Dev Diary: Dragon Chroma Experiment

Introduction

The Dragon's video output has always been "a bit naff", and various people have looked into ways of sorting that out. Probably the most successful is to intercept the VDG, quantise its outputs and convert to RGB.

But I wondered if we could just generate composite video in a different way. And given that both LM1899Ns and 4.433MHz crystals are getting harder to find, it could be a nice bit of future-proofing. You know, assuming we get to the other side of the Collapse with loads of composite video monitors lying around...

You can still order 17.734MHz oscillators (4 × PAL colour subcarrier), so my idea is:

• Generate quadrature square waves from an oscillator
• Low-pass filter them to get sine waves
• Use available multiplier ICs to multiply the VDG's outputs by these sine waves
• ...
• Profit!

So I had a PCB made up¹. For what it's worth, this course of action seems unlikely to lead to the final goal, but it's a laugh, isn't it?

It's designed to plug into the socket of the LM1889N, which provides access to ±5V and the VDG's chroma signals (after they've been processed somewhat for PAL). This socket doesn't have a convenient ground pin so I've made a separate header to let me tap that off the motherboard.

Bottom-right is the oscillator. Between the rows of the board connector sit 2 × 2 D-type flip flop ICs. Only three of the four total are used to generate the quadrature square waves. Two trim pots allow adjusting the attenuation of those square waves.

Next, those waves enter the big array of discretes on the left side of the board. In the middle of those is a single IC containing four pretty decent op-amps. This lot applies a 3-stage low-pass filter to each square wave, leaving us with nice looking sine waves. I used Analog Devices' Analog Filter Wizard to design this.

Finally, to the right of the op-amp IC are two AD835 multiplier ICs. They're quite expensive! This is not a cost-effective solution to the problem! However, they're very simple to use, not requiring a scaling of the input signal and probably reamplification afterwards.

Experiment 1

So here's the board fitted to a Dragon 64 motherboard. Well, my reproduction of a Dragon 64 motherboard, anyway.

You can see I've tapped ground using a little test lead with a clip. It's not the best ground in the world, but it suffices as a tribute. The R pin is similarly connected to the CPU's RESET line. This is to try and avoid any situation where the flip-flops are in an inconsistent state. Y is jumpered to the second ground pin, provided on the header for this purpose.

This experiment replaces the function of the LM1889N, sending its output to the right pin to be mixed with luminance on the motherboard in the same way as the original would have done.

And here's the result. Shockingly good! Ok, that isn't the first result I got. To begin with I connected the board's Y input to ground using another fly lead, and the result there was a lot of noise. But once I jumpered it so it was definitely at the board's local idea of ground and not acting as an aerial, this was the result.

Amusingly (to me) it also eliminates a problem I was having with this combination of motherboard and VDG, where my TV would constantly be confused about whether black & white modes really were black & white, or whether it should be trying to colour them in.

Here's it showing one of Pere Serrat's test graphics. This image exercises the alternate line colour mixing of PAL to produce more colours than the base palette technically supports. It's not really proving anything about the board - the PAL adaptation is done on the motherboard before the signals even get to us - but I thought it would be nice to see.

Experiment 2

Is something I haven't got around to yet. It involves the Y pin mentioned above, and would be interesting if it produced good results...