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Old 07 May 2013, 12:20   #2177
Join Date: Jun 2010
Location: out in the wild
Posts: 1,133
Originally Posted by NovaCoder View Post
Is vsync the same as a 50khz mode (needed for smooth scrolling) or are they unrelated as far as the config tool is concerned?
Make th at "50Hz" ;-)

They're "almost related" - you can do VSync in different ways. The simple method is to run completely syncronous like the old-fashioned 20-year old flickerfixers did, because they had to save memory size and bandwidth.

However, this requires a 50Hz-compatible monitor, which is so hard for most people to find that they chose to buy Indivision AGA MK2. If I'd add a V-Sync mode now that sets you back to requiring a 50Hz monitor, you would not get what you expected.

You can also achieve V-Sync with other input<->output relations. The picture doesn't even tear too badly if you use 62.5Hz like the previous version did. However, just setting the frequency right doesn't cut it; you still need to make sure that input and output is truly happening "in sync", which requires the output stage to wait for the input stage, and that is the part where compatibility with many monitors will break: The last line before the vertical blank will either be extended or truncated, or the number of lines may even change from frame to frame. Many monitors don't like that. We have the feature working in Chameleon, and it's just great on those monitors that accept it (mostly CRTs). The feeling is like "yeah, that's the way C64 games are supposed to look - dump the emulator!".

Indivision AGA MK2 is slightly different in it's approach: There is no crystal, but only PLLs, so I can fully syncronize the output stage. Maybe a 75Hz PAL-V-Synced mode is possible without breaking monitor compatibility: I could V-Sync on every other frame and then phase-shift the output stage from the input stage, so the tearing is outside the center of the screen - and if I continue writing here, I may collect even more ideas that I don't have time for at the moment ;-)

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