[sdiy] Oberheim Xpander Envelopes and LFOs etc

Tristan Upton tu at alphalink.com.au
Sun Sep 30 21:46:26 CEST 2007

Hello Tom, 

 From what I recall, the Xpander uses a control voltage update rate of around 
100Hz. To get around the problem of slow updates the control voltage sample 
and hold has two time constants. One is used for fast updates, like sawtooth 
reset, and a slower one for everything else. I guess that gives enough 
smoothing for the slow update rate to be acceptable. I can't remember the 
clock rates of the CPUs but the Xpander uses two 6809 processors. They work 
as a master and slave pair. 

Best regards,

Sunday, September 30, 2007, 1:56:56 PM, you wrote: 

TW> Hi All, 

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

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

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

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

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

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

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

TW> T. 

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