I'm not very experienced with analog design, so some of this post may contain speculation so it might be incorrect. Since other members probably know some of this stuff better than I do, feel free to correct me or clarify anything I'm unsure of.
To start, I decided to reverse-engineer the original CD4050 circuit posted in the FAQ section of AA:
It seems to work like this:
- The CD4050 buffers the LUM0-2 and SYNC signals from the TIA.
- The different-sized resistors are for a D/A resistor ladder to give the 8 different luminance levels.
- The series diode and resistor are to combine the sync signal into Luma. The diode is apparently to only affect the circuit when Sync goes low, and to protect the CD4050 from driving too much current when it goes high.
- Luma and Chroma outputs both have transistors, and these are configured as emitter followers, not amplifiers as previously thought. As I understand, emitter followers do not provide any gain, and they may even attenuate the signal. Their purpose is to give high impedance to the output signal.
- The 75 ohm resistors at the emitters of the transistors is to match the impedance of the video inputs into the television.
I decided to reverse-engineer the LHE circuit as well, and it appears that he just took other schematics and combined various parts of them. In particular:
- The Luma portion of this circuit is copied verbatim except for the resistors in the D/A.
- The Chroma portion that combines BLK and COL from the TIA is essentially the same as how the original 6-switch does it, except BLK is buffered on the LHE circuit.
- The resistors are roughly twice the value of the CD4050 circuit.
- The series 0.1uF cap and 75 ohm resistor at inputs are suggested from the FMS6400 datasheet (for AC coupling, so any DC offsets go away.)
- 75 Ohm resistors at output are as suggested from the FMS6400 datasheet.
LHE didn't follow the CD4050 verbatim. He changed the size of resistors in the D/A resistor ladder at LUM0-2, which I think aren't critical, but the resistors at LUM1 needs to be twice the value of LUM0, and LUM2 is 2x LUM1. The size seems to only affect output current, not voltage. For now, I'll keep these as they are and experiment with sizes if the output current matters.
There are some design decisions that I don't understand about the the LHE mod. The CD4050 mod has two transistors and his has one. He used the emitter-follower transistor for Luma but used the same method on the six-switch schematic of combining COL and BLK for Chroma, then he added the 75 ohm resistor on both with series capacitors. One problem here is the Luma will have correct 75 ohm impedance due to the transistor but Chroma will not (due to the 600 and 1k resistors.) It's possible that his video chip doesn't care about the input impedance and only the series caps would be needed. If this is the case, and I suspect it is, there is no need for the transistor at Luma, or the various resistors around it.
Another thing I don't understand is why he buffered BLK, but still used the same method of combining the signals as the 6-switch. One issue is that TIA outputs are open-drain but the CD4050 is not. This could mean that you can't just copy the method of combining BLK and COL from the six-switch schematics, as when BLK is high, it will tend to pull up the signal with a CD4050 but the TIA would not. So either a diode should have been added here (similar to the one on the buffered Sync signal) or BLK should not have been buffered.
Also, if everything else is buffered, you might as well buffer COL as well. I read a post where someone asked why this wasn't done, and I think LHE said it was an analog signal, but it is not - it is actually a digital signal, but delayed so it will be out of phase as the phase of the signal determines the color. I read some comments on the video mod thread about the propagation delay of the buffer being an issue, so maybe combining the BLK and COL signals first then buffering them would be the best idea. This of course would rely on fairly even propagation delay at the CD4050 which probably isn't completely accurate but probably better than not buffering at all.
I will start from the basic CD4050 mod at the link above, go over everything and see what can be removed (the simpler the circuit is without removing functionality, the better.) What I've done so far is eliminated the diode, the buffer on Sync, the 10uF and 100uF caps (only regular 0.1uF bypass caps should be needed for the chips), most of the 75 ohm resistors (only series resistors at output were used), combined COL and BLK before the buffer, then ran through a buffer gate. I've also tried to use the same parts wherever possible (such as same cap and resistor values for different places) as this makes for simpler construction. Total savings is 12 parts, I think.
This is what I have so far. Again, feel free to comment.
Also, I'm doing a parallel design with the MAX9512 chip, which appears to have similar capabilities as the FM64xx chips but is cheaper, more available and inputs can be DC coupled so (I think) you save the two series caps at input. The only issue with the MAX9512 chip is its extremely small size so most won't be able to solder it.