[Coco] 384 or 450 scanlines ?

[John Kowalski]

There seems to be a lot of confusion about the way video is formatted
on the CoCo.

The video output of the CoCo and pretty much all  pre-1990s home
computers and game consoles is progressive scan with 262 lines at 60
FRAMES per second (for NTSC) with 192 to 225 of the lines displaying
the picture and the rest of the lines being border and vertical
blanking.

That means, 60 full frames per second.  Each frame contains the entire
image and the images on-screen can be updated/animated at the full 60
fps.

---

Television and computers with interlaced video output 525 lines, 30
times a second.  This breaks down into two FIELDS of 262.5 lines each.
 These are output at 60 fields per second, but it takes two fields to
make a complete 525 line frame, hence it's 30 frames per second, while
also being 60 fields per second.

A quirk: this means, it *is* possible to display motion at 60 fps even
in interlaced video... just that each unique moving image only
individually contains half of the full frame's resolution and only
objects on the screen that are not moving are displayed at full
resolution.

---

The difference between the two...  Non-interlaced / progressive video
generates 262 lines per frame/field.   Interlaced video generates
262.5 lines per field.   It's this half-step - alternating the
vertical sync timing by half a line every field that causes the
displayed video to wobble up and down by half a line each alternating
field.

You can make the CoCo's video output alternate between two different
frames, but it still won't cause the monitor to 'wobble' / interlace
the two images.

You can trick the CoCo to generate alternating frames of 262 and 263
lines, and this may cause some (but not all) monitors to wobble /
interlace them, but it's still not technically standard interlaced
video.

What you'd have to do is find some way to cause the vertical sync
pulse to delay by half a scan line every 2nd frame, which would turn
those 262 and 263 line frames into 262.5 and 262.5 line fields.  If we
could do that, then we'd have true interlaced video.

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