Genesis 1 sound via expansion port to Sega CD

Hi! I've been considering the possibility of a mod like this, and am not sure if it has been actually done before.

Background
I have a VA0 Mega Drive (model 1), the volume of the preamp is rather high on these early consoles and the sound from the headphone jack is fairly distorted due to the fact that it's going through an iffy headphone amp after being preamped quite loud. Pulling audio directly from the preamp chip is said to be much better quality than from the headphone jack, but you would need to mod the jacks somewhere onto the case (or dangle them out). I don't know about pulling audio even earlier than the preamp, I am not sure where the YM2612 (Genesis) and the PSG (Master System) sound gets mixed.

I have a Sega CD model 2 attached pretty much all the time. With a model 2 Genesis, the sound from the console's chips are routed through the expansion port to the Sega CD without any additional cables, and can be outputted in stereo from the RCA jacks on the back of the Sega CD. With the model 1 Genesis, to use the RCA jacks on the CD, you need to patch the audio from the headphone jack of the model 1 Genesis to the back of the Sega CD model 2, externally. Of course, this means that the audio from the console has already gone through the headphone amp and been distorted... though luckily the CD audio does not get sent through the main console if the mixing port is occupied.

Idea
Would it be possible to route sound from the Genesis model 1 through the expansion port to the Sega CD the same way the Genesis model 2 does it? I have a few questions that I haven't been able to answer just yet.

Which pins on the expansion port are used to send audio to the CD?
Should the sent audio be preamped, line level? What does the Genesis model 2 send?

I do have a couple of spare later revision damaged Genesis model 1s to experiment on, if I need to!

If anybody can give some insight on how the sound works, or let me know if I have got something wrong, I would very much appreciate it!

 

Alright, so the mixer cable does not need to be connected to the Model 1. If so, the audio will just be mono, just like the Model 2 would be. And also... let me know the status on those damaged Model 1s. I need one to get my Mega CD working if a friend can't fix it, methinks.

 

Yeah, if the mixer cable is not plugged into the Sega CD then the CD audio is sent to the console and outputted from the main console. Of course, that is outputted mono like you said, and I should have said I'm looking for stereo out. I've considered doing a stereo output mod to the console itself but I'd like to avoid adding in more jacks or changing the pinout of the console when the Sega CD has the hardware for it already.

I should have been more clear about the model 1s, I meant genesis model 1s, not sega CDs! Will go back and clarify all instances of model x. Sorry about that!

 

Ah, no problem. I got you now, and it is something someone more knowledgeable than I can help you with. Good luck!

 

The purpose of the mixer cable is bring the audio from the Mega Drive back into the CD board amplifiers so it can be mixed locally and then pumped out through the "CINCH" (RCA) jacks on the back of the CD unit. When they designed the CD units, they took two of the unused connector pins on the expansion port and decided to use them as output pins from the Mega Drive sound. As MD1 units were already out and could not be modded to achieve that, they could only implement that on MD2s. Hence why the CD unit shipped with the mixing cable.

Regarding the headphones amplifier, it's designed to be the core IC for stereo portable compact cassette players and being designed to drive a stereo magnetic tape playback head it is a high gain amplifier. Therefore, it adds a background of white noise to the sound it amplifies from the MD circuitry. It's a very good amp but it's on the wrong place... lol

 

Great explanation. I did not know that about the amplifier! It is what I figured about the MD1 being unable to send its audio through the port, simply a feature they didn't have planned for the original console. Do you know which pins on the expansion port are the ones being used? I'm not particularly good at deciphering schematics haha

 

The explanation on the amp was about why it sounds somewhat "noisy". White noise is a sound which resembles the noise you listen when a old analog TV is without any signal source, out of tune.

And about the cable, is exactly what you said. I mentioned they were unable to change MD1 units to add the capability of outputting audio through the expansion port as the original purpose they defined for that port was a floppy drive. Why would a floppy drive need to receive the sound coming from the MD?

 

Yeah, a floppy drive wouldn't have any reason to. But, they did end up sending audio through the same port on a later version of the Genesis (the model 2). That is what I'm trying to utilize with the model 1 Genesis, as the Sega CD is designed to receive audio through the port, even if it was not the original design intention.

edit
I found a schematic of the expansion port here which has the audio out pins listed, unlike a previous pinout I was looking at, which should help me figure out a lot

 

My suggestion for you is you study the service manuals and figure out on which pins the CD unit expects the audio output to be in.

 

Thanks. I found the pins the audio is sent on and how it is used inside the CD unit, the pins are of course not connected to anything on the MD1. I took a good look at how the audio is routed using this MD2 service manual as well.

One thing that surprised me are the SL1 and SR1 analog audio pins located at B1 and B3 on the cartridge slot. These seem to allow the cartridge itself to output analog audio... are these ever used? edit: OH! the 32X, of course. Forgot about that!

 

The 32X uses that so you can have the PWM sound on the headphones jack.

 

After taking a look at this schematic for the MD1, it looks like there's nowhere I can tap that has the YM2612(MOL,MOR), PSG, and 32X(SL1,SR1) audio mixed without the CD(SL2,SR2) audio being mixed in as well. On the model 2, the signal is sent to the CD(SL3,SR3) before the CD audio gets mixed in. If I send it with it already mixed in, it will probably be trouble.

But, I can prevent the CD sending the SL2 and SR2 signals if I leave the mixer plug occupied, so I will probably give it a try and see what happens.

 

whatever happened with this? I'd be interested to know if you ever figured it out

 

That's where you get the unamplified, complete YM2612 and PSG audio from.

Source thread: http://www.sega-16.com/forum/showth...ive-1-stereo-mod-no-headphones-volume-control