Re: [stella] Interlace

Subject: Re: [stella] Interlace
From: Erik Mooney <emooney@xxxxxxxxxxxxxxxx>
Date: Sat, 23 Sep 2000 17:51:42 -0400
>Not specifically.  I did however run out of ideas.  What I did was on odd
>frames, I would wait until 1/2 a line later after the end of vertiacl
>blank.  Then I would hit rsync.  I tried moving this around a little. 
>Now that I look back at the code, an uncertainty that i had was that the
>rsync actually gets transformed into a signal that gets the tv to move to
>the next line.  By doing this I had hoped to allow the beam to have gotten
>half way down, then reseted, so that it would be one half of the lines
>down to be the interlaced line. 

Based on what I read in Video Demystified (excellently informative book,
thanks to whoever it was that recommended it here), isn't the trick to get
interlace to begin the second field (60 Hz) of each frame (30 Hz) in the
_middle_ of a scanline?  This means that all the scanlines of field 2 will
be offset vertically by half a scanline's worth of time from the scanlines
of field 1, generating the interlace.

In 2600 terms, you would enable VSYNC in the center (cycle 38) of scanline
#263 (counting from 1) after you enabled it previously.  Then you'd turn
off VSYNC in the middle of scanline #266, and turn it on again at the
beginning of scanline 526 (scanline 1 of the next 30Hz frame.)  RSYNC
isn't involved - there's no interruption or resetting to the horizontal
timing to get a vertical offset for interlace.

Anybody tried this?  My 2600 is far from set up at the moment.  And this
is assuming NTSC, although I'm fairly sure the principle is the same for
PAL except for the scanline numbers.

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