PART 1: Video Text Overlay With a PIC Microprocessor
fields, which gives a nearly flicker-free
image at 59. 94 Hz (nominally 60 Hz)
refresh frequency. This is close to the 60 Hz
alternating current power used in the US.
Once we complete the adding of the text
to a picture signal, another interesting application for the circuit is subliminal messaging. A subliminal message is one that is displayed for a short time — normally one frame
or a 30th of a second, so that it does not register in the conscious mind. Let me say first
that I am a skeptic in this area, but a 1973
Federal Communications Commission
(FCC) investigation determined that an
advertiser had broadcast a subliminal message — “Get It” — in a television commercial,
so the FCC proceeded to outlaw subliminal
messaging in commercial television.
Enter the LM1881N
Figure 2. LM1881N waveforms.
The first chip we will use is from the
LM1881 family of video sync separators.
Costing about $2.50, the LM1881N is an eight-pin integrated circuit (IC) designed to strip the synchronization information from composite video sources such as NTSC or
PAL. An external resistor, R1, allows the LM1881 to be
adjusted for source signals with line scan frequencies differing from the 15.734 kHz US broadcast standard.
Four major sync signals are available from the IC
when processing the composite video signal (a), shown in
Figure 2: composite sync including both horizontal and
vertical scan timing information (b), vertical sync pulse
(c), color burst pulse (d), and odd/even output (e). The
odd/even output level identifies which video field of an
interlaced video source is present at the input. The outputs
from the LM1881 can be used to gen-lock video camera
signals with graphic or text information from a microprocessor. We will only use signals (b) and (c).
cycle time of 200 nanoseconds. The microprocessor is
about $7.00 and has eight kilobytes of reusable programming memory and 368 bytes of data memory.
The programming for the video text display does not
take much of the programming memory (772/8192) or
the data memory (125/368), so there is plenty of space
for adding additional features. Data memory is arranged
into two or four banks, depending on whether the microprocessor is a 16F873 or a 16F877. The program is written to only store data in Banks 0 and 1, leaving Banks 2
Figure 3. Video camera for PICVIDEO.
The second chip in our trio is the Maxim 233 dual-channel, RS-232 driver/receiver chip. This chip provides
the interface between the zero to five volt world of the
microprocessor and the - 12 to + 12 volt world of the RS-232
personal computer serial interface. While this chip may
seem overly expensive at about $5.00, it has the advantage of not requiring any external components (no electrolytic capacitors to buy), and it runs off of five volts only.
The third integrated circuit in the project is the
Microchip 16F873-16F877 microprocessor running at 20
MHz. At 20 MHz, the microprocessor has an instruction