Before I can truly finalize the system and have another set of sample boards made, I wanted to address a problem I was seeing in the video output where a very faint color snow was visible in the background. This also seemed to relate to the method by which I was conditioning the subcarrier output of the LM1889 Color Encoder chip with a simple one transistor emitter follower buffer. There was no inherit gain to this stage, so I had to resort to some trickery with an inline inductor to even have a somewhat reasonable color burst signal, although it was half of what it really should have been. Later I discovered that the inductor was responsible for the noise I was seeing in the chroma signal, and subsequently in the background of the image.

So back to the drawing board it was, with the that emitter follower circuit being changed out for amplification instead. Luckily this didn't require any more components than what I was already using, and I was able to get rid of the inductor at the same time (win-win).

Here's a look at the entire video circuit as it now stands, shown in two parts to better fit the page.

The first part converts the TMS9128 VDP (not shown) R-Y and B-Y color difference signals into a single 3.58 Mhz subcarrier output suitable for the S-Video chrominance signal via the LM1889 color encoder chip, although that chip doesn't provide enough drive on its own to feed into a standard monitor input (we'll address that in part 2). In this particular application the LM1889 is only being used for its color encoder aspect which is half of its normal purpose. It also has an RF modulator section which was utilized by the original Colecovision game console as the sole means of connecting a TV.

The second part of the schematic shows the final signal conditioning and driver circuit going right out to the Atari 8-bit compatible audio/video jack, with the FMS6400 outputting both S-Video and Composite Video. I chose to mimic the Atari connector and pin-out since there are standard cables already available with this in mind.
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​As can be seen, transistor Q3 is now serving to amplify the subcarrier output of the LM1889 to a level suitable for injection into the video buffer/combiner FMS6400 chip,

​With the proper chroma amplification now in place, the color burst is at the correct level for standard video. This can be seen in the captured image of the composite video output being viewed on my hand-held O-Scope.

For this test, the composite output was  terminated with a 75 ohm resistor to ground to mimic what would be the case for a video monitor if it were connected instead.

Normally the color burst signal is suppose to be at the same relative voltage swing as the sync pulse (350-400 mV), which we now appear to be doing thanks to the added amplification of the new Q3 circuit. The output of the LM1889 just wasn't sufficient with buffering only being applied. I think with these changes we're good to go on this aspect. and can now call it a day.

​So after making the modifications, I put Donkey Kong Jr up on my VIZIO S-Video monitor and captured this un-retouched screen shot with my camera.

Captured image from 19" VIZIO LCD TV, via the S-Video input (taken with a Canon EOS Rebel T7)...

Finally the black background is flawless, with no evidence of noise or snow what-so-ever. And the detail and color saturation in the image are quite good at this point.

In order to get such fully saturated colors I did need to boost the color setting on my VIZIO for it to appear as it does in S-Video mode on the LCD screen. However all the other levels were set to 50 out of 100, being the default mid-point settings of that monitor. Later when I fed the composite output to my CRT monitor the color saturation was perfectly fine at the default settings for that particular monitor. I think this is partly due to the differences between how the LCD displays information vs. the CRT. A CRT display just tends to be more vibrant when compared to early 2000's LCD monitors like the one I'm using.

 Update February 2nd 2023

I connected the S-Video output into a cheapo HDMI converter I picked up off of AliExpress and was literally blown away by the result when viewed on my large screen HDTV.

Captured image from 55" LG HDTV, via the HDMI input (taken with a Canon EOS Rebel T7)...

I'm back after assembling, testing, modifying, and scratching my head. So much has happened since my last blog post. First off the CPLD being used for what I'll now call the MSX Enhanced Memory & Sound Module (name got changed to protect the innocent) didn't work on the first power-up. This had me very worried since it was my first go around at designing with a CPLD. However it wasn't long before it started working correctly, only taking a couple of tweaks to make it all better and then passing all the tests with flying colors.

So what is this MSX Module all about?
It takes the stock Colecovision memory configuration from a measly 1K up to a whopping 32K, and it throws in a second programmable sound generator (PSG), giving a total of 6 independent voices when combined with the original sound chip. However it can be put back into a stock 1K mirrored RAM mode with a simple register change, and the extra sound chip won't activate unless needed.

Having this new configuration in play makes it compatible with games that were developed for the MSX and SX platforms, making for very easy ports to the Colecovision system. This also means that it can take advantage of games created for the Super Game Module by AtariAge member opcode.

The ATF1500A CPLD (optional ATF1502ALS)  which acts like a Memory Management Unit (MMU), makes it possible to eliminate at least half a dozen 74XXX series logic chips, and allows for relatively simple CPLD core changes for altering any of the decoding characteristics. For this aspect, I utilized Protel 99 SE to design the inner workings of the CPLD, using it's schematic capture to PLD source code generator, thus keeping me in a graphical design environment not unlike what I use in PCB design.

Here's a peek into what just such a schematic looks like for eventual translation to PLD code. I believe this will be the final version for the project. Wow not too different than an actual physical component schematic!

What Happened Next?
With the proper CPLD in play, the system worked, and worked well. And thanks to AtariAge member PixelBoy, I was able to test right off the bat 40+ SGM/MSX derived compatible games. Of which most all worked, with only one exception turning out to be an issue with being run from an Atarimax Ultimate SD Cart instead of a real cart.

There were also a handful of other game prototypes that had issues, but for the most part compatibility was very good across the board.

However all was not good, and unfortunately I was seeing noise in my video output, which I temporarily fixed by substituting a linear 5V regulator for the switching one that I had spec'd. Of course this produced way too much heat and now required a large heat sink to disperse that extra heat. Good news is this got solved today when I found a very low noise version of a TO220 switching regulator, that actually turned out to be cheaper than the previous noisy one.

QUAD Board Problems
This is a board that fills in the missing circuits for using a quadrature based roller-controller, and better yet, also supports an alternative in a USB Mouse (wireless capable). Unfortunately I had screwed up with the Mouse USB port, having the power polarity reversed. I also had screwed up the orientation of the left and right mouse buttons vs. what a real controller has. So after some trace cutting and rewiring, it worked like a charm.

Then I discovered the game Armageddon, which is very much like Missile Command, having 3 missile silos protecting the city from attack. This game requires a roller-controller for targeting where the missiles will go when launched. However it takes 3 buttons from that controller to launch from all 3 missile silos, and I only had two of those available on my mouse, thus leaving one silo inoperative.

With the help of AtariAge member Chart45 we discovered that the middle mouse button would also pull down one of the unused I/O pins on the CH554, a microcontroller being used to translate the USB Mouse into something that the Colecovision can recognize. This middle mouse button would fill in for the missing button required to launch missiles from the formally inactive 3rd silo. But there was a slight problem, and that had to do with an inexplicable delay after pressing the middle mouse button before you would see a response on the I/O line, which was simply not acceptable. We are currently trying to solve this issue, along with the help of Chart45's brother who is a far better coder than either of us.

Changes are in the Works
There will be a 2nd revision of most all the boards associated with this project, and subsequently a 2nd order will be placed for new sample boards. This is looking to happen about 3-4 weeks from now once all the new aspects and fixes have been proven out. So stay tuned for more news to come on this project.

- Michael