Posted By
Gaia
on 2012-08-15
17:42:38
|  Re: Lord Of The Rings Picture Show
Csabo: the difference between the 312 and the 287.5 PAL lines is 24.5 (about 25) and that is what is called the vertical blanking period. Here the TED generates the appropriate video signals so that the TV's electron beam "finds its way back to the top of the tube"  (ugh...  )
Odd and even frames are different so that you normally get an interlaced 25 fps video frame rate. In this respect the TED's video signal is not really standard since it only emits even frames (if memory serves me well) with consequently no interlace and an ~50 Hz frame rate. At the end of each frame it has to fool the TV to think that it has just finished displaying an odd frame, so that the TV would expect "another" even frame. On proper TV's you can even see the border overlay cut into half in the middle of the screen. The "real interlace" routines actually delay the start of the vertical blanking by half a line (by writing $FF1E) and then make sure the total still adds up to 312 (by writing to $FF1D and - AFAIR - also $FF1E again), making the video signal a "true" PAL one and thus emitting a truly interlaced picture.
See:
http://www.oocities.org/yehcheang/Composite_vertical_blanking.htm
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