Video Clock Superimposer

As a followup to my VCR Pong project, here is a gadget that is actually useful in the Real World! It superimposes the time of day, in “HH MM SS” format, in the bottom right-hand corner of an existing video signal. My friend Scott uses it with his home security system.

In keeping with the tradition of my previous hacks, I use few parts and lots of tricks. All timing signals, including the timebase for the time of day and the clock for the microprocessor, are derived from the incoming video signal. That means if you lose power, or lose video, it loses the time… but in this home security application, that didn’t matter. Lost power would mean the VCR stops recording and starts flashing 12:00 (1:00 during daylight savings).

[b:9d01d86fcf]How does it work[/b:9d01d86fcf]
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.

The code uses the Parallax instruction syntax, so you’ll either need to use Parallax’s SPASM.EXE (available for free on [url=]Parallax’s web site[/url]) or Tech-Tools’s CVASM16.EXE (available for evaluation on [url=]Tech-Tools’s web site[/url]). You can also download the preassembled object file below, in INXH8M format, for use with any device programmer.

Copyright 1998 [url=]David B. Thomas[/url]Parts: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)

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