405 lines from Raspberry Pi 5

[ppppenguin]

Some background to providing a clock for the modulators. These chips were originally designed to use a crystal and not need an external clock. I don't recall what David Robinson did but Darryl used an external clock. It saves two crystals.

ISTR the problem with the external clock turned up in an Aurora that I was sent because it was misbehaving. At the time I was the semi-official support service for Auroras in the UK and Europe. The problem turned out to be that the chip was fussy about its drive voltage. It didn't like being overdriven. So I increased the series resistor to something about halfway between overdrive troubles and failing due to lack of drive. That mod became standard in all future production. Frank will have to say why he used a small capacitor rather than a resistor but it strikes me as a sensible alternative.

This may be a good time to remind new users of Aurora, Hedghog and now this hat that the output voltage is way too high for many TVs. If driving just one set, without using splitters, it's important to use an attenuator. Anywhere from 10dB to 20dB should be OK. Having the full output available can be useful when dealing with a faulty TV.