🚀 go-pugleaf

Set up a counter which counts the horizontal sync pulses; zero it every
vertical sync pulse.  (If you need to separate these, consult any good
book on basic video circuits, or get the National databook - not sure of
the name - which has their video chips.)  You'll need to have at least a
9-bit counter, as there are 262.5 lines in a single NTSC field.

Now, all you need is a circuit which "watches" the output of this counter,
and holds the video (luminance signal) at the black (or blank, your pick)
level for certain intervals; for example, to "letterbox" the video, you
might want to hold the luminance at black between, say, lines 0 and 50
and lines 212-262.  You could do this with sets of 8-bit comparators (such
as the 74LS688) setting and clearing latches which controlled the "black
level" circuit.  Programming which lines get blanked could be done via
DIP switches connected to one set of the compartors' inputs.


This is a bit sketchy, but I think you should get the idea.  Holding the
output at black or blank can be done by clamping the sync tips to a known
level, then clipping the video at the desired limit.


Bob Myers  KC0EW   Hewlett-Packard Co.      |Opinions expressed here are not
                   User Interface Tech. Div.|those of my employer or any other
myers@fc.hp.com    Fort Collins, Colorado   |sentient life-form on this planet.