Getting stereo out from Apple IIGS without de-multiplexing

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Joined: Oct 2 2014
Posts: 3

Hi all!

This is my first post to this forum. I recently got myself a IIGS, not having owned any Apple II before. I was a C64 guy back in the days. Now, my interest spans pretty much all computers of the era. Apple II was very uncommon in my country (Sweden), although 68k Macs were not.

Now to the point. I know there is hardware such as TDX Stereo card and Sonic blaster. I don't know about the Sonic blaster but the TDX design is based on de-multiplexing the sound multiplexed by the IIGS before being output. I haven't studied the IIGS schematics in detail yet and since I'm not really that good at electronics I'm not sure I'll be able to understand them.

Here is my idea, probably stupid, but shouldn't it be possible to tap the left and right audio signals and do whatever filtering and amplifying necessary before it is multiplexed? That way there is no need to de-multiplex the signal afterwards. I mean there should be losses and added disturbance every time the analog signals are tampered with. Someone should even have done this already if it's possible.

OK, now you can shoot me. Wink

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stynx's picture
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Joined: Apr 19 2012
Posts: 182
Re: Getting stereo out from Apple IIGS without de-multiplexing

The multiplexing is part of the DOC sound chip. the sound chip offers several signals that determine which channel is currently active in the audio stream. As far as i know it is possible to decode up to 16 individual channels from the DOC. The IIgs only offers 3 of the 4 available decoder signals (CA0-CA3) on the audio expansion interface which amounts to a maximum of 8 separate channels in the Apple IIgs with decoding. The problem with demultiplexing time coded signals is that you will get small amounts of overlapping even if you are very precise with timing. This (and other events) can results in some audio artifacts when switching is in progress. There are several ways to filter these or introduce 'silent separators' but will result in a lowered sound quality (often encountered with stereo-cards in the IIgs).

If i have gotten something wrong please correct me Wink

PS: The TDX-card is a pretty much straight forward implementation and offers very good quality with a 'simple' design (it takes much work to archive simple designs).

-Jonas

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Joined: Oct 2 2014
Posts: 3
Re: Getting stereo out from Apple IIGS without de-multiplexing

Well, my first post looks like it will not make it to the hall of fame. More like the hall of shame.

If I had gone a bit further and actually looked at the schematics I would have realised that there is no demultiplexing going on at all, but rather the opposite, of course, since it takes the split channels from J25 and does all that which I ranted about in the first post.

Maybe I can blame it partly on this site http://www.drew2gs.com/, where it says demultiplexer. I thought it used the composite audio source and split it somehow. Seemed kinda awkward way to go about it, but the silly hat sits on my head now. Laughing out loud

quickthyme's picture
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Joined: Dec 11 2014
Posts: 35
Re: Getting stereo out from Apple IIGS without de-multiplexing

So the Drew ][ TDX card (yet another unavailable piece of awesome) does demultiplex, but not in the way that you or I originally interpreted. I, too, had the same misunderstanding from having read several bits of lore on the matter. I guess stereo capability has never been that important to me, being partially deaf, but I have often felt that their decision to intentionally cripple the GS in that way was incredibly ironic, because this was supposedly done in order to keep the GS from cutting in to Mac sales, and that was a very Orwellian thing to do. (IIRC, in the novel 1984, those with intellectual superiority were handicapped so that they wouldn't have an advantage over those with average intelligence). I wonder if that original Macintosh commercial was broadcast in stereo?

Anyway...
In retrospect, my understanding of the situation has evolved from having learned that the GS utilized the sound channels for an altogether different purpose than for simply driving a set of stereo speakers, and were apparently part of the secret sauce that allowed its wave table synthesis to play several instruments at once. Perhaps because of this, the GS's MIDI capabilities are far undervalued, even today. (Creative Labs apparently agreed with this assessment when they later decided to acquire the technology for use in their SoundBlaster cards somewhere in the mid to late 90s.)