CoCo1 composite video

Summary

When I got the CoCo, one of the big problems was the super-smeary, snowy video on the RF-out. Even though composite video is generated internally by the video circuitry of the computer, Tandy didn’t end up breaking it out to an actual port. Lots of other 8-bit machines of the era are in the same boat. Luckily, adding a composite video port to the CoCo is very straightforward! So straightforward, in fact, that I did it twice. Theory As I said just now, the Motorola 6847 VDG already generates a composite video signal (by way of the MC1372 modulator transforming it from something-like-YCbCr into composite) and sends it to the RF modulator, which encodes it and the sound with the channel frequency. This is accomplished through some kind of fancy radio math that is scary, but thankfully I don’t have to understand it. This RF signal is transmitted into the antenna input of a television set, and then you can tune into channel 3 or 4 to watch your CoCo work. There’s a great explanation of how RF video encoding works in this now-classic Cathode Ray Dude YouTube video about NES RF, including some inadvertent operation of a pirate TV station. Since I already have composite coming out of the internal chips, I’ll just snipe it from where it comes from and get it to the TV directly. Easy, right? This project started in 2019, which you may note is not the current year. This is one of those projects that has taken me a really long time to get to this point. Discovery My first step was to see if someone else had made a composite mod. Composite is ‘good enough’ for most of my old computers; I have a lot of composite monitors and TVs lying around the place, and it’s probably fairly future-proof. Even new LCD TVs sold at Walmart still have an AV jack in them, and cheap AV-to-HDMI adapters will probably keep existing until I’m in the ground. There is the amazing CoCoVGA 6847 replacement, but it’s quite a bit of money to spend on a CoCo that doesn’t even have a working keyboard yet. Plus...