Chaps,
This RGB-Y,Pb,Pf matrix is now complete, it was no trivial task either! The resultant circuit diagram is shown below. It's about as simple as it can be while still driving 75Ω cables at standard voltages and only using NPN transistors. It uses 10 transistors, all the same, here I used BF199's as had a few...in fact only nine, that's why one is different, but if I had one more I would have used another BF199.
Any transistor you may find in a video IF strip will do, so that's quite a choice, I would have bought some more BF199s but my supplier has ceased stocking them. So the next batch will be KSP10s (Fairchild).
It's pretty straightforward, the resistors at the input produce a Y signal which is buffered and inverted by the differential pair TR202/3 which have an emitter current-source from TR201. The Y signal at TR203s collector then has the composite syncs glued on and buffered to a 75Ω source via TR204/5. The RGB input source is straight out of a PIC micro, no 'HC541 is needed in this application.
The -Y signal at TR202s collector is buffered by TR206 and fed to both colour-differencing circuits. These resistively do the subtraction and are again buffered to 0.7V by a pair of transistors.
This quite a wideband design, the channels are not filtered but if filtering is required for feeding a PAL, NTSC or SECAM coder then TR204/231/241 could easily be made filters with the addition of two resistors and two capacitors each.
There is a small glitch on the Pr channel at the rising edge of the white bar, it's very narrow, around 15ns and probably due to building this sort of wideband circuit on veroboard. This would not be visible on an analogue scope but these digital ones show them up, sometimes to my annoyance!
If this were to be used to turn an RGB NBTV source into Y,Pb,Pr then standard general purpose trannies can be used, any of the BC54x series for example.
The screen-shot below shows how sharp the colour transitions are, the most telling is the green to magenta one. Some of the colours look a bit 'off', I put this down to the screen-camera combination. Looking at the screen live they look spot-on.
This is in no way a matrix for professional applications, but it's good enough for ATV, domestic and hobby use.
Steve A.
An afterthought...the total +12V current is around 80mA including the PIC and its +5V regulator.