Project Genesis - Modding

Now that the repairs and maintenance are complete, I can move onto the final, and most exciting, phase of Project Genesis - "modding." Here I'll modify the Genesis to actually make it better than when it was new. The mods I'm going to make are S-Video, composite, and stereo audio out the back.

Originally the Sega Genesis (Mega Drive) output RF and composite video. My plan is to add a second composite output (more on that later) plus S-Video output which is supposedly much better! This is nothing new, there are tons of people who've performed the S-Video mod and posted their results on the Internet. But all of those posts seem to lack important details. It's my hope to fill in all the missing pieces for anyone else who wants to perform this mod.

To create the S-Video output I need to tap pins 15 and 16 of the CXA1145. Note: not all versions of the Sega Genesis use the CXA1145, so check before you assume this will work. I want to add a second composite output to the back even though the Genesis comes with a composite output. The reason being is the built-in composite requires a proprietary video cable, whereas I'm going to use a standard RCA jack. Finally I plan to add a stereo output jack on the back by tapping the CXA1034P audio chip directly. The schematic for all this should look like this.


S-Video
There are two main S-Video mods I've found on the Internet, David Howland's and Jamma Nation. The Jamma Nation one is newer and uses a much simpler design, so that's the one I'm going to use. However both designs don't give you all the necessary info. I decided to wire up the design on a breadboard first, this allowed me to make changes to the circuit to get the best possible performance out of the mod. Below is a list of the questions about the original designs, and the answers I determined.

1. Some designs use an amplifier for both the luma and the chroma lines, some designs only use an amp on the luma signal. Is the amp on the chroma line optional? Answer: I tested the circuit both with and without an amp on the chroma - I can say the amp on the chroma line does nothing. There was no noticeable difference in the video output with the amp on the chroma line
   
2. Most of the designs call for an NTE85 transistor on the luma amp, but this transistor is no longer in production. What transistor can be used in it's place? Answer: I used a 2n3904 transistor (specifically the 2N3904FS-ND on Digikey) and it worked great.
   
3. Many people who've done this mod state that the capacitor on the chroma line must have the positive towards the CXA1145 chip. Others state that the negative must be towards the CXA1145. Well they both can't be right, so which is it? Answer: I tested the cap forwards and backwards, the polarity made no difference. Since the polarity made no difference, I also tested a bi-polar capacitor and that worked as well. Since polarity didn't matter, in the end I went with the bi-polar capacitor.
   
4. For the chroma line they say to use a 220uF capacitor, but they don't specify the voltage needed. What voltage capacitor should one use? Answer: Using my multimeter on the live circuit I tested the voltage across the capacitor. The voltage never exceeded 2.5 volts. So for this mod I would suggest 6.3, 10, or 16 volts. Anything higher is just a waste. I went with 10 volt in my design.
   
5. Both the chroma and the luma lines call for a resistor, but no one says what wattage resistor to use. Answer: Again I tested this with my multimeter. Both the volts and the current were very low. Even when you multiple them together to get wattage, it was extremely low. So using 1/8 watt resistors leaves enough of a margin. You could also use 1/4 watt if you wanted to.
   
6. The luma line calls for a 27 ohm resistor, and the chroma line calls for a 75 ohm resistor. Varying these resistors greatly effects the quality of the video signal. So are these values truly the optimal values? Answer: On my breadboard I replaced the resistors with trimmers so I could adjust the resistance while the circuit was live. This allowed me to test the values to "see" what looked the best. The people who originally came up with this design did a good job, both of their resistor values were very close. The values I came up with as "optimal" were both within 1 ohm of the original values. So in my final circuit I used the 27 and 75 ohms from the original design.
   

Composite

1. As with the S-Video mod, some claim the capacitor must have the positive facing the CXA1145 chip and others say the negative. So which is it? Answer: I tested the cap forwards and backwards, as before the polarity made no difference. I also tested a bi-polar capacitor which also worked. In the end I went with the bi-polar capacitor.
   
2. They say to use a 220uF capacitor for the compostive, but they don't specify the voltage needed. What voltage capacitor should one use? Answer: Using my multimeter on the live circuit I tested the voltage across the capacitor. The voltage never exceeded 2.2 volts. So for this mod I would suggest 6.3, 10, or 16 volts. I went with 10 volt in my design.
   
3. Again, the composite signal uses a resistor, but no one says what wattage resistor to use. Answer: As before the volts and current very very low. So using 1/8 watt resistors leaves enough of a margin. You could also use 1/4 watt if you wanted to.
   
4. The original design calls for a 75 ohm resistor, but is this the optimal value? Answer: Testing with a trimmer I felt like the best video came from a resistor of 72 ohms. In my final design I went with the original value of 75 ohms as it's the only value I had on hand.
   

Stereo Audio

1. By tapping the CXA1034P directly you bypass the headphone amp and should get a cleaner signal. But is this output too low for the TV, does it need additional amplification? Answer: Surprisingly the audio output directly from the CXA1034P was perfect line-out levels. I did not need any amplification before passing onto the TV.
   
2. Does the headphone slider effect the audio output levels in anyway? Answer: The headphone slider does not affect the audio levels in anyway. So tapping the CXA1034P directly is a great way to get clean stereo audio.
   

Here's the final circuit design I went with:


Here's what it looked like on the breadboard.


Now that I've breadboarded up the design and I'm happy with it, time to solder it all up and squeeze it into the Genesis case. I also need to mount the connectors on the back. Whereas most people drill holes behind the cartridge slot, I decided to use the existing holes on the back of the Genesis. The back of the Genesis had 5 openings; power, proprietary A/V connector, RF output, RF channel selector, and the "EXT" port (which from what I've read was only used for debugging by developers). My plan is to remove the RF unit and EXT port which opens up three holes into which I can mount the S-Video, composite, and audio.

The picture below shows the solder points to clean out in order to remove the RF unit (shown in red) and EXT port (in blue). Getting the RF unit out ended up being tricky. The three point in a row were easy, but the large grounding points are difficult to clear out the solder. I ended up having to use a product call ChipQuik to get the RF unit out. By comparison the EXT port was a snap to remove.


And here's a picture with the RF unit and EXT ports removed.


Now it's just time to solder up all the connections. Here's all the wires connected to the underside of the motherboard.


As you can see in this picture I soldered the capacitors and transistor inline, covering the connections with heat shrink.


And the completed view of the back of the Genesis. I ended up drilling a single hole for the audio jack. The EXT port ended up being too large for the connectors, so that opening went unused.



And finally, the results...

Here are pictures of the composite versus S-Video output. As you can see the S-Video is noticeable clearer. You'll also notice the composite pictures are darker. There wasn't a change in the lighting, nor post image-processing, the S-Video output is just brighter and more vivid.












This mod was a lot of fun, and the quality of the video output was more than I had expected. If you're into retro gaming on the Genesis and if you have soldering skills, you might want to consider this mod. In the end the Genesis cost me $1, and the parts cost about $10.