Gameboy Development Forum

(Hey, I wrote that article on the 8bc wiki )
The 5th channel is an external sound input from pin 31 on the cartridge port. It is there so you can place a synth chip or sample playback chip on a cartridge. However, sound cannot be sampled through this channel, it can only routed. As such it's like the other channels, it can be hard panned to the left or right, or muted or playing on both channel. The best you could as a guitar effect is a gate or a simple (And very CPU-intensive) ring modulator by turning the volume on and off many times per second. Though I imagine that input on this channel will be pretty noisy, and that there will be some voltage spikes when turning the sound on or off for the channel.

Both your questions were actually answered in my previous post.
Clarification: Routing in this case means that there's one input, which you can choose to route to the left or right channel, or both, or neither.
As for effects, the simplest thing you can do is a gate, ie turn the sound hard on or hard off. This can be done rhythmically. Or, by turning it on and off 440 times per second, or any frequency you want, you get a simple ring modulator. If you dedicate the left or the right channel to this, you can even use the 7 step volume setting for the left or right channel to make more complex waveforms than a squarewave for this ringmod thing. This will require extreme precision in timing though!
But it's an interesting idea to say the least.

Don't worry, this forum was created for discussions like this one.
The advantage would be better sound quality. Even though nobody wants to admit it, the sound quality of channel 3 is mediocre for sample playback with its 4-bit sound and the sample playback bug that adds a tonal component to the samples. (Especially on GBA's) A synth chip or a 16-bit or even 8-bit sample playback chip would open a whole new world of possibilites. Hmmm, maybe someone ought to design an OPL tracker for the GB with an OPL chip on the cart.

Sampling means to give the ability for a GB program to read and record the sound. Which this channel can't do. This also eliminates most possibilities to modify the sound as well.
What this channel does is just forward a sound signal and act like a simple mixer. You could just as well connect a microphone to that pin for all I know.

nitro2k01 wrote:

. Hmmm, maybe someone ought to design an OPL tracker for the GB with an OPL chip on the cart.

So I wasn't so damn fool thinking it was possible (thoug I tought maybe using the third channel). So it could be possible linking an OPL3 (FM synth) mix it all with the GB and  for a pure heavenly sound?

I don't know if I'll be able to find the time/resources, but I'd love to help build a prototype OPL cartridge for the gameboy. Knowing that there is a definite interest out there, it gives me some motivation to continue research!

The one thing that's been bugging me is that OPL-2, OPL-3, and even POKEY all require clock rates that are *slightly* different than what I could get by multiplying the CLK pin (4MHz / 4 = ~1MHz). Of course, it just means that one would have to calculate new frequency tables for the various notes, so it's not impossible. The problem with the OPL chips is that I think they both require their own special DAC to output analog, so that takes up a lot of room on the cart. I think the POKEY doesn't need any external DAC and will gladly take ~1MHz clock, so it may be a nice thing to make a test run with to see if it's possible.

I'd really like to have an OPL-3 cart, as it would be much more fun and convenient than hooking up my old DOS computer!

I've been poking around, and it looks like the voltage issues can be solved with a simple op-amp. Ideally one should also have a direct output from the OPL-3's DAC if they wanted to mix it separately. It looks like the 14MHz clock is a requirement for the OPL-3, so that will take up a bit of room, but not much.

I'm still trying to figure out what format the OPL-3 outputs to its DAC (sadly the OPL-3 requires an external DAC, unlike, say the YM2610, which has it built-in). Hopefully I'll have some time to make some sort of parallel port interface to an OPL-3 for testing interfacing, clock stuff, and output levels - should be able to be bread-boarded pretty easy aside from the OPL-3 chip.

Perhaps most of these solutions could be found in the VRC-6 and VRC-7 carts that Konami used on the Famicom?

Curious, if I were to design an OPL cart, would someone be willing to write software for it?

Another thought hit me the other day, although it seems that no one is interested in this forum (sadly).
If anyone is familiar with the Alpha Mod for C64  -http://www.bigmech.com/misc/c64mods/ - then you will know of the feedback mod. This takes the sid's audio out and feeds it back to the sid's audio in, this creates a loop which is regualted by a pot. Some great sounds can be acheived through this mod with the possibility of self oscillation (on the c64).
Ultimatly wondering if a similar modification would be possible on the gb. But then c64 has an actual sound chip, gb does not. Just thought of it with mention of audio I/O.  From volume pot to Vin? not crash hot on gb hardware soz