[YM2149] STF Stereo Modification

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[YM2149] STF Stereo Modification

Postby Zorro 2 » Thu Nov 27, 2003 8:51 am

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                        ST 520/1040 - Stereo Modification                     
                        ---------------------------------                     
                                                                               
Introduction.                                                                 
-------------                                                                 
                                                                               
     If you have a stereo sound monitor, e.g. the Philips CM 8833, you can     
modify the sound output of the ST to provide a true stereo signal.  The sound 
chip, YM2149, has three independent sound channels, A,B and C. In the         
production ST these are simply joined together, hence the monophonic or single
channel output signal to the monitor (or TV via the modulator in the STM or   
STFM).  If you are highly experienced at electronics and are willing to modify
your ST's circuit board, you might like to attempt the following work.  You   
will need to cut tracks on the board.  Obviously the internal metal screen has
to be removed as well.  If your machine is still under warranty, this will     
certainly invalidate it.  I take no responsibility for any damage you cause to
your machine.  It is entirely your responsibility if you decide to undertake   
the work.                                                                     
                                                                               
The circuit.                                                                   
------------                                                                   
                                                                               
     A circuit has to be assembled on a small piece of circuit board.  I used 
a short strip of veroboard. The circuit is needed to place the three sound     
channels in different positions in the sound field.  Channel A will come out   
on the LEFT, channel B in the centre (i.e. fed to both LEFT and RIGHT equally,
this is termed panning) and channel C on the RIGHT.  The result is a true     
spread of stereo sound between stereo speakers or headphones.                 
                                                                               
     I have not given a component layout or a list of components since there   
are so few.  The total cost should be less than 2 pounds and some of you might
be able to build it from your spares box.  The circuit is given below.         
                                                                               
                                                                               
  YM2149   pin 4 >----------------:                                           
                                  |                                           
           pin 3 >----------------(---:  :---------------------------> left   
                      :-----------(---(--:                                     
          pin 38 >----(-----------(---(--------------O---[  ]-----O--> right   
                      |           |   |              |   10k      |           
                      O---[  ]----O   :----:         |            |           
                      |   10k     |        |         |            |           
                      O-----[  ]--(--------O---------(---[  ]-----O           
                      |     10k   |        |         |   10k      |           
                      |           |        |         |            |           
                     [ ]8k2      [ ]1k    [ ]1k     [ ]1k        [ ]8k2       
                     [ ]         [ ]      [ ]       [ ]          [ ]           
                      |           |        |         |            |           
                      |           |        |         |            |           
           pin 1 >----O-----------O--------O---------O------------O-->earth   
                                                                               
                                                                               
       [  ]  or  [ ]               ( bridge, no join      : bend               
                 [ ]  resistor                                                 
                                   O join                                     
                                                                               
                                                                               
                                                                               
ST modifications.                                                             
-----------------                                                             
                                                                               
     The circuit board around the YM2149 has to be cut carefully to separate   
the three channesl. Pins 3,4 and 38 are joined. 3 and 4 are joined at the pins
on the top of the circuit board (chip side). Carefully scrape away the join.   
On the underside there is a strip joining pins 3 and 38. Scrape a gap in that 
too.  The last one is more difficult.  A track leaves pin 38 and goes across   
the board to join to the negative end of capacitor C12 (on the 1040 at least )
which is the coupling capacitor to the sound output amplifier. It is situated 
near the monitor socket.  If you trace its path, you will find it travels     
underneath the YM2149, between the rows of pins (underside of the board). This
is a convenient place to break the circuit to the mono (normal output).       
                                                                               
Fitting the supplementary board.                                               
--------------------------------                                               
                                                                               
     The extra circuit should be connected by short flying leads to the pins   
of the YM2149;  the appropriate leads to pins 1,3,4 and 38.  The stereo       
outputs LEFT, RIGHT and EARTH should be taken by screened leads to the monitor
output socket.  I have broken the tracks at the 13 pin DIN socket so that pin 
1 (the normal mono sound output) becomes the left sound channel and pin 2 (the
composite video output - not used since a stereo monitor is being used!)       
becomes the right sound channel.  Obviously you will have to make up a stereo 
monitor lead too.                                                             
                                                                               
And Finally!                                                                   
------------                                                                   
                                                                               
     The stereo modification will give you glorious stereo on music and sound 
effects, where the programmer has made good use of the YM2149's three         
channels. On some arcade games the effect is stunning!  However, you will find
that sound samples will give very hissy results.  This is because some         
samplers use the individual channels to take parts or bites(!) of the sound   
waveform.  Therefore the sound channels must be joined or added together i.e. 
you must revert to the channels being joined together!  What you need is a     
stereo/mono switch.  This is easily done.  Wire a two pole two way switch so   
that pin 3 can be joined simultaneously to pins 4 and 38 of the YM2149.  With 
the switch one way, the pins are separate (stereo) and the other way the are   
joined together (mono, for sampled sound effects).  Programs that use ST       
Replay will need to be played in mono.                                         
                                                                               
Good luck!  Copyright of Derek Green 1988. For use in public domain, NOT TO BE
SOLD for commercial gain.                                                     
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Postby Slammis » Sat May 15, 2004 10:29 pm

I have a STfm that i modified with this circuit aboiut 10 years ago, and it sounds grat, as loch as you dont play mods or samples on it :-)

/Slammis

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Postby Zorro 2 » Mon May 17, 2004 7:56 am

Slammis wrote:I have a STfm that i modified with this circuit aboiut 10 years ago, and it sounds grat, as loch as you dont play mods or samples on it :-)

/Slammis

Only in stereo MOD... but the STF cannot output clean song digitalised.
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Re: [YM2149] STF Stereo Modification

Postby qq1975b » Wed Mar 04, 2015 10:37 am

I know this is an old topic but I have a question...

On STe, DMA channels are stereo but YM2149 still reproduces mono sound. Is it a good idea to do this mod to an STe? So...you can have DMA and YM sound both in stereo....

EDIT: Here it is well explained too http://rdemming.home.xs4all.nl/Atari/StereoST/
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Re: [YM2149] STF Stereo Modification

Postby qq1975b » Sun Mar 08, 2015 10:36 pm

Besides....on STe, can one use the rca outputs? And just install a switch to activate/deactivate ym stereo sound?
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Re: [YM2149] STF Stereo Modification

Postby Cyprian » Sun Mar 08, 2015 11:21 pm

qq1975b wrote:I know this is an old topic but I have a question...

On STe, DMA channels are stereo but YM2149 still reproduces mono sound. Is it a good idea to do this mod to an STe? So...you can have DMA and YM sound both in stereo....

EDIT: Here it is well explained too http://rdemming.home.xs4all.nl/Atari/StereoST/

I'd like to have YM and DMA in stereo in my STE
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Re: [YM2149] STF Stereo Modification

Postby qq1975b » Mon Mar 09, 2015 8:23 am

Yes...it would be great. Just make YM2149 stereo but adding a switch to avoid sound problems with samples and I would like to know if it is possible to use the already built RCA outputs of the STe. What differences on the circuit diagram must be done?
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Re: [YM2149] STF Stereo Modification

Postby Robson » Fri Apr 03, 2015 3:12 pm

I'm about to do this mod:
http://rdemming.home.xs4all.nl/Atari/StereoST/
but I'm confused. I'd like to use fixed value resistors, however this post says cut all the pins 3,4, 38 but on the schematics 4 goes back to the board directly (monitor audio), therefore I don't think it needs cutting.
I might not understand something so please shine some light on me. If somebody has a working schematic with mono/stereo switch that would even be better. Thanks.
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[YM2149] STF Stereo Modification

Postby catmando » Fri Apr 03, 2015 5:07 pm

Maybe you could see if you can source one of these http://www.atari-wiki.com/index.php/Tweety_Board

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Re: [YM2149] STF Stereo Modification

Postby Robson » Fri Apr 03, 2015 7:43 pm

That is possibly harder to find nowadays than do the mod. I think I figured it out, so I only need to test if my solution works. Thanks anyway.
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Re: [YM2149] STF Stereo Modification

Postby qq1975b » Fri Apr 10, 2015 12:04 pm

Are you doing this mod on a STF/m? or STe?
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Re: [YM2149] STF Stereo Modification

Postby Robson » Fri Apr 10, 2015 3:39 pm

ST FM.
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Re: [YM2149] STF Stereo Modification

Postby Robson » Sun Apr 12, 2015 4:09 pm

I have built and just tested the circuit below. Sorry for image quality and drawing but somebody might find it useful. The thing at the bottom is the switch.
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Re: [YM2149] STF Stereo Modification

Postby qq1975b » Mon Apr 27, 2015 6:45 am

From Robson:

stereo mod.jpg


If I connect the L R and GND wires to the back stereo RCA ports on the STe...will it work? Or do I need another stereo out port? Like on Robson's Atari Black 4160 STf beast mod :) He used an STfm but I would like to install the mod on an STe to make it full stereo (YM+PCM sound).

thanks.
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Re: [YM2149] STF Stereo Modification

Postby Robson » Mon Apr 27, 2015 8:11 am

Checking the schematic of a 1040STE it has different sound chip (LMC1992) so this mod will not work for those machines but do you really need this or I missed something? It seems to me that it is stereo already, you've got Left and Right jacks.

If you want a stereo 3.5mm jack (same thing i used) i guess you can just feed the signals from the RCA outputs to it. Or use a phono to 3.5mm jack cable.

Capture.PNG
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Re: [YM2149] STF Stereo Modification

Postby qq1975b » Mon Apr 27, 2015 8:31 am

The STe plays stereo sound when playing PCM sound but still uses mono out (on both L&R) when using YM chip....that's why I wanted to do the stereo mod.
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Re: [YM2149] STF Stereo Modification

Postby Robson » Mon Apr 27, 2015 9:16 am

deleted.

I'll review it later and give proper feedback.
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Re: [YM2149] STF Stereo Modification

Postby qq1975b » Mon Apr 27, 2015 9:46 am

Ok, post deleted too then :lol:
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Re: [YM2149] STF Stereo Modification

Postby Robson » Mon Apr 27, 2015 9:58 am

I posted something but when I read it back i realized it's not correct, so deleted it.
I am at work now so I will think it through before answering again. :)
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Re: [YM2149] STF Stereo Modification

Postby Robson » Tue Apr 28, 2015 3:38 pm

OK, here is something I have in my mind based on the LMC1992 Block diagram and schematics.

lmc1992.PNG


1. Separate Inputs
The schematic below will use Input 1 of the LMC1992 for the YM chip and leave Input 2 for the AUDIOI signal, which is the Audio IN from the 13pin Din connector:

- Separate pins 3/4/38 at the YM chip (cut and lift the pins) connect them to the small board.
- Separate pins 4 and 26 from 5/25 pins (you can leave the connection there for this 2).
- Trace where and how AudioI and AudioS are connected and break that connection too.



This might not be the correct solution because it assumes that data from shifter selects input 1 for AudioS and Input 2 for AudioI. When they are joined, it doesn't matter what the shifter data is (selecting 1 or 2)


ste_sound.jpg


2. Losing AudioI
Other possible solution is to lose AudioI. I'm not sure how often is it used through the monitor socket as input. So connect 4&5 and 25&26 to the L/R from the YM chip.

ste_sound2.jpg


3. No AudioI in stereo mode
Maybe the best solution is the schematic below, which allows you to have AudioI in Mono mode, but not connected in stereo. This needs a 2 state/9 pole switch.

ste_sound3.jpg


Keep in mind that none of these solutions are tested!
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Last edited by Robson on Wed Apr 29, 2015 8:16 am, edited 1 time in total.
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Re: [YM2149] STF Stereo Modification

Postby qq1975b » Tue Apr 28, 2015 4:02 pm

Hi Robson,

Thank you very much for taking the time thinking about the stereo YM mode in a STe .

If I understand correctly, going directly to try version 3, I have to:

Separate pins 3/4/38 at the YM chip (cut and lift the pins) connect them to the small board.
Separate pins 5 and 25 from LMC1992 (cut and lift pins too?) and join 3YM with 5LMC and 38YM with 25LMC
Trace where and how AudioI and AudioS are connected and break that connection too (it won't be easy...).
Connect AudioI from DIN13 to switch

It seems a lot more complicated than the STfm version...it is due to the existence of the LMC that merges YM sound with PCM sound, right?

thanks.
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Re: [YM2149] STF Stereo Modification

Postby Robson » Tue Apr 28, 2015 4:22 pm

Yes, STE has an improved sound system therefore this needs a bit more thinking.
Personal note: if you want to use the ste for sound than it is useful to do this mod, but when I tested my ST with games it was not a pleasant experience to my ears listening those tracks in stereo, so I switched back to mono :).

I don't know how the LM1992 pins are connected, it needs to be looked at first before saying cut and lift the pins. You might not need that to break the connection if it is easy to cut the wires on the boards, but if these are connected under the chip, than you need to lift that pin up (all of them, not only 5/25).
It is the same thing to break the connection between Audio S and I, just cut or de-solder AudioI pin at the connector and wire it to the switch so it won't go to the board at all. But again there might be better points to separate them if you trace the route down.
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Re: [YM2149] STF Stereo Modification

Postby Robson » Tue Apr 28, 2015 7:53 pm

It seems that I missed 1K resistors from all the drawings between YM pin 3 and the earth, same way as pin 4 and 38 have them. I'll update visuals tomorrow.
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Re: [YM2149] STF Stereo Modification

Postby qq1975b » Wed Apr 29, 2015 6:31 am

Robson wrote:Yes, STE has an improved sound system therefore this needs a bit more thinking.
Personal note: if you want to use the ste for sound than it is useful to do this mod, but when I tested my ST with games it was not a pleasant experience to my ears listening those tracks in stereo, so I switched back to mono :).


That is an important personal note.It would be interesting to know if other people that did the mod agree with this feeling. I would use the modded STe for games and demos basically...so maybe it is better not to touch it. :shrug:

By the way, here it is a picture of a STe motherboard:

http://www.atari-wiki.com/images/c/c0/S ... fan_jl.jpg

LMC chip is on the top left, close to the RCA connectors and YM is next to the right side of the SIMM connectors.
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Re: [YM2149] STF Stereo Modification

Postby Robson » Wed Apr 29, 2015 8:21 am

Because I used a 3.5mm stereo jack as output it was easier for me to plug a headphone in when tested. On the headphones I don't like when I hear tingling on one ear and something different on the other. If you use the RCA out to connect the STE to a decent sound system, the whole thing might be different.
On the other hand if you do have a stereo out you'll still have a choice to switch back to mono whenever you want.
I'd be interested in reading what others think too.
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