Blowing overscan? Blowing scanline count? Is it bad to do so?

[Tag365]

Why is it a big deal to blow up the overscan and scanline count? Does it cause issues with rendering to the screen? Does it cause the screen to lose signal from the Atari 2600?

[Verdant]

Yes, getting the number of scanlines wrong will cause issues. The code executing on the 2600 is tied to the timing of the output video signal much more intimately than almost any other video game console. Analog TVs were designed to accept a picture stream which included a specific number of horizontal refresh signals between each pair of vertical refresh signals, and some older TVs could be quite sensitive, having problems displaying a stable image if that number was off even by one.

 

The 2600 automatically takes care of the horizontal refresh signals, but vertical refresh signals are only sent when your code tells the TIA to do so. That's why your code must keep track of how many scanlines have been sent (or how long it's been) since the last vertical refresh. Those horizontal refresh signals will keep going out whether your code wants them to or not, which is why even the overscan timing is important even though you're not trying to display anything there.

[+Gemintronic]

On 7/26/2024 at 11:30 PM, Tag365 said:

Why is it a big deal to blow up the overscan and scanline count? Does it cause issues with rendering to the screen? Does it cause the screen to lose signal from the Atari 2600?

 

It's a very big deal.  Making the TV screen roll makes ones efforts look unprofessional.  And, my games already look unprofessional!

 

There are times when a screen shake is appropriate and warranted.  Say, a devastating sequence of blows from an enemy.  If your code goes *just* slightly over and creates that effect it's a feature not a bug.

 

THAT being said, you can never count on old school TVs and analogue capture boards in modern TVs to have the same sync tolerance.  So, you're playing with fire.