[Coco] 384 or 450 scanlines ?

[John Kowalski]

Mathieu Bouchard wrote:

>Huh, what would cause it to be 261.5 + 262.5 ?

Well, it would have to be the (theoretical) circuit that modifies the
Vertical Sync pulses that changes the video format from two
(progressive) frames of 262 lines each into two (interlaced) fields of
261.5 and 262.5 lines each.

Normally, the CoCo will generate the vertical sync at the start of the
last line of every 262 line frame.

In order to trigger interlacing on the monitor, you'd need to generate
(on every 2nd field) a vertical sync pulse that starts halfway through
the last line of the screen instead of at the beginning of the last
line.

Since the modified pulse happens at a half-way point across a scan
line, you will effectively be displaying 262.5 lines instead of 262
(because you just stole half a line from the next field).  And the
next field will then effectively have 261.5 lines.  It's this shifting
of the vertical sync... at the start of the last line on even fields,
and *halfway through* the last line on odd fields... that makes a
TV/Monitor interlace the screen (because their scan timing shifts by
half a line.)  The display scanning will hop up and down by half a
line each field, which will fill the otherwise empty space between
individual scan lines with picture information..  giving you a total
524 (interlaced) lines instead of 262 (progressive) lines.  (524
because the CoCo isn't quite NTSC spec.   NTSC would run two fields of
262.5 lines each rather than 262.5 + 261.5)

-- 
John Kowalski / Sock Master
http://users.aei.ca/twilight/sock