Linards Ticmanis wrote:
josephoswaldgg@hotmail.com wrote:It would be possible to re-sync, but if you were more than one cycle out of sync, it would take more than one field to re-establish precise sync.It depends on the application. If you are done doing your video-related work very early in the field, you could reacquire on the bottom of thefield.And of course if you just want to mix two modes in a nonstandard way, you can leave a few screen lines of black between your graphics and text (or between your lo-res and your hi-res, or whatever). Then you wouldn't care where exactly in that range the switch happens (as the pixels on both sides of the exact switch point will be black), so no cycle-exact lock is needed.
Thats a good way to get some timing margin! Of course, each cycle of uncertainty will require another <character time> or <7 hi-res pixel time> of margin. Time flies when generating video. ;-) The fastest possible VBL polling loop (lda vbl/bmi *-3) can only locate the VBL edge within 7 cycles, so either a 7 character margin is required on each side, or better lock must be obtained (perhaps with "vapor lock" bytes) if the screen is to be split left-to-right (in the middle of lines). If splitting the screen top-to-bottom (whole horizontal lines), then 7-cycle accuracy is sufficient, since the horizontal blanking time provides more than sufficient margin. -michael Music synthesis for 8-bit Apple II's! Home page: http://members.aol.com/MJMahon/ "The wastebasket is our most important design tool--and it is seriously underused."