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  dhs.nu special feature #6
Published June 2012

ST/STe overscan dimensions
Evil / DHS, ae(at)dhs.nu
 
Method 1 (classic ST)
  • 230 bytes linewidth, using an extra 71/50 switch at the end of the scanline to stabilize the overscan
  • Each 16px segment uses 8 bytes (4bits/pixel) and 230/8 = 28,75 16px spans. So obviously 28*16 (448) px width!
  • Problem is, the shifter doesn't output all the data, even if the linewidth is greater
  • With Atari monitors or other limited monitors, this was impossible to see, so some guys assumed you had 448 px visible graphics as that's what the linewidth suggested
  • In real life the shifter outputs 416 px, and the stabilizer code at the end of the scanline sabotages about 6-8 px, so about 408-410px USEABLE resolution
Example picture 1, full 512 cycle scanline and 313 lines.

The black area are cycles not used for graphics output by the shifter (on STe, this area is used for DMA sound for example) and the white area is the visible screen.
The red line at the top of the white area is called "syncline", the time it takes to synchronize with the electronic beam, this uses up one scanline out of the 274 visible ones. So this line can't be used for a fullscreen picture.
The red lines in the right border symbolizes the stabilizer code, trashing about 6-8 px.

Conclusion: 408-410 pixels per scanline, 273 scanlines with overscan.
Safe useable resolution: 408x273

Note: In the later years a new linewidth has been found, so it's possible to show 274 overscanned lines. But this was unknown back in the 80's/90's.
Method 2 (STe only)
  • Very similar to the classic ST overscan, but doesn't do the right border stabilization, which saves some CPU
  • Graphics output is somewhat different than the classic ST overscan
  • Linewidth is 224 bytes, which is even 28 16px spans, which once again can make one belive there are 448 good pixels
  • But as for the classic ST overscan, the shifter doesn't output the whole linewidth
Example pixture 2, full 512 cycle scanline and 313 lines.

As one can see, the bad pixels in the right border are eliminated as there is no stabilization code.
However this method shifts the entire screen 8 pixels to the right, making the 8 leftmost pixels of the screen unusuable.
The syncline is the same as for ST.

Conclusion: 408 pixels per scanline, 273 scanlines with overscan.
Safe useable resolution: 408x273


Appendix, full PAL dumps from famous overscan screens


The famous Level 16 Union demo fullscreen. These guys obvoisly noticed that the screen ended after 416 displayed pixels and that there were some trashed pixels in the right border, limiting the graphics in the right border.


The parallax distorter that had most people scratching their heads for a good while. Here the trashed pixels in the right border is easy to spot. This would also happen for a still image. That's why the last 6-8px are unusable.


And last, a dump from Blood by Holocaust. Again, the last pixels in the right border are left unused, simply because it would be trashed, like in the DSOTS screen above.
 
Anders Eriksson - June 2012
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