VBLANK and underscan/overscan

[Windless]

Hi,

 

  I'm trying to code a kernel with a non black barckground. If I understand correctly, stella will always display 228 lines regardless of NTSC / PAL / SECAM detection. The 36 extra line are displayed normally, i.e. if you switch VBLANK on, they will show black but if you don't you can normally use 228 lines in NTSC.

 

  Did I understand correctly ?

  Is this close to what will happen on a CRT monitor / TV or will the NTSC TV only display around 192 lines, ignoring (most of) the extra 36 lines or not showing borders if you VBLANK ?

 

Thanks,

w.

Edited September 4, 2022 by Windless
understood a few more things

[+Andrew Davie]

Not quite an answer to your question, but the typical 192 usable lines is only a recommendation, not a rule.  It is fine to use more of the VB or OS area for actual display, but the more you do the less likely all CRTs will actually have that "stolen" area visible on the screen.  I haven't taken it to extremes like you are implying. One note, though - if you reduce the total line count from 262, then you are effectively proportionally increasing the frame rate -- and here, too, you hit liitations of CRTs, which expect a ~60Hz NTSC signal of 262 lines.  So to summarise - there's give and take and you can vary those OS and VB areas' size and also the size of the "actual content" area.  You can also vary the total number of lines, to some degree, at the cost of changing the frame rate.

 

[Windless]

mmm, ok, I made a few more test and the fact that stella in NTSC mode display about 228 lines doesn't seems to be tied to the fact that PAL has 228. In PAL mode, it also has extra lines.

 

So if you are coding an intro screen with a non black background, I is the proper way to :

 - make an (NTSC) kernel with 230 colored lines and 32 VBLANKED lines (e.g. 14 UNDERSCAN and 16 OVERSCAN ?) so you don't get black borders in stella

 - make you don't display any important information in the 20 first / last lines of the kernel (because thought you colored them, they might no be visible on some TVs)

 

?

 

[Thomas Jentzsch]

Stella displays that many lines, because there are some games which even go beyond the limits of common CRTs. That leads to black borders on games which obey the common limits and switch to black outside the visible area. But you can use any color there. 

 

The problem comes with VBLANK. Which was required for many CTRs, else the screen would roll. However I do not know how many lines are required here. But IIRC, it has to be before VSYNC to have an effect.

 

BTW: If you use more than the recommended 192 lines, consider that old CRTs were not flat and had rounded corners. So avoid the edges when it comes to displaying important stuff.

[Ecernosoft]

On 9/4/2022 at 3:49 AM, Andrew Davie said:

Not quite an answer to your question, but the typical 192 usable lines is only a recommendation, not a rule.  It is fine to use more of the VB or OS area for actual display, but the more you do the less likely all CRTs will actually have that "stolen" area visible on the screen.  I haven't taken it to extremes like you are implying. One note, though - if you reduce the total line count from 262, then you are effectively proportionally increasing the frame rate -- and here, too, you hit liitations of CRTs, which expect a ~60Hz NTSC signal of 262 lines.  So to summarise - there's give and take and you can vary those OS and VB areas' size and also the size of the "actual content" area.  You can also vary the total number of lines, to some degree, at the cost of changing the frame rate.

 

Its similar to the 7800- there are "technically" 240 lines but only 192-224 are visible. Some TV's don't do the full 224 and others do (Usually on the flatscreens).

MARIA is weird... Very weird. 😃