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Old 20 October 2019, 12:29   #15
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Join Date: Jun 2010
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Originally Posted by Toni Wilen View Post
Yes, this topic is about almost undocumented way to change refresh rates that does not require ECS.
Changing vertical refresh rate is easy - CPU can nicely write to VHPOSW/VPOSW extending frame lenght (on real TV i've get progressive 625 lines at 25Hz - CRT flickered obviously but TV electronics worked correctly), i even extended frame beyond 625 lines on real CRT - flicker was worse but still OK, then i've made even more than this with my ancient analog oscilloscope going over 800 lines still stable refresh. Based on this i assume that OCS timing is very flexible and easily over 1000 lines should be doable at a cost of the reduced framerate - reduced framerate is not a problem from LCD perspective but it can be issue from human interface perspective (lower refresh rate will affect perceived latency and of course significantly reduced smoothness for or moving objects, mostly mouse pointer but not only). Limit is i think only counter length and chipset capabilities (OCS if i recall correctly has limit to 1024 lines and ECS/AGA extended this to 2048 lines) - but perhaps it can dealt in same way
Issue is horizontal timing as it is most demanding from time domain characteristic (lot of Amiga things seem to be hardwired to HSync).
As i wrote before - whenever vertical was easy to modify then horizontal was almost immediately penalty - or system hang, or loosing display, at best horizontal line distortions (it looks like only few lines in middle of screen disturbed for a while).

ECS/AGA is easy peasy when compared to OCS.

Originally Posted by Toni Wilen View Post
I meant "normal" as in the default (actually hardware default is short fields but all KS ROMs set it to long at boot) unless it was changed.
This was my point - nothing is fixed - by altering single bit short or long field can be created as such Amiga display can be easily 624 or 625 or 626 lines and this is even more tricky when interlace is activated as field dominance can be easily altered during display and as such it may ruin deinterlacing as such creating real interlace animations (i.e. 2x25 not 1x25) can be very challenging in Amiga.
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