PARTS LIST AND CIRCUIT DESCRIPTION

Video Clock Superimposer
David B. Thomas
dt@dt.prohosting.com
http://dt.prohosting.com/pic/vidclock.html

C1	.1 uF
C2	470 pF
C3-4	.1 uF
C5	.01 uF (shown as 10nF)
C6	.1 uF
C7	47 pF
C8-13	.1 uF
Q1	2N4124, 2N2222, 2N3904 or equiv. NPN
R1	680 ohms
R2	681K
R3	4.7K
R4-7	10K
R8	1K
R9	2.2K
R10	75 ohms
R11-12	10K
S1-2	normally open momentary pushbutton
U1	LM1881 sync separator
U2	74HCT4538A one-shot
U3	74HCT4046 pll
U4	74HCT4020 divider
U5	74HCT4051 multiplexer
U6	PIC16C711 microcontroller (Microchip)

The LM1881 separates the composite sync signal from the incoming
video.  The 74HCT4538A one-shot eliminates the vertical sync and
equalizing pulses, to provide a nice clean horizontal sync.  (Yes,
Elantec makes some chips that do this in one package.)  A 16.1 MHz
signal from the VCO of a 74HCT4046 PLL is phase locked to the
horizontal sync via the 74HCT4020 configured to divide by 1024.  That
16.1 MHz signal then serves as a clock for the PIC.  The PIC in turn
operates the 74HCT4051 multiplexer at just the right moments to put
the clock pixels up in place of normal video.  Having the PIC
syncronous with the video signal allows crisp overlaying with no
jaggies.

Someday I might get some PC boards made.  The circuit can be built on
a breadboard, but it tends to be flaky, owing to the extra
capacitance.  If it almost works on the breadboard, try shortening
leads as much as possible.  It it still only almost works, try
soldering it up on an experimenter board.  It works, I promise!

The code is freely available for download.  If you prefer, I can mail
you a preprogrammed controller chip (you cover the cost of the chip
and postage).