PART 1: Video Text Overlay With a PIC Microprocessor
the TV camera and the RF modulator, while a 7805 voltage
regulator supplies five volts to the integrated circuits.
Jaggies and Jitter
The initial design used a 20 MHz crystal oscillator to
drive the microprocessor. One of the problems I ran into
was what some people call the jaggies, shown in Figure 6.
This was a horizontal back and forth motion of the characters as if they were doing the hula. With cheap black and
white TVs and cameras, there is a certain amount of noise
in the circuit, but — even with better grade equipment — the
hula was still there. I believe this was due to the interrupt
latency of the Microchip microprocessors.
The Microchip specification states that, “For external
interrupt events, such as the INT (RB0) pin, the interrupt
latency will be three or four instruction cycles. The exact
latency depends on when the interrupt event occurs relative to the clock cycle.” This means that there may be a 0.2
microsecond difference in when the data on the screen
starts for different scan lines, based on whether the latency for that scan line was three or four instruction cycles.
The 0.2 microsecond is the time it takes the microprocessor to output one dot on the screen, which is about the
amount of hula action shown in Figure 6.
Figure 4. Schematic diagram for PICVIDEO.