The following rant is a bit off topic, but illustrates why I hate YUV in retro home computers:-
The original ZX Spectrum (not +2 etc.) only produced YUV internally, which was fed into an LM1889 video modulator IC for conversion to composite. The color palette was the same as digital RGBI - 8 saturated colors with darker versions via the 'intensity' bit. Timex Corporation produced their own enhanced version of the ZX Spectrum for the US market, called the Timex Sinclair 2068. This also used the LM1889, but the color palette was different. One annoying difference is that 'bright' black became dark gray. This is a problem for some ZX Spectrum games that expect black to remain black when the color is 'bright'.
I bought a TS2068 off eBay a couple of years ago. As very few titles were produced for it, I wanted a way get the correct colors for ZX Spectrum games. Now you may be thinking "the TS2068 has digital RGB outputs, so just hook them up to an RGB monitor and you're good to go!". But in their 'wisdom' the TS2068's designers decided
not to include the intensity signal.
Another problem with my TS2068 (which probably affects all of them) is that the composite signal was very noisy. But why? One reason was the switch-mode DC/DC converter which was putting ripple on the 5V supply. I replaced it with a modern converter module and the varying diagonal lines disappeared, but it still had strong vertical bars that appeared to be synchronized with bus operations.
This is when I realized the purpose of the peculiar video circuit that had 'ground' from another part of the motherboard coupled into it as a signal. They were trying to null out voltage variations induced into the Y signal inside the ULA. I ripped out this circuit and soldered a heavy copper wire between the ULA ground and video ground, which cut the interference in half. The picture now looked a lot cleaner, but still wasn't good enough for what I wanted to do.
My idea was to extract the intensity bit from the YUV signal, but this isn't easy because each color has its own intensity so Y isn't just 3 levels (black/gray/white) but has 16 different levels. To determine whether a color was 'bright' and 'dark' my circuit would have to distinguish between these 16 Y levels, but this turned out to be impossible because the interference exceeded the difference between brightness levels.
I cursed the Timex engineers for trying to fix the video interference problem by bodging the circuit rather than fixing the cause, but more for neglecting to bring out the intensity signal. If only they had brought
all the digital RGB signals it would not have been a problem. But no, they decided to generate the analog Y signal
inside the ULA, with predictable results - and no practical way to clean it up.
Custom ULAs were expensive to make, especially when they included analog circuitry. Imagine what those engineers thought when they tested the first production run and discovered they had massive video interference. "We can't afford to chuck all these ULAs", the boss probably said, "just bodge the boards and hope nobody notices!"
