That exists, but it's not open source. But really, how is an FPGA Gameboy emulator any different than a well written regular emulator?

I always multitrack when recording and never use any sort of sync device other than one chain containing a single note that is 01 in length in all four channels. Poor man's sync. And I never have issues with floating bpm.

That being said, I think Bright Primate is looking for a live solution, right? The best option here would be NeX's original suggestion of two GBs running the same song data, synced and hardpanned. Which is great as long as you don't mind losing your O-commands, of course.

I got mine for free... hehehehehe!

The answer is to rebuild the GameBoy from the processor up.  Reverse engineer the GameBoy's custom Z80 processor, recreate it in an FPGA, and modify to your hearts content, in this case, assign each sound channel to a different pin.  Of course, this is only an option if you are a glutton for punishment who doesn't mind spending at least 6 months on a project.

Low-gain: How is DMG-DMG sync any less solid than MIDI if all you want is to keep the boys internally consisent?

Once again... Using Midi Clock is a lot more solid than depending on the sync mode on a DMG. but if you dont have the $$ to fork out for the midi interface then i guess you can't do it that way... But then again, if you had the money to spend on an interface for your puter that has more than 2 inputs, then you probably have the extra $$ to drop on a midi solution for the DMG.

Anyways... Midi clock FTW!

Now, as for multilpe GBs and a single LSDJ cartridge, this isn't feasible since the access lines on the GB cartridge are done in an asynchronous parallel style, meaning you'd inevitably have access conflicts. The only way such a simultaneous access could work is if each GB CPU was programmed to have synchronized sharing of the bus lines, which would be incredibly cumbersome. Low-Gain's solution sounds like it'd be pretty good though, 4 channels with hard panning on 2 gameboys. If you needed further panning you could mix the panning live by hand I suppose.

Low-gain gave a better solution. If there is no panning, you can use only 2 gameboys sync'd and pan the channels. So you would have 4 mono channels, specially if you use an rca cable then you can pass the channels thru the effects easily.

Easier and cheaper than 4 gameboys and 4 carts plus the box, unless you already have 4 gbs and carts.

If you want them all stereo/panning, independent and live, flash the rom with your tracks to 4 carts, mute according to your needs, sync using low-gain's 4-way GB hub and play ...quite manual, but simple.
edit: not an exact answer to the original question, but a way around it actually

sync 1 gameboy via midi clock and do recording passes..
if your song doesn't consist of any panning. then you can pan 2 channels out on either L or R and only have to do 2 passes of recording.

shouldn't be too tough.

Just a thought about this. Would it be possible that all four game boys could use the same cart? Then you'd have the same content on every game boy but individual outputs.

You can do it, but remember there will be a small delay between two synced boys, so if you have to effects or something that are supposed to be triggered in the exact same time, they may sound a bit off. (Like the same mlody playing detuned on two channels, a snare playing on two channels and stuff like that)

When recording, what you could do is use a sync device and record one solo'd channel at a time. When playing live, I would recommend splitting the song up between just two boys and using a regular link cable. That gives you a total of 4 mono channels, or 2 mono channels + 1 stereo channel, or however you want to use it. Heck, I've even recorded a single GB and grouped the instruments in two groups, one in the left channel and one in the right, with decent results. It's all about how much effort you want to put into it and what you expect in return.

And by popular demand, no, there's no way to physically break out the individual channels, they're mixed inside the CPU chip and the best you can get is two audio channels per boy.

ah, well sort of bad news from NeX:

also

yeah, that makes sense, but then loses the "live mode" functionality right? ahhh, gameboy dev seems pretty tricky ;P

EDIT:  Re-read your post... that's totally doable with 4 units + 4 carts + a sync box of some kind, I thought you were referring to 4 stereo outs from one unit.

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I'd love to see that happen but I'm guessing it's one of those "if you could do it someone would have by now" things.  There's only stereo out from the sound chip itself so there's nothing to break out, all the sound is already mixed when it gets to the analog stage... right?

Please correct me if I'm wrong 'cause that's one mod that I'd consider worth doing.

so i think that at this point, the one thing can could really move forward the "gameboy sound" for both live applications and recording would be the ability to record/mix all four channels separately but simultaneously, ie. have a stereo out each for WAV, NOI, PU1, PU2. The ability to run effects to seperate channels, compress bass, put reverb or delay on leads, actually set levels... all in real-time... would be pretty much amazing.

How possible is this? Would this be something that could be accomplished break-out-box style, having a separate unit with 4 gameboy soundchips slaved to the link port of a master DMG running LSDJ or whatever? or could you just link a master DMG to 4 regular DMGs and cripple/mute the channels that you dont want on each?  i am hoping some of the dev crew can enlighten me!

maybe this is something that has even been done before, that im just not aware of?

thoughts?