Subject: [stella] NTSC - PAL porting guide From: Eckhard_Stolberg@xxxxxxxxxxxxxxxxxxxxx (Eckhard Stolberg) Date: Sat, 8 Mar 1997 19:51:43 +0100 |
Hello there, I would like to play all your new NTSC games also on my PAL system. Therefore I have decided to write a little conversion guide. Things to keep in mind when writing 2600 games: 1. Do a Vsync in every frame. This way NTSC games will atleast display a stable picture on PAL systems. Early games only did the Vsync once and then went on counting 262 lines for each frame. While this works on NTSC systems and saves 3 lines for game logic, it only produces rolling pictures on PAL VCSs. 2. Have the Overall Lines/Vblank lines ratio right. NTSC normally has a ratio of 262/37 while PAL has 312/45 (plus 3 lines of Vsync for both). It is possible to reduce the number of overall lines, if you don't want to use them for game logic or display and therefore you can have a smaller number of Vblank lines. But if you get the ratio just a little bit wrong, there is the risk of loosing the colour signal. Because of this, I think, it is best use the normal number of lines for both TV systems. If you use a Tim64T timer loop for Vblank and overscan, converting between both systems should be as easy as just changing the timer value. 3. When converting from PAL to NTSC, you should take into account, that NTSC systems can only display a smaller number of lines. Therefore you should limit yourself to 192 lines of displayed graphics in PAL mode too. Or you should create your graphics in a way that allows you to remove enough lines to fit on a NTSC screen. Also the number of lines that can be used for gamelogic is smaller on NTSC systems. That is why you should do cycle counting for the worst case, if you are using any timeconsuming calculation loops. 4. NTSC systems operate at 60hz while PAL systems use 50hz. Because of this you have to redo all frame based timing. For example: if you are moving your spaceship in NTSC 5 pixel in every frame you need to move it 6 pixel per frame on a PAL machine. Or if you are doing anything every 6th frame on NTSC, you have to do it on every 5th frame on PAL to get the same speed. 5. This is not in the programmer's guide. PAL systems use a different form of colour encoding than NTSC ones do. The TIA chip seems to be the same on both systems, which leads to two totally different palettes. Based on Ed's SoundX source code for NTSC and on my own observation for PAL they are: NTSC PAL grey grey gold grey orange gold red orange yellow green pink orange brown purple green blue purple pink - red blue blue green blue2 purple light blue turquoise turquoise lilac green blue light blue green purple blue yellow green blue orange green grey light orange grey I am not very good at discribing colours with words, so it would be nice, if someone else with a PAL system can verify or correct this. I hope to see a PAL Okie Dokie or a PAL EdTris soon. :-) Thanks, Eckhard Stolberg -- To unsubscribe, send the word UNSUBSCRIBE in the body of a message to stella-request@xxxxxxxxxxx
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