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View topic - Anyone ever put FM sound into a game gear? I'm planning to try, looking for tips

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  • Joined: 22 Feb 2016
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Anyone ever put FM sound into a game gear? I'm planning to try, looking for tips
Post Posted: Thu Jan 14, 2021 3:27 am
Finally getting around to planning a game gear build, hoping to do the consolized thing to make it functional for SMS on a TV too. The last couple things I'm looking into are an FM mod, and maybe eventually some info on the LCD glasses adapter.

I think I have the wiring and schematics all figured out for modifying an SMSFM unit, but I guess I was just hoping to talk to someone who might have done this mod before, with the SMSFM unit or without. I'm not even sure I have specific questions at this point, but I'm committed to give it a try at least and will probably update here with any progress when I finally attempt it.

I can find hints of people trying it, a message from "viletim" himself saying he had wired one of his SMSFM unit into a game gear just to test it eleven years ago, other messages on various forums of people asking for advice, but no messages/pictures/videos of finished projects.

Wiring the module into the game gear seems pretty straight forward but with a few considerations (not mentioning the space needed to fit it in there).

First, the game gear apparently doesn't have a "joykill" pin, so two games won't work without patches.

Second, the game gear is stereo and the SMSFM module is designed to mix in and pass through mono sound. I could just settle for mono sound, there's really only a handful of game gear games that utilize it well, but I think I've managed to copy down the mixing part of viletim's SMSFM board so I can hopefully add a few components to it and make it stereo. This stuff is right at the edge of my knowledge and skills though, so we'll see.

Third is exactly WHERE to mix in the FM sound. using the guide for generic SMS consoles, I followed the audio schematic and found caps that I think is where I would mix it in, on the audio board, caps C5 and C7. Although comparing to an SMS console, I wonder if before the audio board wouldn't be better, it would be easier to keep everything on the main board.

Anyway, I appreciate any advice anyone has and regardless I'll be sure to update if I manage to get things working. Thanks!
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Post Posted: Thu Jan 14, 2021 5:40 pm
I don't know how you mix the FM and PSG part but I would send the FM to both audio channels and leave the PSG as stereo instead of mixing PSG into mono...

but, again, I have no idea what this requires
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Post Posted: Thu Jan 14, 2021 9:45 pm
I could just wire it mono, which would be super easy, but like you said I'm trying to keep psg stereo while adding fm sound. The issue is that the smsfm was designed for mono sound. To get the mixing circuit working right, I have to basically double that part so one mixes FM into the right channel and one mixes it into the left.

I also posted this question in other sites and someone directed me to a page here titled "Game Gear Cartridge FM Audio?" By bsittler back in may. Last activity was around December so it's pretty recent. Don't know why I couldn't find it...

Not built into a gg, and it doesn't use the mixing circuit, but this at least confirms to me that it works like I thought it would and also tells me where to add it into the circuit.
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Post Posted: Mon Feb 15, 2021 6:23 am
Last edited by asdfqwer426 on Tue Mar 09, 2021 9:31 pm; edited 1 time in total
Well I finished up the addition. Wiring in the module is easy enough and I found the perfect place for it in the front of the system. There is no "joykill" pin, which supposedly breaks FM sound support for two games based on what I read. "times soldiers" and "wonder boy III". Although time soldiers seemed to work and wonder boy needs a region switch I haven't messed with yet so maybe the info is outdated and/or the SMSFM has been updated.

Edit: March 9, 21 - didn't think this warranted an extra post. I don't know why but the lack of "joykill" or "jyds" pin which is reportedly needed for"time soldiers" and "wonderboy iii" does not seem to effect this mod at all FM still works and is enabled by both games correctly as far as I can tell. Of course wonderboy III still has the region issue, but using a patched rom or by grounding the TH pin on the link port takes care of that.

I tested every known game and only had a couple issues. I'm still testing a couple out but other wise it works great.

To mix in the FM sound I looked closely at the game gear sound circuit and what I wanted to do with the SMSFM and a GGTV. By default the game gear is a mono console. it ONLY will output stereo when a pin in the headphone port is grounded. otherwise all sound is output on the left channel, which is output to the speaker. because of this I decided to just embrace mono and mix the FM sound into the left channel. I removed C67 from my 1asic board and mixed the sound in there. I used point T9 as the GGTV audio point, which is right after C67 in the left channel making my GGTV output mono only.


When I plug in headphones, game gear audio still works correctly, but FM sound only comes out the left. also, since the SMSFM mixing circuit is only designed with one channel in mind, it can't cut off the PSG audio for the right channel when you use headphones, so some games like outrun wind up with some extra sounds coming out the right channel in certain spots like other pre-2015 SMSFM units that didn't cut the PSG audio out (from what I've read).

While the game gear's low quality speaker and amp isn't doing the FM sound justice, the GGTV lets me plug it into a TV and really enjoy the sound. Even my wife who isn't really into older games and things commented on how great "outrun" sounded with FM.
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Post Posted: Tue Feb 16, 2021 5:27 pm
That's amazing work! Congratulations on achieving something others of us have so far only dreamt of
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Post Posted: Wed Feb 17, 2021 5:41 pm
Thanks! I did gobs of research about the FM stuff while I waited for an the parts to arrive, so when I finally sat down to do it went very quick.

I'm adding controller ports next, got one port cut out last night and hope to cut out the second tonight.

I do have one minor issue, I bet one of you regulars might have an idea what's up.

When I return to the everdrive menu after paying a game, the FM sound board if it's enabled will output a random tone or screech until I power down or pick a new game. Additionally the signal to reenable PSG audio is missed, so if I switch from an FM game to a PSG game the audio does not turn back on. I've been able to diagnose a bit with headphones, since the right ear has no FM stuff come into play.

I have pretty limited knowledge but I imagine the chips aren't getting the signal to shut down, either because there is no joykill pin, or because something in the everdrive gg just doesn't account for it.

So I guess my question is, do you experts think the lack of joykill threw off the fm module, or is it just the everdrive gg menu?
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Post Posted: Wed Feb 17, 2021 6:25 pm
there's nothing writing back to the FM chip select port (audio control port) so the FM stays enabled and the PSG disabled even if you start a new game that uses PSG
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Post Posted: Wed Feb 17, 2021 9:40 pm
Thanks sverx, I knew one of you would know the exact issue. I asked around and this doesn't happen with a master everdrive afaik, so maybe I can request krikkz add that bit of code to the gg version.
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