>"Paul Christenson [N3EOP]" <PJC130@PSUVM.PSU.EDU> writes:

> Is the PIC fast enough to be useable as a character generator to overlay
> text/graphics on NTSC video?  About 10 chars/line is all I need; nothing
> fancy.  (I want to overlay date/time on a camera feed.)

Fast enough to generate characters?  Yes.

Overlay?  Yes, but with external hardware.  The PIC clock will need to
be synchronized to (a multiple of) the horizontal sync, or you will get
unacceptable jitter in the horizontal positioning of the characters.  A
PLL circuit using a 74HC4046 would probably do the job.  You'll need a
sync separator to extract the horizontal sync signal from the incoming
video; the canonical chip for this purpose is a National LM1881, but save
yourself some trouble and use the Elantec EL4581 instead.  The Elantec
part is a much-improved replacement.

I generate characters on video (but not overlay) in my PIC-Pong (tm) and
PIC-Tock (tm) projects.  Information about these, and source code for the
latter, are available from my PIC page on the web:

        http://www.spies.com/~eric/pic/

I get about ten characters per line running the PIC at 18.432 MHz.  The
technique used requires all of port B to be dedicated to the video.

You'll also need some code for the PIC to handle vertical synchronization.
My PIC-based closed-caption decoder has code which could be adapted for
that purpose.  Documentation and source code are available.

I've started thinking about a closed-caption encoder.  The latter will use
 basically the same circuitry as a text overlay would require.

My closed-caption decoder originally used a PIC16C61 (or a '71 if you add
code to disable the ADC) and an external dual comparator.  Over the recent
holiday Rich and I got it working using the internal comparators of a
PIC16C622.

Cheers,
Eric