ResidentDante
Member
Interesting! Thanks for the tip!I explained it incorrectly. The 48 missing lines aren't drawn on your bezel outside of view, they are not sent by the console because they are outside of the timing.
It's critical to understand how a CRT works. The electron beam scans line after line from top to bottom at a constant pace. The Sync signal tells it when it's time to go one line down or go back up. The 240p of vertical resolution is a consequence of the refresh rate: because the horizontal speed is fixed, there is only so many lines you can draw before having to go the next frame. If you want to draw more lines, you have to slow down the refresh rate. PAL has more time to draw the picture (1/50=20ms), therefore the beam can draw more lines. It doesn't matter if the code was made to compute 288p worth of data, at NTSC rates when the beam arrives at the 240th line it's time to go back up and draw a new picture. Consoles are designed to force the computation of a new frame when it's time to draw it. If they didn't, you'd have the last 48 lines on top of the following frame and a resulting rolling picture. It's all about what you can put in between the SYNC ticks.
Mastering SYNC is critical. If you try to fit to large a picture, you'll get a screen rolling. If you try to feed a different Sync to a display that asks for a fixed Sync, you'll get a rolling picture. NTSC TVs are locked to 60Hz and will accept nothing else. Using a PAL console on it makes the picture roll. A PAL TV is more lenient and will accept pretty much anything between 50Hz and 60Hz. But the picture drawn on screen still has to work within the Sync boundaries defined by the console.
tl;dr you can't display 288p at 60Hz.
For more information on how consoles compute and display, and how devs in the early days had to fight against the Sync to get their game on a TV, I recommend the book Racing the Beam : The Atari Video Computer System which explains in lots details how videogames are made on a machine that doesn't abstract the rendering away from the devs.
How does PAL-60 work though? Or was that just introduced at later stage so it was basically a 480p image at 60fps? I would've thought a PAL-60 compatible CRT would have managed 288p@60fps.