Stella vs. A Real TV

[Lumi]

Hi! I'm just getting into Atari 2600 programming, and I have a question about how certain behavior might differ between a real TV and Stella. I don't have a Harmony cartridge, so I can't test it for myself.

 

Basically, I'm putting together a title screen, and after each major update I recompile the code to see whether or not it worked, working my way down the display. What I noticed is that in Stella, I don't have to draw all 262 scanlines of the frame - I can draw, say, the top half of the title screen, then go into vertical sync and Stella will immediately start the next frame, leaving the missing scanlines black.

 

It's very useful for development, but I haven't seen any example code that does this, so I have to ask: what does a real TV do if you go into vertical sync before the frame is "complete"? Would it cause unwanted side effects, and if so, is there any way I can make Stella reflect this behavior so I'll know if I'm drawing the wrong number of scanlines?

 

Thanks!

Edited November 26, 2015 by Lumi

[DarkCart]

I'd assume it look something like this

 

[Omegamatrix]

The most common effect is the screen will roll.

[+SpiceWare]

A number of TVs are forgiving about an incorrect number of scanlines - you can see that by the various scanline counts used in games back in the day.

 

They're not as forgiving about variable scanline counts though - such as 262 this frame, 261 next frame. When that happens the screen will jitter up & down. I recently implemented that for Stella and stephena committed it for version 4.6.5. In 4.6.6 keyboard shortcuts (CMD-J on Mac, ALT-J on Linux & Windows) were added to enable/disable the jitter routines.

 

Modern TVs aren't as forgiving about the scanline count though, so do target 262 scanlines. The jitter emulation will catch it if you occasionally go over/under.

[Lumi]

Thank you, that's exactly what I was looking for!

 

I also just realized I could use CMD/ALT+L to display the number of scanlines. That's a useful feature, too.