Re: Mysterious Sound Card

[dempson@atlantis.actrix.gen.nz (David Empson)]

In article <15MAY199604312451@vax2.concordia.ca>,
Mitchell Spector <spec@vax2.concordia.ca> wrote:
> In article gt5460c@acmex.gatech.edu (Martin Tuck) writes...
> > 
> >        I recently bought gs#3 ($225 for a WOZ with a zip gsx) which came
> >with a sound card of which I know nothing.  The sound card is a
> >SuperSonic by MDIdeas.

> 	The MDIdeas SuperSonic is a stereo output board, many companies
> produced simular boards for the Apple IIgs over the years. It does not
> add new sound capabilities, in fact it doesn't even produce a stereo 
> signal.

Yes it does.  It has left and right output channels which can produce
different audio output, so by definition it can produce a stereo
signal.  :-)

> What it does do is "grab" the stereo signal already built-in to the
> Ensoniq 5503 on the motherboard, plus add a bit of signal filtering 
> and amplification along the way I believe.

Signal filtering and amplification, yes, but...

> The pins at J25 are the Ensoniq  I/O molex connector, stereo-out and
> sound-in are available through here, since Apple negelected to add
> proper port connectors in back of the GS.

That is not correct.  A single pin on the 7-pin molex connector
provides the sound output signal, which contains multiplexed sound
from up to eight channels.  Three further pins contain the channel
number, and another pin is a strobe for the address.

The remaining two pins are analog ground and the single channel analog
input.

Thus the 7-pin connector provides a multiplexed sound output
containing up to eight channels, and a mono sound input.

To extract stereo audio from this connector requires an analog
demultiplexer.  Only the least significant bit of the channel number
is used, so all even channels go to one speaker, and odd channels to
the other speaker.

A more advanced sound card could produce up to eight independent sound
channels.


It is not possible to digitize stereo input directly using the
Ensoniq, because only one channel is provided.  If the card supports
simultaneous digitizing of two channels, it must provide its own A/D
converters.

> One thing to note is that the 'Volume' control setting in Control
> Panel has no affect on the stereo card (it only controls the internal
> speaker, which is actually quite annoying when your trying to enjoy
> music through external speakers. :) 

Some cards (e.g. the SoundMeister) also have a volume setting.

-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand