Jeff King <jeff@MICH.COM> wrote:

> Not quite a direct PIC question, but wondered if anyone had
> interfaced any device(s) to generate a video text overlay
> over either NTSC or PAL video? I believe Philips has some
> IC2 devices to do this, but wondered if there was any "real
> world" experince on this list?
>
> Specificially, I'm just looking to display simple text (doesn't
> need to be anti-aliased) over top of video. Of course, it would
> need to be in sync with the underlying video signal.

Byron A Jeff <byron@CC.GATECH.EDU> replied:

> Go take a read around the www.circellar.com web site. There was a design
> contest winner that had a wind speed indicator with video output that used
> a single PIC. It probably could be adapted to your overlay project with
> the use of a sync detector. In a nutshell:
>
> 1) Use the sync detector to detect starts of frames and lines.
> 2) Use a analog switch that will pass either the regular video or
>    pure white.
> 3) Program the PIC to switch the switch at appropriate times.

What you will find is that there is an unacceptable amount of jitter in
the horizontal positioning of your overlaid video.  Generally the tightest
synchronization you can get to the horizontal sync will be two PIC
instructions, or perhaps one instruction using some hardware-dependent
tricks.  If the PIC is running at 20 MHz, that's 200 or 400 nS.

If you really want to use a PIC to generate a video overlay, you need to
use a PLL to generate a master clock for the PIC that is a phase-locked
multiple of the horizontal sync.

In general it is much easier to use the video overlay chips that are designed
for this purpose.  Also, they can generate better looking characters; the
PIC can't provide very good horizontal resolution.

But for specialized applications, using the PIC and PLL approach may be
useful.  That's how my closed-caption *encoder* works.

Cheers,
Eric