Recently we featured an unusual Commodore 8-bit computer on the bench of [Tynemouth Software] — a Commodore 64 in a PET case. One of the unique parts it had was a board which took the composite output from the mainboard and split out the sync pulses for the monitor, and now they’re back to give it a full reverse engineer.
Perhaps the first surprise is why this board is necessary at all, after all one might expect an 8-bit machine to have those signals already at hand. It seems that the VIC chip inside the 64 did the combination to composite internally, so no such luck for the Commodore engineers. The board they designed then is a complete and very well-engineered sync splitter.
The technology of a video signal has its origins in the 1930s, so it’s not hard to extract both vertical and horizontal sync pulses with little more than a few passive components and a couple of transistors. The trouble with such a simple approach is that the output will work, but it will be messy and crucially, not have quite the required timing. The Commodore board uses the same approach as a simple discrete circuit of having a pair of filters with a time constant selected to catch the relevant sync, but extends it with extra logic. There are one-shots designed to provide clean pulses of exactly the right length, and gates that provide blanking to remove the chance of pulses ending up where they shouldn’t. The video path is the only part which might differ from a conventional sync splitter, because as the output from the 64 is all-digital, it takes a TTL-level through a gate rather than a more conventional analogue path.
You can see the rest of the machine in our original write-up, and we’re reminded that the boards haven’t been cleaned at their owner’s request, to preserve their patina.
