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Published 20 hours ago • verified publisher icon schlae

Use of MicroChannel audio feature

Open elosha opened this issue 2 years ago • 4 comments

MCA seems to include an audio bus: Signals AUDIO_GND (contact B1) and AUDIO (contact B2).

  1. However these are not used by SB MCA, what is the reason?
  2. Could the PCB be patched to connect them, i.e. to use PC Speaker for sound output?

Thanks :)

elosha avatar Mar 25 '22 15:03 elosha

I have not tried it, but in theory it is possible to do. You have to use a series resistor since the audio signal line is an op-amp summing junction. Conversely, you can also go the other way so that the SB can mix the PC speaker sound in and direct it to the line output jack. The main reason I didn't add this feature is that I have no adjustable mixing chip on this design.

schlae avatar Mar 25 '22 16:03 schlae

Thanks for the info! That's cool! 👍

On my wishlist for a possible future rev:

It would be nice if we could have a trimmer + signal routing jumper, or at least a MCA audio bus pinheader and also audio in/out pinheaders (like SoundGalaxy). One could also attach a CD-ROM audio cable then.

Sure it wouldn't be actively mixable, but a simple trimmer for one-time adjusting audio levels would theoretically be fine.

Thanks for considering 🙏

elosha avatar Mar 26 '22 09:03 elosha

I noticed the same feature in the MCA spec and got curious about it (including why no MCA sound cards ever seemed to use it). It looks like it was mainly intended for modem use — sending the initial squeaks and squawks from an internal MicroChannel modem to the PS/2’s speaker until a successful connection was made.

Having said that it would be very interesting to see how it functioned in a sound card context!

pleonard212 avatar Jun 29 '24 03:06 pleonard212

probably because the internal speaker is not very good. the audio pin is pretty simple, it goes to a 620 ohm resistor to ground and then has a series coupling capacitor and series resistor to the input of an LM386 audio amp. the resistor helps mix this signal with the PC speaker signal.

you can either drive the pin with a buffer or you can listen to the pin with an amplifier. there might be a way to do both at once, if the audio path from the card ties to the pin before the final output amplifier stage.

schlae avatar Jun 29 '24 04:06 schlae