exxos's Atari, Amiga & retro forum

I'd like to add a stereo audio output (for attaching to a VGA monitor with two (stereo) built-in speakers), this in addition to the two (stereo) phono outputs and single (mono) pin (pin 1) of the DIN13 video output connector.
How is this best done without messing up the existing outputs (i.e. no signal loss etc. if they're all attached to their intended devices (monitors, amplifiers) simultaneously?

I looked up the STe schematics and see that pins 14 and 16 of the LMC1992 audio are the L and R outputs, and the monitor mono output (pin 1/DIN13 connector) is a combination of these. Can I attach the new audio output (stereo for the VGA monitor) directly from those pins, or would I need to run it through an op-amp circuit equivalent to the ones used for the mono output signal (the two LF347 chips)?

I would just lift the right side of R540 R541 and then you have stereo outputs..

I would just use two external RCA Y splitters on the two L/R phono outs. (RCA Male to two RCA Female)

https://www.aliexpress.com/item/1005002807881411.html

If you choose to go inside, consider performing the DMA/YM volume modification.

https://exxosforum.co.uk/atari/last/STEDACFIX/index.htm

alexh wrote: 14 May 2025 08:32 I would just use two external RCA Y splitters on the two L/R phono outs. (RCA Male to two RCA Female)

https://www.aliexpress.com/item/1005002807881411.html

Won't this cause impedance differences if those outputs are simultaneously connected to two different amps (the VGA monitor's stereo speakers and a an audio interface/audio recorder etc)?

If you choose to go inside, consider performing the DMA/YM volume modification.

https://exxosforum.co.uk/atari/last/STEDACFIX/index.htm

Thanks for the link! I didn't know about all those audio related fixes, so that's useful info indeed.
The one I already had on my to-do list was to attenuate the rather high level from the phono connectors by adding a resistor to each of them ( a trimmer would probably be a better idea for more precise adjustments, possibly with a test signal present. I can't remember the resistor values, but it's a well known issue and probably mentioned in numerous forum threads.
I'll be re-reading that page in case I missed anything, but it doesn't seem this particular problem is mentioned, unless it's the "STE LMC mixer audio fix" near the bottom of the page, but it seems to address a different matter and not the generally too high output (i.e. compared to say a CD player, radio tuner etc. attached to the same stereo system amp).

exxos wrote: 14 May 2025 08:17 I would just lift the right side of R540 R541 and then you have stereo outputs..

Good idea! But I don't want to lose the monitor (DIN13 connector) audio out when attaching an Atari monitor.
I want to keep that as it is and add a stereo output for the VGA monitor's speakers.
(The computer will be recased and have a self-designed Atari/VGA video output board -similar to Ubeswitch but with two output connectors (a new DIN13 and a VGA connector).

Pacman wrote: 14 May 2025 11:15

alexh wrote: 14 May 2025 08:32 I would just use two external RCA Y splitters on the two L/R phono outs. (RCA Male to two RCA Female)

Won't this cause impedance differences if those outputs are simultaneously connected to two different amps (the VGA monitor's stereo speakers and a an audio interface/audio recorder etc)

I am afraid I don't know enough about analog electronics WRT to Audio to answer that question reliably.

HOWEVER splitting the analog signal to two places internally or splitting it into two places externally using nothing more than wires is surely the same?

For £4 inc postage I would try it and see :)

alexh wrote: 14 May 2025 12:02

Pacman wrote: 14 May 2025 11:15
Won't this cause impedance differences if those outputs are simultaneously connected to two different amps (the VGA monitor's stereo speakers and a an audio interface/audio recorder etc)

I am afraid I don't know enough about analog electronics WRT to Audio to answer that question reliably.

HOWEVER splitting the analog signal to two places internally or splitting it into two places externally using nothing more than wires is surely the same?

For £4 inc postage I would try it and see :)

That's true :)
And you're right about splitting the audio internally or externally will make no difference.
I'm sure it'll work (audio will be present on both outputs), but there might be some level drop when connecting/disconnecting both outputs at once.

I've been reading through various discussions on these audio issues as i was only aware of the distorted/very high STe rear phono connector problem, and as you rightfully suggested earlier I should look into those while at it. I'm sometimes amazed at how wrong Atari got things while designing an otherwie such a great computer. Must have been in a real hurry, or communications between designers wasn't at its best.

To summarize I see three or four STe related audio problems that I need to look into:

- D/A converter output noise
- D/A converter crackle/pop noise
- Sound generator (YM-2149) vs. DMA (sampled sound -not sure which chips this is) mismatched output levels
- Stereo outputs (rear phono sockets) overly loud and distorted output

As for the last problem: since I don't know where the problem originates, perhaps I could feed a steady audio signal (i.e. from a test-oscillator) at a stage before the LMC-1992 mixer chip, then check (using an oscilloscope) at various places to find out where things go wrong.

I think you need to check to see if your STe motherboard suffers from any of these issues before attempting anything. I was happy with mine except for the mixer levels.

Pacman wrote: 20 May 2025 15:19 - Sound generator (YM-2149) vs. DMA (sampled sound -not sure which chips this is) mismatched output levels

LMC-1992 mixer chip has 4 input lines (stereo) plus one (also stereo) for SoundDMA. According to Atari STE spec, the first input line is for raw YM signal, the second one for YM -12dB. But the final version of the STE does not haveYM -12dB , and when you switch LMC1992 to input 2 then you will hear nothing

https://www.atari-forum.com/viewtopic.p ... 66#p252366

Pacman wrote: 20 May 2025 15:19 - Stereo outputs (rear phono sockets) overly loud and distorted output

Atari has to high an output voltage level for both: Audio and video TV SCART standard. For the last one we have to use 75Ω resistors on R/G/B lines.
I know someone has also used resistors mounted in the audio cable.

alexh wrote: 20 May 2025 16:06 I think you need to check to see if your STe motherboard suffers from any of these issues before attempting anything. I was happy with mine except for the mixer levels.

Sound advice (pun intended) :D
Except for the rather loud RCA jack stereo output I haven't noticed any audio issues. But then again, apart from some demos, I haven't really used my STe much for games or other audio related stuff, so I need to look into this properly.
I'll probably do the mixer level fix.
On that subject -are there any good programs around for adjusting the SMC-1992 controls?
It would help me understand how it all works.

Cyprian wrote: 20 May 2025 16:33

Pacman wrote: 20 May 2025 15:19 - Sound generator (YM-2149) vs. DMA (sampled sound -not sure which chips this is) mismatched output levels

LMC-1992 mixer chip has 4 input lines (stereo) plus one (also stereo) for SoundDMA. According to Atari STE spec, the first input line is for raw YM signal, the second one for YM -12dB. But the final version of the STE does not haveYM -12dB , and when you switch LMC1992 to input 2 then you will hear nothing

https://www.atari-forum.com/viewtopic.p ... 66#p252366

Here, you're talking about software settings for the LMC-1992, determined by programming done in games, demos etc, right?
(NOTE: see updated comments further down where I think my assumption here may be wrong, but again that may be wrong -so please correct me if that's wrong.... hmm, confusing :lol: )


I downloaded the test program from the forum link you provided, and tried it in Hatari. But it'll be more interesting to try it out with my two STe computers (currently none of them are up and running, but hope to fix that soon).

I've been trying to make sense of the schematics, and below is the LMC-1992 section from a Mega STe (the connections are (apart from the placement of the filters at the RCA jack outputs) the same as with the STe, but with slightly different values. The Mega STe schematics I've found are more readable than the poor scans of the STe schematics that are available online).
I've added the signal names to the LMC-1992 (from its datasheet), and also got some help from Info coach -Atari STe -The GST shifter where there's a useful audio signal-flow diagram on the right hand side (fig. 5). So if I understand it correctly, "AUDIOS" (lower left side) is where the (Yamaha YM-2149F) programmable sound generator chip's output comes from. And "AUDIOI" is where the DIN13 (monitor connector) input comes from. Both of these are mono and simultaneously fed into "input 1" and "input 2" (both L and R of both).
But while AUDIOS (YM-2149F) and AUDIOI (audio input) are connected together, and furthermore connected directly to input 1 (LMC-1992 pins 4 and 26) I'm trying to make sense of what the resistor and capacitors (C639, C640, R631, R625, R632, R627 in the schematic above) do. According to the LMC-fix Exxos page (scroll down near the bottom) Atari mistakingly lets input 2 (LMC-1992 pins 5 and 25) effectively go to GND, resulting in silence.
Now, I noticed that in the Info-coach audio signal-flow diagram, input 2 (LMC-1992 pins 5 and 25) aren't even shown, and if your scroll further down on that same Info-coach page you'll see an LMC-1992 pinout (fig. 6) you'll interestingly notice that pins 5 and 25 are marked NC (no connection)!
Obviously this is an error, but since there's also mention of the Atari STF in the accompanying text it makes me wonder if that particular ST (and perhaps the other ST models including the ST and most likely also the STFM which I understand is basically an STF with a TV-moduator output)... all used the LMC-1992 as well, but connected just to input 1 (LMC-1992 pins 4 and 26).
But I digress :lol:
Nevertheless, it makes me wonder why input 2 was all of a sudden added with the STe. And what's the purpose of having input 2 anyway?
You said input 2 is -12dB (I suppose this is in relation to the input 1 level), so it appears Atari wanted to have two output level "presets" from the YM-2149F:
- LMC input 1 (pins 4 and 26): normal (full) output level
- LMC input 2 (pins 5 and 25): reduced (-12dB reduction) output level

EDIT: I stand corrected with my comment earlier in this same reply where I wrote:

Here, you're talking about software settings for the LMC-1992, determined by programming done in games, demos etc, right?

So it seems input 2 was intended to be reduced with the aid of the hardware circuitry (I'm guessing this is what the resistors and capacitors between inputs 1 and 2 are intended for -but please correct me if I'm wrong).

Moving on and summarizing...
If I'm not mistaken, sampled audio (DMA) is fed into the LMC-1992 mixer chip at pin 9 (L select in) and pin 21 (R select in). These are in fact a stereo signal pair as far as I can see. So stereo for sampled sound, and mono for YM-2149F sound generator sound (as well as anything fed into the DIN13 audio in pin.
So to summarize it appears we have 3 signal input sources going into the LMC-1992:

- input 1 (pins 4 and 26): YM-2149 and DIN13 audio MONO
- input 2 (pins 5 and 25): YM-2149 and DIN13 (reduced (actually erronousely grounded audio) MONO
- L and R select in (pins 9 and 21): DMA (digitally sampled) audio STEREO

Have I understood it correctly?

Pacman wrote: 20 May 2025 15:19 - Stereo outputs (rear phono sockets) overly loud and distorted output

Atari has to high an output voltage level for both: Audio and video TV SCART standard. For the last one we have to use 75Ω resistors on R/G/B lines.
I know someone has also used resistors mounted in the audio cable.

Yes, the video signal is also overly high according to normal standards. I'm working on a VGA output board and resistors have to be added to the R, G and B lines in order to match VGA standards (I don't remember the values off-hand). Seems the Atari designers were a little sloppy.

My apologies for the rambling in my previous posting.
But it would be very helpful if someone could clear up the purpose of input 2 (-12dB) of the LMC-1992?

My take on this (after reading several other threads on the subject):
Because the LMC-1992 isn't really a mixer (as in the ability to blendi audio levels), it rather just switches on/off its inputs.

So, to reduce the YM-2149 input they (Atari) had to duplicate the connections (from input 1), then reduce the levels (in hardware) by -12dB before entering input 2, and allow for switching from input 1 (normal) to input 2 (reduced).