Old NTSC tricks: 240p?

[Adam Sampson]

John Foust <jfoust at threedee.com> writes:

> I'm trying to understand at a low level how some early computers 
> and game consoles generated a non-standard form of NTSC.

There's a pretty good description here, with diagrams of the video
waveforms involved for both PAL and NTSC (both use the same idea):
  http://martin.hinner.info/vga/pal.html

In brief: a video signal consists of a series of fields (about 60 per
second for NTSC). Each video field starts with a series of "vertical
sync" pulses that returns the electron beam to the top of the screen
(there's also a "horizontal sync" pulse at the end of each line, which
moves the beam back to the left of the screen). There are different
vertical sync pulse sequences for odd and even fields, so the monitor
knows to offset the even field by half a line. A normal video signal
alternates between odd and even fields; in the kind of non-interlaced
signal you're talking about, every frame starts with the odd field
vertical sync, so the monitor always pulls the electron beam back to the
same place.

Cheers,

-- 
Adam Sampson <ats at offog.org>                         <http://offog.org/>