So would 720x528, but I think it's mathematically inconvenient for Vegas)

Rob Mack
riredale wrote on 10/30/2004, 11:15 PM
The reason the number is 655 is because DV is not based on a 4:3 aspect ratio. It's a bit wider than that. If I do a search on this tidbit later tomorrow I will find the technical document that lays this out (this was discussed at length on this board about two years ago).

You will notice this effect for yourself the first time you import a 4:3 still photo into Vegas. There are narrow vertical black bars on each side of the image, a byproduct of the fact that the DV window is slightly wider than 4:3.
PH125 wrote on 10/30/2004, 11:32 PM
Wow, all that is just confusing. It's too bad that we can't just all upgrade to completely digital signals and tvs that diplay the entire 720x480 image, and not certain areas based on the particular tv. On cable news networks, the one tv in my house diplays the news ticker exactly at the bottom of the screen, and the other flatscreen diplays that plus some of the black slack at the bottom that the broadcasting network uses for safety.