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0 pointsthought_alarm3y ago0 comments

> Might not actually be RGB, did not look at the schematic. But having RGB before modulating into NTSC's color difference signals is not uncommon

The standard NTSC Apple II does not generate color this way, but the European PAL models do.

The native Apple II video signal is 560x192 1-bit monochrome pixels. The PAL video circuit converts video signal into a digital stream of color pixels, 4 bits per pixel. It then converts the digital pixels into analog YPbPr signals by attaching various resistors to each of the 4 bits. The YPbPr signals are then fed into a PAL encoder chip.

Presumably, the original authors used these YPbPr resistor values to determine the colors sent to their Bulgarian PAL/SECAM encoder.

This is a schematic of the PAL color encoder for the Apple IIe: https://imgur.com/CQH7vNS

1. The native monochrome video signal is SEROUT in the top-left of the schematic. It is fed into the LS164 shift register.

2. The LS175 buffer captures each 4-bit color pixel from the shift register.

3. The output of the LS175 is then fed through a various resistors to generate the YPbPr signal which are fed into the TCA650 PAL encoder.

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anyfoo3y ago· 1 in thread

Thanks, that's great context. Do I read it right that the NTSC Apple II generated "artifact colors" (heard that before, but did not know if it applies to the Apple II) through effectively monochrome pixels that just work out to the desired color in NTSC, but since that would not work for PAL (because of the phase flip each line, and because the colors would be different), PAL Apple IIs have some extra circuit to convert artifact color pixels into YPbPr and then modulate that using the TCA650?

thought_alarmOP3y ago

Yes, that's right. This page has more info: https://nerdlypleasures.blogspot.com/2021/10/apple-ii-compos...

The PAL color circuit was initially an expansion card for the Apple II+ in 1979. The same circuit was built-in to the later Apple IIe and IIc models.

A novel part of the design is that the TCA650 is actually a PAL decoder chip that is meant to split a composite video signal into YPbPr components. The Apple II drives the TCA650 in reverse to generate the composite signal.

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