Atari STE and Mega STE sound fixes

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Recently (2015-2017) there have been identified three separate sound issues in the Atari (Mega) STE:

  1. STE DAC playback has bad noise and high pitch whistles
  2. The YM sound is much louder than the DMA audio sounds (and the Microwire setting for -12dB PSG volume makes it muted instead)
  3. Default STE sound (both the YM and DMA) output on the output jacks is too loud


Bad noise and high pitch whistles

The YM sound is much louder than the DMA audio sounds

The official LMC1992 bit setting, controlled via the Microwire is documented like this: 

 Mixer control: x x x  x d d  (left 4 bits are ignored)
                         0 0  = DMA + (YM2149 - 12 db)
                         0 1  = DMA + YM2149
                         1 0  = DMA only
                         1 1  = reserved

Setting "00" mixes the output of the YM2149 and the output of the DMA-sound, but the YM2149 sound is being downsized by 12 db.
"01" mixes DMA and YM2149 linearly, "00" means DMA sound output only.

When one wants to mix the YM and DMA sounds together, the obvious %01 setting gives YM output which is too loud. So the next logical choice is %00 to lower the YM output. However, this doesn't work, it leads to a muted output.

Since all software supports the original PSG/DMA mixing (i.e. with the wrong YM volume level), one option is to make that -12db for PSG always active. To do this, swap lines leading to pins 4-5 and 25-26. Then the default Microwire setting will be set as -12db for PSG.

Please note that with this fix the programmer has to set "%10" (former "DMA only") to get "DMA + (YM2149 - 12 db)". With swapped lines mentioned above the lowered output would be on "%01" and the original "DMA + YM2149" on "%10". So it's not backward compatible in neither case.

Sound on the output jacks is too loud

Simpler fix

Add external dividers to both channels. No need to open machine and 2 smaller resistors can fit in the RCA connector. 1K and 150 Ohm will give approx. 6x lower amplitude: 

Output jack (centre) --|1000|-+--|150|-----> ground
                              |
                              +------------> input

It's possible to do this inside the STE, too.

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