Sega Genesis Audio Mod Idea

Started by Richter X, February 17, 2008, 03:11:58 PM

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Richter X

After doing some searching around it seems that the PAC-S10 only outputs analog audio, which is converted to digital inside the Laseractive itself. This has me thinking. "Why does it get such good audio then?". My guess is whatever circuitry it goes through on board the PAC-S10 drastically cleans up the audio before heading to the internal ADC.

Unfortunately, the only way to find out exactly how or what it goes through is from some sort of schematic. I found one for the Laseractive itself, but not the PAC-S10 (unless I wanna get charged by the schematic hoarders). Perhaps someone could either trace the signal, or get ahold of said schematic and maybe we could find a way to get flawless Sega Genesis audio in the Model 2.

Flowchart of some kind.
Pioneer Laseractive CLD-A100 Service Manual


Tiido Priimägi

I'm the author of this mod, ask any question you want about it... I'll do my best to answer them.
Mida sa loed ? Nagunii aru ei saa ;)

timofonic

#3
Here are my questions...

How this mod works? I mean... what does for improving the sound? I just want to understand how it works your mod, maybe it could be quite useful for improving sound on other systems too.

It's possible to replace the amount of capacitors and such by using some kind of packaged equivalent? I mean some kind of IC.

Will it be possible doing it for MD1/Genesis1? If yes, do you plan to do the MD1/Genesis1 version? What about video noise on this? What's that video noise and why occour?

Do you agree that someone creates a wiki page with your mod on GameSX? And about someone drawing the schematic in a more computerish way? :)

Tiido Priimägi

Well, this mod replaces the existing rather crap audio mixing circuit of MD2. The sound is pretty much the best possible... all the 52KHz YM2612 makes comes out from my MD2 and the stereo separation is 99.9%
I have no idea how would you ICfy it... its fully analog... and there's so little components anyway...
You could use it for MD1, but you need to boost up YM2612 levels a lot, since they're lot more quiet than MD2s. And the the video noise is inevitable, the YM2612 is in the same chip as VDP is, and the sound outputs are pretty close to video outputs... and the level of the noise is so low you barely hear it in silence. Original mixing circut cuts all the higer freqs so the sound is bland and there's no such noise.
Also, 32X and MCD sound is not added yet since I don't have these !!! f anyone's willing ot donate me some (broken)HW, I'd be more than happy to accept it.

As for someone re-drawing the scematic... I'd rather let you wait until I add 32X and MCD too.
Mida sa loed ? Nagunii aru ei saa ;)

albino_vulpix

Question regarding the Mega Drive 2 audio mod: Where do I get the PSG, FM L and FM R signals?
I also noticed that on your MD, there is one BA10324AF IC whereas there's 2 in mine. IS this important?

Tiido Priimägi

I removed one of the OP Amp chips during the mad attempt at improving existing circuit (and its not required to remove it). FM L comes form pin56 and FM R from pin55 and PSG from pin 178 of 315-5660. You have to make sure that the pins aren't connected to existing circuit (a small cut on PSG line, and resistor removal for FM lines does the job).
Mida sa loed ? Nagunii aru ei saa ;)

duo_r

TmEE can you list the parts here because I had a little hard time reading them on the handwritten diagram. This is a very interesting project! So it is still analog audio but cleaned up?

Is the audio pre-amped and you are amplifyiing it? Would you be willing to look into a similar mod for the Turbo Duo?

Tiido Priimägi

I have got the mod worked out fully, I haven't updated the schematic yet though. I will add it in this site's Wiki when I have few problems in real life taken care of.

It is all analog and the final output is going tol be around 40% more quiet than before. Its no real problem as the gain in audio quality is totally worth it IMO.

As for other game consoles, I need the system first before I could do anything... so far I've only got Sega stuff. I am gonna work out something on SNES when I get some controllers and cables. I will probably work out something for Saturn if there is anything to do... for example, MegaCD needs no mod, it sounds good enough already.
Mida sa loed ? Nagunii aru ei saa ;)

duo_r

#9
before I do mods I like to make sure I go all the parts clarified. Please confirm this as the part list:

Crystal Clear Audio Mod for Sega Genesis V.3
tmeeco@gmail.com

6 X C945 (any NPN will do)
3 X 4.7 kΩ
3 X 100 kΩ
2 X 22 kΩ
2 X 2.2 kΩ
2 X 200 Ω
2 X 330 Ω
2 X 470 Ω
2 X 10 kΩ
2 X 10/15/22/33 kΩ (and a note that use any resistor, smaller size preferred)
(1 X 0.1 uf ceramic cap)

I didn't see anything on the wiki yet......think u could throw up more of a tutorial?

Questions - where am I pulling psg, L and R audio from on the board? I saw mention of something needing to be disconnected - any more details?

Do you feed this to RCA's or back to the Av port? Some tips on how you do that?

Tiido Priimägi

You have correct part list. The C945 transistors "real" marking would be 2NC945.

A good schematic of the mod :
http://www.fileden.com/files/2008/4/21/1876835/CCAM.png

Where to put your wires :
http://www.fileden.com/files/2008/4/21/1876835/MD2SCAN.jpg

How it would look :
http://www.fileden.com/files/2008/4/21/1876835/MD2SCAN2.jpg

In that particular MD2 on the scans, the audio ends up in the 2 audio pins on the AV out, note that mono audio out will be missing, so mono AV cables and RF modulator will not have any sound. On one MD2, I did use RCA out, and on my primary use MD2 I have it on a DIN5.

http://www.epicgaming.us/tiido/ACDCMD2/ACDCMD21.JPG
http://www.hot.ee/tmeeco/MYSMD2.JPG

The resistors on the audio out lines could be omitted (the 10/15/22/33Mohm), you need to experiment with your equipment, if anything gets distorted sound without the resistors, then you need some. I need to work out some better final ampplification/separation in the mod...

I intend to add this to the Wiki at some point, but right now I've got school to worry about...
Mida sa loed ? Nagunii aru ei saa ;)

duo_r

#11
Thanks a ton, that really helps. Now I have a US Genesis, I assume it will be the same? I think there are some differences on the MB from Mega Drive and Genesis. What was your total part cost?

Also, any chance u could take some higher res photos (and brighter) of your mod? Little hard to see. Thanks so much for the update!

Quote from: Tiido Priimägi on December 15, 2008, 11:44:36 PM
You have correct part list. The C945 transistors "real" marking would be 2NC945.

A good schematic of the mod :
http://www.fileden.com/files/2008/4/21/1876835/CCAM.png

Where to put your wires :
http://www.fileden.com/files/2008/4/21/1876835/MD2SCAN.jpg

How it would look :
http://www.fileden.com/files/2008/4/21/1876835/MD2SCAN2.jpg

In that particular MD2 on the scans, the audio ends up in the 2 audio pins on the AV out, note that mono audio out will be missing, so mono AV cables and RF modulator will not have any sound. On one MD2, I did use RCA out, and on my primary use MD2 I have it on a DIN5.

http://www.epicgaming.us/tiido/ACDCMD2/ACDCMD21.JPG
http://www.hot.ee/tmeeco/MYSMD2.JPG

The resistors on the audio out lines could be omitted (the 10/15/22/33Mohm), you need to experiment with your equipment, if anything gets distorted sound without the resistors, then you need some. I need to work out some better final ampplification/separation in the mod...

I intend to add this to the Wiki at some point, but right now I've got school to worry about...

viletim

Tiido,

Instead of replacing the mixing circuit entirely, have you tried to make it more linear by modifying some components? I traced out the audio circuit of my SMD II. There are highpass and low pass filters in place which would give an overall bandpass effect.

Your circuit has some problems, the worst one being the output impedance is far too high. It's even higher than the input impedance of some audio amplifiers! Lower the output Z to < 1k and you'll reduce the noise level (which must be relatively high at the moment).

I had a listen to the audio samples at:
http://arcade.ym2149.com/megadrive/index.html
and it does sound much better without the filtering. But is this always the case? I presume all Mega Drives have the filter and it must have been taken into account by the programmers.

Tiido Priimägi

#13
I've played a ton of games and none of them have suffered from clean sound output... man reason for the mod is to get more or less 100% channel separation and sound that's free of distortions (just listen the before and after : http://www.sega-16.com/forum/showpost.php?p=79831&postcount=60 ).
I tried to modify the original audio mixer, but I was not really able to get the sound free of distortion and the stereo separation problem still remained. Your schematic is much nicer than somethign I drew ages ago (BTW, how do you know what capitance those surface mount caps have ?)

As for my thing, I cannot really comment on the amount of noise, I have no o-scope, and there won't be more noise than if there were with nothing attached to the (6 meter) cable prior to doing recording... the output level is quite ab it more quiet than regular line levels... I'm not very good at analog stuff and my attempt at building something that should give better amplification have ended up with no improvement...
The resistor values have come from extensive trial and error, if you change anything, you would get poorer sound...

Quote from: duo_r on December 16, 2008, 06:12:56 AM
Thanks a ton, that really helps. Now I have a US Genesis, I assume it will be the same? I think there are some differences on the MB from Mega Drive and Genesis. What was your total part cost?

Also, any chance u could take some higher res photos (and brighter) of your mod? Little hard to see. Thanks so much for the update!

MD/Genny boards with same model number are near identical, only differences being in region signals grounded/VCCed differently in each region. There can be differences in the RGB encoder chip area aswell, but that depends on which chip is used.

To build the mod, it costs me maybe 10$ on parts (including PCB, which apparently is so big you can make 20 mods with it).

I have some older pics which should be enough to make this tiny 10x10 hole PCB yourself, you just have ot be careful when doing the solder bridges there, it is easy to do wrong bridges, and then its pain to get them undone....
Mida sa loed ? Nagunii aru ei saa ;)

viletim

Quote from: Tiido Priimägi on December 16, 2008, 08:23:03 PM
I've played a ton of games and none of them have suffered from clean sound output... man reason for the mod is to get more or less 100% channel separation and sound that's free of distortions (just listen the before and after : http://www.sega-16.com/forum/showpost.php?p=79831&postcount=60 ).

Wow, the distortion on the original is really aparent! I'll have to do some attentive listening next time I play around with my Mega Drive. Does it do that on both model 1 and 2?

Quote
Your schematic is much nicer than somethign I drew ages ago (BTW, how do you know what capitance those surface mount caps have ?)
I just pulled them out and measured them with my little L/C meter.

[/quote]
As for my thing, I cannot really comment on the amount of noise, I have no o-scope, and there won't be more noise than if there were with nothing attached to the (6 meter) cable prior to doing recording...
Quote
That's just it. Notice that when you plug the same cable into some other audio device (eg, a cd player), the noise level will be much lower than it would be with a floating cable. You have a common emitter transistor amplifier at the output of your circuit. Its output Z is equal to the collector resistor. The only practical way to reduce the output Z is to add another emitter folower stage. Probably a better idea to just redesign it with an op-amp.

Quote
the output level is quite ab it more quiet than regular line levels... I'm not very good at analog stuff and my attempt at building something that should give better amplification have ended up with no improvement...
The resistor values have come from extensive trial and error, if you change anything, you would get poorer sound...
I was afraid of that. Your biasing scheme (ie. the way you keep all the transistors in their linear regeon) is very fragile. Sega's mixing amp has coupling capacitors at three stages while yours is DC coupled all the way through. A coupling cap at the input along with a voltage divider bias would make your amp a lot more stable. You'd sacrifice the input Z, but it probably doesn't matter much in this case.

duo_r

#15

sorry if this is a dumb question, but does the Sega CD improve the audio output? I was listening to a game hooked up to the Genesis 2 directly, and it really sounded poor. It was a CD game Lunar 2, I could not believe the scratchiness. I then hooked the audio output to the Sega CD audio out and the sound was much clearer (makes sense, direct output). Does the Sega CD output almost eliminate the need to mod a Genesis 2? Sorry if that is a dumb question.

Tiido Priimägi

Quote from: viletim on December 16, 2008, 10:18:48 PMWow, the distortion on the original is really aparent! I'll have to do some attentive listening next time I play around with my Mega Drive. Does it do that on both model 1 and 2?

MD1s except the very last one (the VA7 board) have relatively good sound output, but it does vary from model to model, same boards are not quaranteed to sound same. VA7 is a MD2 in MD1 shell basically. MD1s main problem for me is the too filtered sound, YM2612 own output it as crisp as what you can hear from emulator (except in case of MD1, you have a lot of aliasing noise, MD2s have better implementation of YM2612 in their ASIC which is much more accurate).

QuoteI just pulled them out and measured them with my little L/C meter.

Something like this would become handy...

QuoteThat's just it. Notice that when you plug the same cable into some other audio device (eg, a cd player), the noise level will be much lower than it would be with a floating cable. You have a common emitter transistor amplifier at the output of your circuit. Its output Z is equal to the collector resistor. The only practical way to reduce the output Z is to add another emitter folower stage. Probably a better idea to just redesign it with an op-amp.

I don't get decrease in noise when I plug in anything else, the noise is around -60db and stays so.
I am actually thinking of using some op-amp (probably something dedicated for audio, maybe an headphone amp) to get the output level to line levels and don't have to worry about distorted sound when using some devices...

QuoteI was afraid of that. Your biasing scheme (ie. the way you keep all the transistors in their linear regeon) is very fragile. Sega's mixing amp has coupling capacitors at three stages while yours is DC coupled all the way through. A coupling cap at the input along with a voltage divider bias would make your amp a lot more stable. You'd sacrifice the input Z, but it probably doesn't matter much in this case.

The YM2612 output is very very easy to distort, it took me quite a bit of time to get good resistor values for the transistor so as to have no distortion in sound... the input Z must be high ???.

Quote from: duo_r on December 17, 2008, 10:16:45 AMsorry if this is a dumb question, but does the Sega CD improve the audio output? I was listening to a game hooked up to the Genesis 2 directly, and it really sounded poor. It was a CD game Lunar 2, I could not believe the scratchiness. I then hooked the audio output to the Sega CD audio out and the sound was much clearer (makes sense, direct output). Does the Sega CD output almost eliminate the need to mod a Genesis 2? Sorry if that is a dumb question.

MCD won't improve MD2s audio, only thing that benefits is the MCDs own sound as it does not go through the not so good MDs mixer.
And another important note about the mod : All audio will come from MDs out, when you use MCDs out, you only have MCD sound. When I'll add some better output stage for the mod, you can route its output to MCDs input signals on MCD connector... if you do it now, the MD sound will be too quiet compared to MCD sound. I've not done too much comparsion between MCD sound through MD and MCD out, but by just listening, there's no loss in quality for MCD sound, no filtering happens and the sound is still crisp and clean. I have a recording somewhere, I'll dig it up...
Mida sa loed ? Nagunii aru ei saa ;)

Segasonicfan

Wow, this is really quite an extensive audio mod.  Very good stuff!  I like viletim's schem since it uses opamps for everything which is very nice for audio since there are a lot of great ones to choose from now (thanks to TI's newer OPA series of opamps).  My question is has anyone tried this on a CDX?  I am looking at my mobo now but I'm not spotting the YM2612 which is a  odd since the CDX is a MD2 in almost every way.  I guess it could be incorporated into a custom chip, but the main processor is still the same (315-5660).  Anyone looked into this already?

Tiido Priimägi

#18
YM is inside the 5487/5660/5700/5708, aswell as VDP, PSG and I/O management and audio is mixed with 315-5684, I need to make some pinout of the chip, based on schematics of nomad and 32X I drew sometime ago.

Viletim's schem is how things are in original config in MD2... if you replace the opamp with something with higher bandwidth and adjust filtering stuff, you should be able to get nicer sound of the setup. I prefer to use a transistor as a buffer for YM signals, since you can easily have some 250MHz transistor and no need to filter the outpout of YM directly (which is the cause of distortion too, since the cap will overload the YM's very weak output).
Mida sa loed ? Nagunii aru ei saa ;)

Segasonicfan

That would be really great if you could make some pinouts/develop this audio mod for the 315-5684.  The CDX is my Genesis console of choice to this day due to it (still) being the smallest "full spec" Genesis with Sega CD/32X/SMS support.   
I still have to overclock and import mod mine and some clean audio with that would be phenomenal.

-Segasonicfan