Apple II Colour Demodulator

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Third attempt. This encodes directly to PAL and uses only TTL IC's. The same technique is used to extract the 4-bit colour stream from the display output, but those colours are now ANDed to a colour carrier to produce the correct PAL signal. There's a delay made from three inverter IC's to produce the colour burst, which is switched into the circuit with a diode switch. I included a chroma trap for the text signal to reduce colour bleeding on 80 column mode to make it more legible on low resolution PAL sets. The values of the inductor and capacitor could probably be optimised. The resistor output network is calculated to be as close to 75 ohms as I could manage.

The schematic is available here

A second attempt at the same idea has yielded much better results. This method uses a digital decoding technique to give a four bit number for every colour pixel on the screen. This is then put through a resistor matrix to obtain a colour palette that looks pretty much identical to the original NTSC colours. As a bonus, because the colours are decoded four bits at a time, there aren't any NTSC artifacts on the screen. Each pixel is a solid colour. The output is a standard component television, as was the original.

Schematic of this version is here

This project came about after not being able to find a PAL modulator for my Apple //c and myself bragging to friends about how easy it should be to make a simple NTSC to component demodulator. So I made it. It took a day.

The current implementation uses the original NTSC output of the Apple //c as the luminance signal on a component television and the circuit supplies the red and blue difference signals.

This is the first implementation of the circuit on bread board to make sure the idea would work. Only the red difference is wired.

Final implementation.

Here is the schematic

Email me if you have any questions or suggestions!