2600 Composite mod
Posted: Mon Mar 19, 2018 1:32 pm
This board I purchased ready made and apparently is based on the "Longhorn Engineer mod" which seems to be the best mod which people go for. Finding information is hard as there are so many clones and so many "lost" sites it was a epic to work out what this mod is actually doing.
So to start with, it solders direct to the TIA. So the motherboard CD4050 isn't used (which I later removed). The board uses its own CD4050.. which seems a little pointless... Some sites replaced it with a HC device, I would assume as its faster, it would give a sharper initial signal.. Though its not a simple swap as the pinout does not really match other chips..
Basically what we have is the 4050 driving its own resistor ladder and then it is amplified with a transistor. At this point I'm getting confused as the 4050 is outputting 5 V, the resistor ladder is actually dropping the voltage, where then the transistors used I presume amplify the voltage for the next stage. It would have seemed simpler just to adjust the resistor ladder and not have to use a transistor. In any case, I remove the transistor and it made no difference at all to the video quality or brightness. So not really sure why it is even there. I can only assume some original version of this circuit is used to drive the monitor directly from the transistor, later it was modified with a buffer IC after it. Which is fine, we basically have two buffering circuits in that case does not really make much sense.
Another thing I'm not totally sure why, is that the colour pin (TIA pin 9) isn't buffered at all. I would assume it would be better to have all the colour signals all perfectly in phase and buffered.. Is something I need to check on further..
The next stage uses a video conditioning chip generally seems to be the FMS6400. It seems a little hard to find now and is actually obsolete, but it does do the composite signal merge and does buffering also. It also has some internal filters which should help clean up any noise in the signal.
I used pole position my testing.. Title screen with it having a chequered white and black background may be good for testing colour bleeding. The mod seems to do pretty well with this aspect. I did experiment around with it a little and it managed to make it worse but it seems any sort of load on the TIA is the cause this colour bleeding issue.
Main game screen with it having a lot of solid colours like the blue sky makes it a good test for any noise in the signals. It is probably difficult to see, but there is vertical bars in the blue sky seem to be every few pixels all the way down the colour blue. This could be down to my fantastic wiring, but I have been unable to find a cause for this otherwise.
There also seems to be some sort of juddering on the white part of the image. The main school at the top seems to be juddering a little bit, even though the rest of the image appears to be okay. I also see the same in the middle of the title screen flag, where it seems to be juddering slightly. Again I am not sure why that is happening.
Overall, it is not a bad modification that it does seem to have some little annoying niggles. Other than this juddering problem and noise problem, I think this is down to PCB layout issues. Clearly the people designing the circuit and not read the layout guidelines for anything, or even have any experience with HF PCB layout design. So I think if a proper PCB layout is done I think some of these little issues will go away anyway. I could do with doing some more tests, but someone may have blown up something on the board not currently working
I think the 4050 is not exactly needed, but I think buffering it will definitely not hurt. There is a similar series of chip just does video filtering and buffering, it as a few inputs and outputs, so I think this would be a better choice than the older 4050. I think the transistor can be done away with along with some other parts. I don't see the need in a pre-buffer to the video driver chip itself. The likely I will adjust the resistor ladder values in the hope it will add a little bit more quality to the image.
I need to start designing a new PCB next...