Inconsistent scanline requirements

[flame]

hey everyone

 

reading various documentation and examples I am starting to see an inconsistent requirement in things such as the under scan which sometimes asks for 36 or 37, where the overscan is 29 or 30

 

so my question is, does it matter so long as the total is 262 stable ?

Edited March 15, 2020 by pvmpkin

[+Andrew Davie]

3 hours ago, pvmpkin said:

hey everyone

 

reading various documentation and examples I am starting to see an inconsistent requirement in things such as the under scan which sometimes asks for 36 or 37, where the overscan is 29 or 30

 

so my question is, does it matter so long as the total is 262 stable ?

It doesn't matter.

But also, 262 doesn't matter. NTSC can happily display 276, for example.

You do need to have an even number of scanlines for PAL.

But TVs can cope with a fair degree of variation in scanline count. 

The more scanlines, the lower your frame rate ends up. It's all co-dependent.

[NoLand]

Just make sure to have an even number of scanlines per field on PAL, otherwise color is lost. (Certainly on emulators, if PAL color loss is a real thing on actual CRTs, is a bit open to debate, compare this post.)

 

I once made a statistic of figures provided in the Atari 2600 scanline list at Digital Press, and came up with these numbers:

 

  TV    Scan Lines   Observed 
System   Standard   Min    Max   Median

 NTSC      262      238    290    262
 PAL       312      258    336    312

So, while most games historically stuck to the standards, there are significant deviations, some of them by rather popular games.

Edited March 19, 2020 by NoLand

[flame]

In my current game project, I am hitting 262, but somewhere it jumps up to 263. How "bad" is this. I see no visual difference but I haven't been able to test on actual hardware yet. I wonder what the margin of safety is, or if it matters much ?

[+SpiceWare]

Inconsistent scanline counts will cause jitter on real hardware.

[flame]

That's what I was afraid of, I have no way of testing this currently !

[JetSetIlly]

On 3/15/2020 at 11:59 AM, Andrew Davie said:

It doesn't matter.

But also, 262 doesn't matter. NTSC can happily display 276, for example.

You do need to have an even number of scanlines for PAL.

But TVs can cope with a fair degree of variation in scanline count. 

The more scanlines, the lower your frame rate ends up. It's all co-dependent.

 

Out of interest, where does the value 276 come from?

[+Andrew Davie]

12 hours ago, JetSetIlly said:

 

Out of interest, where does the value 276 come from?

It's the value used in Boulder Dash, and sufficient for my use. Most TVs will display all of a 276 scanline image.

You can vary plus or minus.  276 is not set in stone, or related to any special requirement. It's just a good size that works.

For PAL, make sure you have an even number of scanlines. I use 312, I seem to recall.

More scanlines = slower frame rate. Less scanlines = higher frame rate.