[sdiy] Oberheim Xpander Envelopes and LFOs etc

[Dave Manley]

Scott Gravenhorst wrote:
> Tom Wiltshire <tom at electricdruid.net> wrote:
>   
>> 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.
>>     
The service manual shows two 68B09 micros.  The clock into each is 8 
MHz.  Wikipedia says the 68B09 ran at 2MHz, so there must be a divide by 
4 in there, probably to generate some internal multiphase clocks. 

More to the point, the manual also states the S/H update rate at 10mS 
(or 100Hz as someone else has said).  Of interest is the way the dac 
output is smoothed.  The circuit is a dac driving a 4051 driving a 
0.033uF cap to ground, series 1M, and a 0.0068uF cap to ground to TL084 
voltage follower.  This is the slow smoothing circuit.  The dac also 
drives a  second 4051 which is connected directly to the 0.0068uF cap 
and voltage follower node.  This allows a fast non-smoothed update 
selectable under processor control.

-Dave