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 Sega Genesis model 1 and the cxa2075

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Drakon
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PostSubject: Sega Genesis model 1 and the cxa2075   Tue May 08, 2012 10:03 am

Lately I've discovered that the cxa2075 in the model 2 genesis isn't generating an image that's as perfect as I thought. So I decided to revisit messing around with that video circuit in the model 1 genesis. My modding / hardware knowledge has come a long way since I last worked inside the mdoel 1 so as to be expected the results have been pretty great so far.

I first tried working with my va5 model 1. I removed all the previous wiring which was a huge mess mostly because it's one of the first systems I ever mucked around with. After cleaning the internals up I remembered that the model 1 is terrible when it comes to rainbow banding. I've had a lot of people swear up and down that replacing a capacitor fixes the rainbow issue but nobody seemed to know which capacitor it was. Earlier I had been mucking around with the model 2 genesis. I noticed that when feeding the colour subcarrier line from the system into the video encoder that the image stability was much improved but the rainbow banding appeared. When running the video encoder off of a separate crystal oscillator I didn't really notice any rainbow banding but there was other stability issues with the image. So this lead me to believe that the rainbow banding is mostly caused by the colour subcarrier line that comes straight out of the vdp chip.

I figured if there was a capacitor to replace that would fix the rainbow issue the capacitor would probably be somewhere near the vdp chip. Ironically the capacitor that was closest to the vdp chip where the colour subcarrier came out from had been crushed. I have no clue what crushed that capacitor....but maybe it was gods way of pointing me to the right part to replace. The cap was a standard 10 uf cap so I replaced it, wired up everything nice and neat, turned the system on, surprisingly there was no smoke coming from it. Next I attached the cxa2075 circuit into the header I wired up, turned it on, everything worked fine! First thing I noticed....the vertical bars were pretty weak...pretty much as weak as my model 2. Second thing I noticed, there was barely any rainbow banding....I don't know if this's because I replaced this one capacitor or something else.

The VA5 vdp outputs rgb that's pretty good but the va6 and later vdps output noticably better video quality. So today (when my client stops hounding me) I'm going to do the same thing to my va6 and see if I get the same results. Also with the va6 I'm hoping to get rainbow banding before replacing that capacitor (hopefully it's not crushed) so I can do a before and after comparison. I'm glad that I got such good results as I love the look of my model 1 megadrive.
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PostSubject: Re: Sega Genesis model 1 and the cxa2075   Tue May 08, 2012 4:04 pm

As promised I did some farting around today and again had some great results. I cleaned up my va6, fixed a few things to restore it back to factory condition, and went to work on it. Installed the cxa2075 circuit and indeed the s-video picture looked a little better than my va5. The rainbow banding on the va6 was actually just as good as the va5 but no caps had been replaced so I guess that had to do with the cxa2075 circuit.

With the cxa2075 I got a bit of jailbar effect going so I decided to see if I could fix that. I tried adding a low pass filter to the luma line in the s-video output but that made the bars worse (strange). I then tried adding a low pass filter in the chroma line and that made the bars weaker. The filter isn't strong enough to completely get rid of them but it was strong enough to make them not annoying or even noticable unless I'm infront of the tv. I would have tried a stronger filter but I used my strongest capacitor so I'd need to buy stronger ones to do that.

Anyway here's a screenshot comparison between the s-video from the built in cxa1145 vs the cxa2075 when I bypass the built in encoder. Keep in mind both of these screenshots are taken from the same system just running different video encoders with different circuits for each.





Strangely adding the low pass filter to the chroma line made the rainbow banding a little worse but since it got rid of the jailbars it's a good trade. Also the rainbow banding isn't strong enough to be annoying when playing games so no big deal. I absolutely love the colour encoding of the cxa2075 it has an even bigger pallette range than the cxa1645 which is amazing.

Another thing to take note of is the model 1s which came with a cxa1145 work a lot better when you bypass the built in encoder to a cxa2075 circuit. My model 2s which came with the garbage samsung encoder are possible to bypass to a cxa2075 but when I plug the s-video output in my capture device from the model 2 with the cxa2075 I had all sorts of strange interference going on. This's probably because the signals were designed to be fed into a samsung chip instead of a sony chip like the model 1s that came with sony chips. The bizarre thing is that the model 2 with the bypassed encoder looked fine on my tv, but on my capture device it was insanity. The cxa1145 based model 1 with the same mod looks just as good on my tv and capture device.

It's also important to bypass the built in cxa1145 because if you encode the rgb output from the cxa1145 you'll be encoding the rgb pallette of the cxa1145 which isn't as vibrant as the rgb from the vdp chip.

I consider this job done.
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PostSubject: Re: Sega Genesis model 1 and the cxa2075   Fri May 11, 2012 12:47 am

Today was pretty productive. I downloaded a schematic of the sega genesis pcb. I took a look at the circuits between the vdp and the cxa1145 video encoder. I tried messing with the colour subcarrier circuit but couldn't get the rainbow effect to go away. One thing did catch my attention though.....the rgb lines had power (vcc) connected to all three with just a resistor in between. I thought it was a bit odd that you would connect power to video signal lines, even with resistors in between this didn't make a whole lot of sense. Naturally I tried disconnecting the voltage from the rgb lines and wouldn't you know it....the picture quality GREATLY improved.

I asked tiido why on earth sega would wire up their console video circuit this way. Tiido said it's probably because the old cxa1145 is expecting the video signals to be in a certain voltage range and the vdp outputs a signal that doesn't have enough voltage. So what sega did was they just mixed electricity with their rgb video signals. Therefore part of the signal going into the video encoder is just electricity and part of it is the actual video signal. Of course mixing elecricity with the video signal was mudding up the video quality and decreasing the image quality. Disconnecting voltage from the rgb lines coming from the vdp was fine on my genesis console because in my genesis I bypassed the crappy cxa1145 and wired in a cxa2075 circuit. The cxa2075 is a much newer chip that can handle video signals fine even if the voltage is a bit low.

The difference is quite noticable the sharpness has increased and the colours are much more vibrant. I didn't get time take any screenshots but I'll try that tomorrow and see how it looks from my capture device. I next decided to look at a schematic of an early super nintendo pcb revision and wouldn't you know it...after the rgb goes through a transistor amp before it goes into the video encoder the old snes has the same thing....voltage connected to the rgb lines with only a resistor in between.....what the heck doh
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PostSubject: Re: Sega Genesis model 1 and the cxa2075   Fri May 11, 2012 10:09 pm

Wow, what the hell were they thinking. What a terrible solution to get the voltage within a certain range. Interesting find there Drakon.
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PostSubject: Re: Sega Genesis model 1 and the cxa2075   Fri May 11, 2012 11:25 pm

glenn101 wrote:
Wow, what the hell were they thinking. What a terrible solution to get the voltage within a certain range. Interesting find there Drakon.

Yeah I did more tinkering today. It turns out that the built in cxa1145 displays picture fine even without the voltage wired into the rgb lines so I truly have no clue what that voltage is for. I did notice without the voltage there that the s-video from the cxa2075 at the very top of the screen has a blue bar that's like 1-2 pixels total height. Really though that's the only thing I noticed besides that everything looks perfect. Strangely enough my va5 has a better quality image than my va6 when you disconnect voltage from the rgb lines. So now my va5 is my primary system.

I also FINALLY figured out how to decrease the rainbow banding to the point where it's so weak that it's no longer annoying. For some reason adding more resistance on the chroma line of the s-video output from my cxa2075 circuit increases the rainbow banding. The default circuit has a 75 ohm resistor on the chroma line so I removed the resistor and the rainbow banding greatly improved. The rainbow banding is still there but it's super weak, so weak that you can barely notice it. Also the low pass filter I was using to decrease the jailbars was messing up the colours and also weakening the colour strength. I removed the low pass filter and the picture is so good looking that what's left of the jailbars are far too weak to be annoying. So finally I've got a pretty much perfect looking s-video image from my model 1 genesis. The image isn't 100% distortion free but I've weakened all the distortion to the point where it's not annoying in the slightest. I'll take some new screenshots when I get the time my gf is sick so I'm busy taking care of her.
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PostSubject: Re: Sega Genesis model 1 and the cxa2075   Sat May 12, 2012 1:04 pm

Okay I finally took a screenshot of my new setup and the screenshot yielded quite interesting results.

For reference here's the cxa1145 s-video image with no mods done to the system:



cxa2075 before I found the voltage fix and when I had a filter to get rid of jailbars (that later proved to be screwing up colours so I removed the filter)



And here's the latest cxa2075 image quality:



I should try adding a low pass filter to the luma output on the current setup and see if it helps with the vertical bars at all. The current cxa2075 where I disconnected voltage from the rgb lines the brightness is exactly the same as the cxa1145 when the voltage was still wired up....interesting. What's really great though is you can clearly see that the rainbow banding is massively improved!

The rainbow banding on my cxa2075 circuit went from this:



To this:



Literally all I did to fix this rainbow banding was I removed the 75 ohm resistor attached to the chroma output line from the cxa2075 circuit. The more resistance you add to the colour line the worse the rainbow banding becomes....I have no clue why. From tinkering heavily with this system I know a good amount of the rainbow banding comes from the colour subcarrier line but also there's rainbow banding is in the rgb lines. Leaving the colour output from the cxa2075 circuit at full strength keeps the rainbow effect at its weakest. This is fine because the colours look pretty perfect at full strength I only had the 75 ohm resistor wired up because that's what the datasheet suggests but the resistor isn't needed for the circuit to work.

So I've got the brightness perfect, the colour encoding much more vibrant than the built in encoder, and the rainbow banding flaw is pretty much gone. If I can find a way to build some sort of filter for those vertical bars this mod will be as good as it can get. When I tried using a low pass filter on the luma output before it made the bars worse so I have no idea if I'll be able to fix those bars or not. Filtering the chroma line caused the colours to go off so that was a bad idea in the end.
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PostSubject: Re: Sega Genesis model 1 and the cxa2075   Sat May 12, 2012 8:15 pm

Did some more testing today and figured out how to cure the jailbars. Unfortunately my "cure" isn't any form of circuitry. I remember in the past when having the s-video wired into the cxa1145 that using a shorter s-video cable made the jailbars go away on my tv. I also remember that with my genesis 2 with a cxa2075 that the image looked fine on my tv but on my capture card there was jailbars and issues all over the place. I tried the same 6 foot only s-video trick with my model 1 genesis that's using a cxa2075 and the jailbars again went away on my tv. Then I wired the cxa2075 genesis 1 into my capture card with only a 6 foot cable in between and this's what I got:



It seems that using a shorter cable actually makes the jailbars worse and if you make them so bad they overlap each other in such a way that it looks fine on a tv. sort of like how when you see a car rim spinning at a certain speed it appears to not be moving at all because it's matching the speed of your visual perception. Previously I had thought that using a shorter s-video cable was the solution but when you put the signal under my capture card microscope you could clearly see that this was definitely NOT a fix. But since the picture looked better when I was using 12 feet of cable I figured....let's try adding more s-video cable. When tinkering with the rgb nes back in the day I remember that adding 12 feet of rgb cable to the signal blurred it and blurred away certain video issues. The 12 feet of extra rgb cable didn't even have to be connecting the tv to the system it could just branch off and connect to nothing and still you would get the same effect. So I took a s-video splitter, and added around 20 feet of s-video cable connecting to nothing, and here's what happened to the picture:



Success!! Now if only I could wire up the video circuit in such a way to re-create this effect. This's a ghetto fix for sure but you can clearly see that it indeed works.

The theory behind why this works is because since it's an analog signal sending it over a greater distance weakens it a little. The weakening of the signal is just enough to filter out the stupid jailbars.
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PostSubject: Re: Sega Genesis model 1 and the cxa2075   Sat May 12, 2012 10:06 pm

Okay I did some further testing. I wired up the extra length of s-video cable between the console and the tv and the bars were just as bad. So this trick only works when you add a lot of cable length branching off and connecting to nothing. Also whatever is fixing these bars is happening in the chroma line. With my jailbar fix setup if I disconnect the chroma from the tv and leave just the luma connected the bars come back in full force in black and white.

*edit* okay with this setup both the va5 and the va6 look perfect so that's pretty sweet.
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PostSubject: Re: Sega Genesis model 1 and the cxa2075   Fri May 18, 2012 1:22 am

Drakon's sega genesis crystal clear video mod





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PostSubject: Re: Sega Genesis model 1 and the cxa2075   Fri May 18, 2012 3:29 am

Looks good! Do you have time to make a video when you compare an original Genesis vs. your modded with both audio and video upgrade? Would be awesome.

The game you are playing, Castlevania - Bloodlines (only for Genesis/Mega Drive) is one of the best Genesis games out there, and also one of the best Castlevania games too... Im actually speedrunning this game and Im holding the World Record with the Eric character.

Anyway, keep it up. Im interested in Genesis/MD mods so Im glad your stuff worked out. Smile
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PostSubject: Re: Sega Genesis model 1 and the cxa2075   Fri May 18, 2012 8:36 am

ProD1gy84 wrote:
Looks good! Do you have time to make a video when you compare an original Genesis vs. your modded with both audio and video upgrade? Would be awesome.

The game you are playing, Castlevania - Bloodlines (only for Genesis/Mega Drive) is one of the best Genesis games out there, and also one of the best Castlevania games too... Im actually speedrunning this game and Im holding the World Record with the Eric character.

Anyway, keep it up. Im interested in Genesis/MD mods so Im glad your stuff worked out. Smile

Yeah I'll make a comparison video and see how it looks. Even with my upgraded video circuit it was difficult getting the picture free of distortion. All sorts of random things would effect the vertical bars in the picture. If I unplugged the audio jack from the headphone jack connecting to my amp the video bars would get worse. And adding 12 feet of s-video cable branching off connected to nothing really improved the vertical bars. I'm going to guess that bizarre grounding tricks help but I really have no explanation.
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PostSubject: Re: Sega Genesis model 1 and the cxa2075   Fri May 18, 2012 11:28 am

Video:



The left side (the one that has audio) is the cxa2075 with my s-video fix. The right side is the built in cxa1145 with just a s-video cord from the system to the capture device. Both are s-video. I didn't change any of the capture device settings for these two videos and both are running from the same console.

You can see that the cxa2075 circuit has more vibrant colours, also it has a sharper picture but you can't really tell that in this youtube video. The vertical bars are much more noticable in the 1145 setup. Also the rainbow banding is worse in the 1145 setup which is interesting.

While making the video I discovered that the s-video cable trick also helps the vertical bars on the cxa1145 the same way it helps with them on the cxa2075 so that's pretty cool. However the cxa1145 has worse rainbow banding, less vibrant colours, and in my opinion the s-video picture isn't as sharp.
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