2014
TTL PAL Sync Generator
I was in need of making a new sync generator for my tennis console as the original one was quite terrible, and my plugin module was just a prototype. I built the first module in order to achieve stable line and frame pulses, but the one-shots making up the sync pulse lengths, still on the main board, also needed some attention. So I soldered together this new module that plugs right into the two old 74121 sockets.
It consists of an oscillator running at 16MHz which gets divided down to 15625Hz(line frequency) and 48,82Hz(frame frequency) by the 74HC4020 and 74LS390. The two original 74121's were replaced by a single 74123 containing two one-shots. The PAL video standard says that 4,7µS is the pulse length of the line frequency, but I found that modern TVs are very forgiving about this. My TV for example, accepts all from 3.5µS to 10µS. I chose 4,2µS (after warmup it reads about 4.6µS). The frame pulse length I'm not too sure about, I've heard that it lies at about 1mS, so I chose a pulse length slightly below that (630µS).
As usual, schematic below.
Main Schematic |