Video Circuit Basics - FIGnition FUZE Hardware Reference Manual

Fig 3.4

3.2.2 Video Circuit Basics

To generate video, FIGnition must be able to produce three voltages: a 0v Synchronisation
voltage, a 0.3v voltage for black and a 1.0v voltage for white. It does this using two diodes
and two resistors.
The synchronisation voltage is easy, when PD1 and PD3 output 0v, a 0v synchronisa-
tion voltage will appear on the output.
Black is next. To output black PD3 (Sync) outputs a 1 and PD1 outputs a 0. The diode on
PD3 lowers the voltage by 0.7v (taking it to 4.3v) and then the 1KΩ and 75Ω resistors act
as a potential divider (see Appendix A). The voltage on the potential divider will be:
4.3x75/(1000+75) = 0.3v.
But what about the 470Ω resistor - doesn't it affect the circuit? In this case, no. Because
the voltage at the junction of R5 and R6 is greater than the 0v voltage at PD1, the diode is
reversed biased and acts as a large resistance. This means no current flows [Fig 3.5].