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Tapping a stereo signal from genesis model 1
Malakai - Jul 3, 2005
  Malakai Jul 3, 2005
The topic pretty much speaks for itself. I know most of you who have done a composite or s-video + RCA stereo output mods on the sega genesis model 1 have probably just tapped the headphone input for the stereo source. I was wondering if anyone knows where to tap the source before it is amplified/controlled?

 Malakai Jul 4, 2005
Well, I've modded two Sega Genesis systems, with a composite video and stereo/rca mod, on 1 model 1 and a model 2. Of course, on the model 1, I tapped the headphone source, because I did not know where to tap a true/unamplified stereo signal.

On the model 1, which I did last, I actually did a dual video and audio output, and also have an s-video connector mounted, but have not done the s-video mod yet.

Anyway, from comparing the two different systems, the model 1 seems to not only have slightly better composite video quality, it also seems to have far more superior sound quality than the model 2. Do they each have a different video and audio chips? I don't remember what the model two had, but I think the model 1 had a yamaha sound chip.

 lordofduct Jul 4, 2005
I don't know if this could be any help... never tried it, but it would be the route I'd go if I were too. Malakai could probably add in some thought on this one.

the YM2612 (FM synthesizer used in the genny...) has the pinout of

Code: 
  
   +--()--+

GND | 1 24| 0M(CLK)

 D0 | 2 23| Vcc

 D1 | 3 22| A.Vcc

 D2 | 4 21| MOL

 D3 | 5 20| MOR

 D4 | 6 19| A.GND

 D5 | 7 18| A1

 D6 | 8 17| A0

 D7 | 9 16| /RD

  ? |10 15| /WR

 /IC |11 14| /CS

GND |12 13| /IRQ

   +------+

Well... if you used pin 19 as the ground for both channels and used pins 21 and 22 for L and R respectively you "should" get a stereo signal...

 Mask of Destiny Jul 4, 2005

  
	
	

		
		

			Originally posted by Malakai@Mon, 2005-07-04 @ 11:51 PM



 Anyway, from comparing the two different systems, the model 1 seems to not only have slightly better composite video quality, it also seems to have far more superior sound quality than the model 2. Do they each have a different video and audio chips? I don't remember what the model two had, but I think the model 1 had a yamaha sound chip.


It's possible they used a different RGB->Composite video encoder on the Model 2. Video looks a lot better on my Model 2 when used with a 32X (which has a better encoder).

As for the sound, the YM2612 was integrated into the VDP for the Model 2. Steve Snake also mentioned something about the power supply being weaker on the Model 2 causing more distortion or something.

 Malakai Jul 6, 2005
I have an extra genesis that I'm going to try to do this on, probably sometime tomorrow if i can clear out some workspace, and i'll let you all know how it goes.


Code:
Originally posted by lordofduct@Mon, 2005-07-04 @ 10:14 PM

I don't know if this could be any help... never tried it, but it would be the route I'd go if I were too. Malakai could probably add in some thought on this one.

the YM2612 (FM synthesizer used in the genny...) has the pinout of

Code: 
  
   +--()--+

GND | 1 24| 0M(CLK)

 D0 | 2 23| Vcc

 D1 | 3 22| A.Vcc

 D2 | 4 21| MOL

 D3 | 5 20| MOR

 D4 | 6 19| A.GND

 D5 | 7 18| A1

 D6 | 8 17| A0

 D7 | 9 16| /RD

  ? |10 15| /WR

 /IC |11 14| /CS

GND |12 13| /IRQ

   +------+

Well... if you used pin 19 as the ground for both channels and used pins 21 and 22 for L and R respectively you "should" get a stereo signal...

[post=136299]Quoted post[/post]


 Mask of Destiny Jul 6, 2005
If you tap the YM2612 directly you won't hear any of the sounds from the PSG which most games use to a degree. On my model 2 they're both fed into some opamps and mixed somewhere in that vicinity.

 Malakai Jul 6, 2005
Well, I just tried that setup, and it doesn't work at all. It'll get a few screens sometimes, and the genesis will freeze up, and there is no thing but a buzz that comes through both the MOL and MOR. Are these really for left and right output, or are they a digital signal or something else?

I checked, double checked, and triple checked the connections, and it's all soldered and connected correctly. So, I guess we just need to find where all of the connections turn into analog, and somewhere before it is being amplified. I wonder if it would be somewhere else on the chip, or another place on the circuit board.

UPDATE: If there is an analog signal on the MOL and MOR, then I'm guessing that you would have to cut those two connections from that chip to the circuit board, but if mask of destiny is correct, and the PSG sound isn't connected to it, there would be two problems. First, you would cut off any/all sound to the rest of the circuits, atleast from the yamaha chip, and secondly, you'd miss the PSG sound. In other words, you may get FM (and PCM?) sounds from the yamaha chip , but not the PSG, and no sounds from the headphone jacks or from a DIN A/V cable connection, without adding toggle switches atleast.

I'm not even going to try try disconnecting those pins and adding toggle switches. Using the headphone output is better than that choice, and you never know when/if someone will figure out where to get the analog stereo signal that I'm looking for. Maybe these threads will find their way to some electronics/technical buff that will already know or will start working on it.

The next mod should be to do a built in power supply mod :/

 lordofduct Jul 7, 2005
Trial and error my brother... trial and error.

 Mask of Destiny Jul 7, 2005
Checked the schematics. MOR and MOL are definately analog. Perhaps you should try capturing them right before they enter into the amplifier chip (CXA-1034 IC12). MOR is pin 8, MOL is pin 1, PSG is pin 7 and SLI (presumably for sound input from cartridge/sega CD) is pin 2.

 Malakai Jul 7, 2005
Well, I got some good news and some bad. The good: from the sony chip (CXA-1034,) I was able to get an analog connection.

The bad: the signal isn't strong enough to to be a viable option. In other words, it would probably need to hook up to a preamp. I guess it could be tested in a stereo system that had a phono input, and I'll probably test that option out soon, even though I don't consider that to be any viable option,

 Malakai Jul 7, 2005
After doing some searching, forum reading, and looking out pinouts and information of some of the chips in the genesis, I found something that I thought would be significant.

The Sony CXA1034x chip apparently is the mixing chip of the sega genesis (model 1 atleast). From what I can gather (if I understand correctly) is that it mixes the pcm/fm with the psg and possibly a digital channel and then apparently sends it out as a stereo signal, to the amp(s)/preamp(s).

I found some information about one of the small texas instruments chips that is in the sega genesis, the LM358x chip. Since it features two channel analog output, I thought it may have been what I was looking for. I soldered wires to pin 1 and 7, and got an analog music/game signal from pin 1 but not from pin 7.

I will double check everything again tomorrow and see what else I can come up with, but I'm going to guess that this is probably the spot where it sends a mono signal to the DIN connector on the back of the Genesis. If it is, I guess I'm back to the beginning again lol. Also, if this is true, I can't understand why they just didn't add stereo sound to the genesis, because both the sound chip and the OPamp supports stereo sound. It wouldn't have cost Sega a dime more to have manufactured it that way.

 Mask of Destiny Jul 7, 2005
The CXA1034 is a stereo power amplifier (at least according to one parts site). I can't find a datasheet, but it would appear that they're using it as both a mixer and an amplifier (IN A and IN B are tied to ground and 4 other inputs are used for the FM, PSG, and cart/sega cd sound channels). I'm pretty sure all audio signals going in to the chip are analog.

The LM358x feeds the downmixed signal into the video encoder chip (no idea why) which then feeds the DIN connector. I'd imagine it's mono because in 89 there were very few stereo TVs and I think that DIN connector was somewhat standard at the time (or at least Sega had used it before).

 Malakai Jul 12, 2005
Well, I think I just killed two birds with one stone... Modded my xbox, used slayer's auto installer, which has a genesis/32x/cd emulator, and now I have composite, s-video, analog stereo and digital optical output (well, i haven't officially tested the digital optical output yet, because I haven't got the cable or done the digital coaxial mod... I've tested it out on a few games, and they play pretty nicely.. I think this was ported from gens, not sure though...