[sdiy] Oberheim Xpander Envelopes and LFOs etc

[Tom Wiltshire]

Thanks to everyone who replied.

To summarise what I've learned:

The Xpander mod processor is a 68B09, with a clock of 8MHz, probably  
running at 2MHz. The modulation update rate is 100Hz, or every 10mS.
The service manual explains the Sample & Hold smoothing, where an  
extra RC smoothing stage can be switched in or out under uP control.  
This allows it to output reasonably smooth ramps without rounding off  
the abrupt changes.

The service manual page with all the answers is here (took me ages to  
find this):

http://matrix-12.tripod.com/xpsvc28.jpg

It's an interesting system. They used a 14-bit DAC, which is better  
than it needs to be and probably compensates for the appalling update  
rate somewhat. At higher rates (~20KHz) I've found that at 10-bit  
resolution zippering is only just audible, and by the time you're  
using 12-bits at 25KHz, it's gone altogether (as long as the  
modulation width isn't too enormous, I suppose). At 25KHz sample  
rate, even a 1mS attack transient includes enough points that the  
shape of the curve is determined by the uP and not the filter.

Obviously you can trade-off sample rate against sample resolution.  
Better resolution decreases the amount of digital noise in the output  
signal and reduces the amount of work the filter has to do removing  
it. This allows you to get away with a slower rate. What matters is  
the size of the little steps in your output, that give-away of  
digitally produced waves.

What I find amazing though is that highly regarded instruments like  
the Oberheim Matrix 12/Xpander or the Waldorf Wave use an envelope  
update rate of 100Hz or less.

I'm sure that a system that used 12-bit resolution at a 1KHz rate  
would be acceptable. If you pushed the rate to 2 or 4KHz, you'd have  
something that is much better than a host of famous and extremely  
expensive instruments!

Thanks again for all the comments folks!

Regards,
Tom