Why NES screen captures aren't identical sizes

[Cosmonautical]

To start with, here's an old quote:

Originally Posted by DRW

O.k., one more question: On a normal TV monitor (not PC) with NTSC, which one is it? The upper or the lower image?





If you don't see it: The difference is the height. Look at the blocks on the ground.

To elaborate, you might've noticed different screen heights while watching videos on youtube, as he did.

I'm not sure how I forgot to post again in the first topic, but I do know relating information that could explain the extra pixels phenomenon.

To start with, older "SDTV" signals are actually much taller than standard CRT screens display. They are interlaced, which means the tv draws every-other line in one frame, and then the lines in between those lines the next. This gives it a larger appearance than what the tv is doing; drawing a signal that is actually half the height of what is seen, it spaces the lines out to appear taller than the signal is, and the image changes fast enough that you (hopefully) don't notice the difference.

Now, the number of scanlines that make up the two frames is actually taller than what your TV screen displays. The actual number is 525 lines, but older tvs could handle a maximum signal of 486 lines. From what I know, the tv draws 243 lines, and then(in the time that it would take to draw the remaining amount from half of the 525 line signal) adjusts itself to draw the next 243 lines. The rest of the lines of the video signal should be dumped, while the tv lines up to draw the next frame.

You might be thinking why that doesn't explain why there's that much more image data that you're not seeing. Well, not only does the display not render the entire signal, TV's didn't always actually show all 486 lines that they had time to draw. The image actually runs off the screen. Emulators that are designed to simulate an NTSC environment have likely been designed to run the NTSC signal as you're used to seeing it - smaller than it is.

So, that wraps it up, right? Wrong. NTSC is (or was, but that's another matter) the standard format for North American television broadcasts. Other areas use tvs that function in PAL format.

The difference? Well, the difference comes from the frequency of the power used in our areas. The US standard was 120v Alternating Current, which means it's distributed in a 60hz frequency. Now, you don't have to understand that in itself, because what it means to you, the gamer, is that the image can easily be displayed at a smooth 30 frames per second. The PAL standard, though, was setup to run at 25 frames per second.

That all makes PAL sound crappier, right? It's true that PAL TVs are slower to changeout their pictures, but they actually use the extra time it would take to do 30fps to draw more pixels. A PAL signal has many more pixels than a NTSC signal; PAL is transmitted with 625 pixel lines, 575 of which are displayed on-screen. That means the standard PAL signal was higher-resolution, clearer looking than NTSC.

That's not the whole puzzle unraveled, though, because games like SMB that were made in NTSC format don't have more image data in their PAL version. The PAL versions actually display on PAL screens with black bars at the top and bottom, to count for all of the extra lines that the NTSC system couldn't display. PAL screens are about as tall as NTSC screens, though, so this means the game picture looks squashed. Well, PAL is still showing those 486 lines or more that the game system was capable of rendering. Some of those lines didn't show up on NTSC at all.

NES emulators, of course, aren't restricted to the same standards as old PAL tvs, and don't need the black bars at the top or bottom. So when someone plays an NES game in PAL mode, it will be taller, as in the picture above. The game data isn't different at all, shouldn't be different, but the emulator is also emulating the amount of pixels the screen was capable of producing.

So, there you have it, as far as I've figured out.





As for how the difference in colors between NTSC and PAL is caused, that's something less mysterious and altogether more technical. Rest assured, the two formats use rather different color signals which are accomplished in different fashions. PAL does have better color than NTSC, but in this day and age there's really little difference between the images displayed, since we've all upgraded past the days of the NES in somewhat unnoticable ways, even if we haven't all gone all the way to HD.

[Valigarmander]

...Neat.

[Cosmonautical]

Yeah, I don't expect much of a response to this, but I thought it might interest a few members in the very least.