DLI question

[rdefabri]

I know the Atari limits you to 192 scanlines on a display - what happens if you try to exceed that with DL commands?  Does it work or do you get an error?

 

I'm watching a tutorial where the presenter replaced the 24 blank lines at the top of the screen in GR.0 with 24 lines of GR.0.  Am I losing it or does that equate to 216 scan lines?

 

I may have misunderstood, I think he said that you could use that but you run the risk of having that area "lost" if viewed on an old TV screen...which shouldn't really be an issue these days.

[+Stephen]

You can have up to 240 lines on a display, with the caveat that the last line cannot be a hi-res mode.

[rdefabri]

2 hours ago, Stephen said:

You can have up to 240 lines on a display, with the caveat that the last line cannot be a hi-res mode.

Makes sense.  I’m assuming many programs / games take advantage of this?

[Rybags]

"TV safe" area is generally under the 240 scanlines on NTSC CRT TVs and sometimes even on PAL ones.

Generally if you put non important data there or restrict to about 224 scanlines centred on NTSC you should be OK.

 

Plenty of games use over 192 scanlines.  I would say very few if any use the full 240.

[thorfdbg]

Actually, it was a bit less than 240 lines, at least for my (PAL) system. If I recall correctly, 238 was about the maximum you could do without getting image distortions because image data run into the vertical blank, then causing severe image distortions.

 

[Rybags]

Some TVs have the V-height adjustment.   Then you had those stupid "deep image" TVs that in fact just did a V-stretch which packs less scanlines into the same area.

 

The actual amount the TV shows shouldn't affect VBlank - you still have the scanning going on but it occurs offscreen.

Though I do remember a friends TV that had the shorter visible display having trouble when there was a bright background in the overscan area so maybe some TVs do have some sort of self-determined VBlank processing (and that was an early 80s job well before they had digital internals)

[thorfdbg]

Actually, I don't think that this was really a problem of the TV. I believe it is more a problem of the sync and frontporch generation. If you generate a 239th line, the front-porch (what goes upfront the sync pulse) does not have a 0-level, and then the TV is confused. The lines 238 and 239 are, in my understanding, the front-porch area that need to stay black.

[Rybags]

Those scanlines are well before VBlank though.   The 240 visible area is pretty well in the middle of 262/312 for PAL/NTSC.

 

Black and blanking level - that is another nonstandard thing of Atari and probably most computers and consoles of the time.  The standards have "blacker than black" as the blanking level but our computer uses the same luma voltage for both.