[sdiy] Oberheim Xpander Envelopes and LFOs etc

Tom Wiltshire tom at electricdruid.net
Sun Sep 30 13:56:56 CEST 2007

Hi All,

I've been spending the weekend doing a bit of research into the  
oberheim Matrix synths and the Xpander.

The Xpander's multi-mode filter design is really something special,  
and has been mentioned here before (check the archives - http:// 
search.retrosynth.com/synth-diy/search/        mar 97, sept 04) but  
one thing that I can't find the detail on is the envelopes and LFOs.

The Xpander uses a pair of processors, with one of them being  
dedicated to generating all the envelopes and LFOs (and lag  
processors, and ramp generators, and tracking generators and, and,  
and..!!). Basically, all the slow control voltage processing is done  
in software. This processor is (I read somewhere) a 6809 running at  
16MHz. This isn't clear at all from the schematics I've seen,  
although 68000 series is a possibility.

What I want to know is what output sample rate is this processor  
managing to produce. It is dealing with 5 envelopes and 5 LFOs per  
voice, plus all the other stuff, for each of 6 voices. This is 30  
LFOs and Envelopes, which has got to take a while to work out. I've  
done similar stuff on more modern processors, and felt like I was  
running out of time generating only 4 envelopes. However, I was  
working with a 25KHz output rate.

I suspect this is one of the areas where corners can be cut far more  
than most people would accept. The Waldorf Wave also uses software  
envelopes, and these are only updated at about 53Hz! (http:// 
www.unofficial.waldorf-wave.de/wavetech.html). Yet the Wave is held  
in high regard. If anyone could do a similar test on a Oberheim  
Xpander, I'd be interested to know the result.

So the question is "How low can you go?". 53Hz might be really silly,  
but would 1KHz be enough? 5KHz? I know some of the software sound  
languages (csound or puredata) use rates of 6KHz or so.

What counts as a reasonable sampling rate for LFOs and Envelopes?


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