Sound inputs in the SNES cartridge port.

[MaxWar]

Does anyone know how those inputs are supposed to work?

I am talking about the pins #31 and 62
Source: http://www.caitsith2.com/snes/flashcart ... nouts.html


According tho the page Linked above:

Audio Inputs - whatever the cartridge puts on these lines will be mixed into the SNES's audio output.

I have yet to get this to work though. Tototek AD adapter output sounds to these pins, i know for a fact. It is standard line level audio. However It does not get mixed to the output to the AV port. I tried this on 2 different SNES.

I have also tried connecting the output of the AV port directly to those pins to get the sound of the AD adapter to output and it seems to work without a hitch. But this would effectively feed the output of regular SNES games back to these inputs when using a standard SNES game, However I hear no impact on the sound of regular SNES games.

Maybe this mixing circuit needs to be activated somehow? I also doubt this is digital audio since there is left and right.

So in summary I have 2 main questions:
-How is this features used/activated?
-Any known examples of cartridges that use this feature?

[lidnariq]

The redrawn schematic by jwdonal shows those all six audio channels (Left and Right each of EXT, CART, and SPC700) being mixed explicitly into the final stage. And the SPC700 has to stop asserting MUTE. If that doesn't work, maybe try hunting down the analog output stage and making sure the traces have continuity?

[Near]

> -How is this features used/activated?

Unmute the S-DSP first, otherwise you get no audio.

> -Any known examples of cartridges that use this feature?

Super Game Boy (mixes with Game Boy audio)
BS-X Satellaview (mixes with streaming satellite receiver audio) [technically through the expansion port]
MSU-1* (mixes with redbook audio)

(* MSU-1 is implemented in the sd2snes by ikari.)

[MaxWar]

First of all, a big Thanks for the replies!! You guys are a great help.
This schematic rewrite is also great, A big improvement over those barely readable originals.


I checked the /MUTE pin of the DSP and it is indeed High during normal operation but it is low when the AD adapter is used.

I am trying to figure the best way to switch it high when Im using the AD adapter. I do not think I can just force it high, it would probably just short the system.
Here is the concerned part of the schematic: (/MUTE at the transistor base connects to the DSP )


I am not any kind of electronic engineer so I am not familiar with this specific type transistor usage.
I am familiar with transistors as common collector AMPS and switches, but with the switched device connected to either C or E. There its connected to Base and I do not understand how it works yet.

I could always cut the Mute line right at the pin and switch it but there must be a nerdier/cooler/better way of doing this. Any ideas?

(EDIT, I just realized that there is 33k resistor between the base and the pin, so I guess I could try to force it high, but then would not the 4.3k pull up resistor force it high all the time since since its much bigger then the 33k?? What kind of sorcery is this!)

EDIT 2, I measured resistance between pin 39 of the DSP and ground and +5v, gets in the MegaOhm range with both. That does not add up with the Schematic. Maybe my SNES revision is different, Its a GPM-02 .

[lidnariq]

Ideally, you should have a little piece of software that the SNES uploads to the SPC700 that will disable the mute.
There's probably enough information at wiki.superfamicom.org to piece that together:
SPC 700 Reference, Transferring Data from ROM to the SNES APU

Oh, and in the case you've circled, the transistor is just serving as an inverter. (You could also think of it as a common-emitter amplifier, but when the input is digital, that's a bit misleading)

When MUTE is asserted, the second and third transistor (Q14, Q15) short out the adjacent resistor and reduce the gain of the adjacent opamp.

If you really want to break your SNES to make this work, the cleanest thing would be to desolder R49. But be careful; that part may have moved or changed number between revisions of the SNES, I don't know.

[MaxWar]

Wait, damn, then the /MUTE pin is an output?!
I somewhat Assumed it was an input. But hey that makes more sense now.

Btw I would like to find a non-software solution ideally.
Going to sleep on this, night time!

Thanks again for the help.

Edit : I have no problem with desoldering parts and moddifying this SNES. Its already quite modded, btw I supposed I could give you more info on what im working on.
I finished the Video part of my project, now working on sound)
http://www.assemblergames.com/forums/sh ... e-AV-cable

Edit 2: ( no im not asleep yet) I think that the simple way would be to just feed +5v at the base of the transistor with a switch and a pull up. the 33k would protect the DSP chip. ( I already have an automatic switch in my mod, it would be dead simple ) What do you think?

[lidnariq]

Well, the SNES is already running some code to relay the data from your controllers to the GBA in the adapter. So I think the "best" solution would be removing the SNES program IC (labelled 29LV040TC-70 in the picture on Tototek), dumping it, and putting in a little stub that fixes the mute behavior.

Actually, you should be able to dump it without desoldering it using a Kazzo or something. And you might be able to flash it without desoldering it, too, but that requires a better picture of the board than Tototek themselves provided.

(As an aside, huh, 512KiB is a lot of data for the little shim they have there. I wonder what all of it is?)

And I'm not exactly certain why they didn't do that by default. I guess they figured there's no advantage to being able to get sound through the SNES but not video, so they may as well force you to plug in a full set of cables instead of just video.

Regarding your "add a switch" question: Yeah, I guess that would work. It'd be nicer if it was provided by some IC that detected when the Tototek AD cartridge was plugged in. And equally effective would be putting your switch in series with R49.

[MaxWar]

Well, the SNES is already running some code to relay the data from your controllers to the GBA in the adapter. So I think the "best" solution would be removing the SNES program IC (labelled 29LV040TC-70 in the picture on Tototek), dumping it, and putting in a little stub that fixes the mute behavior.

Actually, you should be able to dump it without desoldering it using a Kazzo or something. And you might be able to flash it without desoldering it, too, but that requires a better picture of the board than Tototek themselves provided.

(As an aside, huh, 512KiB is a lot of data for the little shim they have there. I wonder what all of it is?)

And I'm not exactly certain why they didn't do that by default. I guess they figured there's no advantage to being able to get sound through the SNES but not video, so they may as well force you to plug in a full set of cables instead of just video.

It seems to me the Adapter was designed to play on modern clones like the Retro Duo. When on a real unmodded SNES, the sound sucks terribly, its weak and thin. It gets drained by dual 200 ohm pull down resistors on the Cart connector audio input and does not even get amped.

To make matter worse, the output is mono, mirrored to stereo. The adapter shorts the two sound input pins of the SNES together when the adapter is inserted, which makes the two pull down resistor twice as effective as they are now in parallel. In other words you are dumping a good part of your audio to ground.

I tried completely isolating adapter audio output from the SNES as a test, and damn it sounds MUCH MUCH better.

[lidnariq]

Huh, the GBA is stereo, though. Does their clone not support it, or did they just munge up the wiring?

[MaxWar]

Huh, the GBA is stereo, though. Does their clone not support it, or did they just munge up the wiring?

There seems to be only one sound trace on the adapter board, that goes through a Y just before the output.
Cannot say if it has untapped stereo capability or not. But yeah this sucks..

[MaxWar]

I tried my idea of putting a switch on the base of the Transistor ( IT was Q13 and not Q15 on my board revision )

It works well, the sound of the AD adapter gets mixed in but it still sounds bad. Maybe its a little better than default sound out of the adapter on a SNES, but not as good as when the sound output is completely isolated from the SNES. I'll have to think of a more invasive solution I guess.

Maybe I could try removing the pull down resistors on the inputs, or replace them with higher value, but then how would that affect a regular game boy player uh?

[lidnariq]

Maybe I could try removing the pull down resistors on the inputs, or replace them with higher value, but then how would that affect a regular game boy player uh?

Hard to say. Even looking at scans of the SGB it's not clear exactly what they're doing in the output stage. Most likely, it'd get louder, and maybe so loud it would clip.

It seems likely that the right option would be to provide a current buffer on the cartridge audio output. How hard would it be for you to put a simple amplifier in the cartridge? I don't know, maybe a single transistor common-collector amplifier would work here.

[MaxWar]

Maybe I could try removing the pull down resistors on the inputs, or replace them with higher value, but then how would that affect a regular game boy player uh?

Hard to say. Even looking at scans of the SGB it's not clear exactly what they're doing in the output stage. Most likely, it'd get louder, and maybe so loud it would clip.

It seems likely that the right option would be to provide a current buffer on the cartridge audio output. How hard would it be for you to put a simple amplifier in the cartridge? I don't know, maybe a single transistor common-collector amplifier would work here.

Its weird, being connected to the SNES makes the adapter sound not merely weaker but also thinner. There is like No bass left at all. So Im wondering if simply amping the signal in the adapter would be sufficient.

Also, I would Ideally like to keep the modification inside the SNES, if possible. Anyway, Just for the occasion I just finished repairing a Super Game boy I had gotten in a game lot. Turns out some other toy had leaked battery acid on it and I had to repair a dozen traces, but it works now. My only GB game is tetris but that should be enough to experiment with changing resistor values.

[lidnariq]

Its weird, being connected to the SNES makes the adapter sound not merely weaker but also thinner. There is like No bass left at all. So Im wondering if simply amping the signal in the adapter would be sufficient.

Well, the audio path, as far as we can see it, in the SNES is something; 100 ohms to ground, 10k resistor and 1µF capacitor. That would normally just look like a highpass filter inside the SNES at 16 Hz, so that can't be it.
There's got to be a capacitor in series with the audio trace on the GBA side; tracing through your images it looks like it's C35 or C36. They need to be bigger, to reduce the highpass effect.

[MaxWar]

I made some interesting discoveries.

First, I did try removing the pull down resistors on the SNES audio input side, they are 200 ohms to ground actually.

It then tried both the Adapter and the super gameboy and it was not as bad as we expected I guess. Removing those pull down resistors really seems like it makes a bass boost + volume boost. I even kinda liked it with the Super game boy, I mean, Tetris Bass boost was sort of a refreshing novelty.

With the AD adapter, bass was equally boosted, maybe a bit too much.

I then tried with 1000 ohm pull downs and it felt about right. It was cool with Super game boy, sounding more badass than usual. With Ad adapter though, it was still a bit thin, because they count as 500 ohm instead of 1000. And The adapter is not designed for that to begin with. I think its designed to for straight output.

But that is not all, I then opened the AD adapter and tested every parts on the AD adapter to find where the hidden audio trace was going and I found it.
It goes to a pair of caps, where it then splits to two traces that seem to bury under the glob top. ( STEREO ?! )
Those caps are C28 and C29.



I was ultra excited that maybe this thing had stereo after all. I removed the caps and soldered the pads to a temporary RCA outoput and tried it.
They are definitely two separate channels it would seem, one has more noise than the other for some reason. But they still output the same waveform, I checked.

It seems like they actually intended stereo with the adapter. But possibly they had some issues and they deactivated it. Maybe it can be reactivated with a pin...
There are several empty part spots on the board; resistors, transistors, caps. I wonder what else this adapter could be hiding.

This is the version 1.0 board. Maybe the Super Retro Adapter is a better version? I am tempted to get one... But I am risking getting the same board twice... There are no reviews of PCB pictures of it yet and it annoys me...