Neo Geo experts: Strange Audio Issue with CMVS

[Howard_Casto]

So I'm building a supergun for a neogeo mv-1b board but I've ran across an odd issue.  When pulling line out stereo directly from the ym3016 chip (pins 10 and 11) I noticed that the coin-up sound effect wasn't playing.  What's odd is that when hooking a speaker up to the amplified mono on the jamma connector I can hear the coin up sound.  I'm getting audio from both pins but for whatever reason one or two sfx like coinup sounds in certain games just aren't going through.  I thought the ym3016 was essentially all the audio before it gets amplified.... is there another chip on the board I should be tapping into instead for the stereo mod?

Any suggestions would be appreciated. 

[keilmillerjr]

http://www.jamma-nation-x.com/jammax/tutorials.html

[Howard_Casto]

I ran across that last night, thanks.  Turns out the neogeo has a third, seldom used audio channel called the SSG(software sound generator).  It's seldom used because it can generate some square waves and that's about it.  I can't tell if the circuit on jamma x is amplified audio or not though.  The 6.8k resistors and caps make me think that it is amplified audio... looks like a voltage divider.  Surely the ssg pin is unamplified somewhere down the chain.   I don't mind building the circuit, but it seems dumb to merge the channels and then reduce to line-out levels when two of the three channels are already available at line out levels.  Maybe just build half of the circuit for the c-channel if I can't find it at line level?

[Howard_Casto]

I looked into it a bit more.  YM2610, which is on the backplane of my pcb, generates the SSG out of pin 27.  The tech specs don't specify if it's amplified or not so I'll have to check.  The audio I'm getting out of the other chip is perfect as-is, so I'm thinking if this also sounds ok at line levels I might just stick a pair of diodes on each channel to prevent cross-contamination and wire it into those as-is.  I'll get out the old multi-meter to make sure. 

[Howard_Casto]

Ok I realize I'm probably talking to myself at this point but anytime I ask a technical question I like to make sure it's answered in the post in case someone else has a similar problem.  Line "C" on the Jamma X tutorial indeed leads back to pin 27 on the ym2610.  Is it amplified?  Sorta?!?  It's slightly above line level and oddly enough can be controlled via the volume pot for the amp, but it definitely is amplified again before going to the speaker.  I hooked it up to one just to make sure and it was barely audible even with the volume pot set to max.  Even though it's above line level, it's just barely so it should be ok for my scart adapter for now.  Hooked it up to the left signal along with the digital left and the volume sounds just about perfect. 

I will eventually dig out the proper caps and resistors to be safe but really I think it just needs a couple of diodes.  I prefer the line level outputs for the two digital signals so when I do build it I'll do like I said earlier and just build the part for the SSG. 

So hey, maybe that'll help somebody.

[Titchgamer]

Sorry cant help with any of these probs howard.

But theres some clever people you can ask on the NG forums including James from Jamma Nation X

[Howard_Casto]

Well like I said, I think I have it sorted.... it's just weird.  According to docs the SSG is analog only for some strange reason and that leads to the inconsistencies compared to the digital signals I tapped into for regular left and right audio. 

Really haven't ran into any real problems on this build.  Amazed to get a piece of hardware that actually works as intended for a change. 

[Titchgamer]

First time for everything!

I have just started my supergun build to!

[Howard_Casto]

Cool.  I've been taking pics of mine... decided to do a dedicated consolization.  I'll put it up over in the console forum when I'm done. 

[Titchgamer]

Cool.  I've been taking pics of mine... decided to do a dedicated consolization.  I'll put it up over in the console forum when I'm done.

Look forward to it!