[sdiy] Oberheim Xpander Envelopes and LFOs etc

Scott Gravenhorst music.maker at gte.net
Sun Sep 30 14:38:02 CEST 2007


Tom Wiltshire <tom at electricdruid.net> wrote:
>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.

At 16 MHz, 68000 is more likely, IIRC, the 6809 came in only 3 flavors, 1.0 MHz, 1.5
MHz for 68A09 and 2.0 MHz for 68B09.

>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.

Nonsequitur comment: FPGA does this without breaking a sweat and with far more slow
objects than 4 and at sample rates beyond human hearing...

>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?
>
>T.
>
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-- ScottG

-------------------------------------------------------------

-- Scott Gravenhorst
-- GateMan I - Xilinx Spartan-3E Based MIDI Synthesizer
-- PolyDaWG/8 - 8 Voice FPGA Polyphonic MIDI Synthesizer
-- FatMan: home1.gte.net/res0658s/fatman/
-- NonFatMan: home1.gte.net/res0658s/electronics/
-- When the going gets tough, the tough use the command line.




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