[sdiy] Mopho modulation DAC configuration, update rate, etc

[Dave Manley]

It's too late to look at the encoder scan rate :-) , I spent the evening 
trying to tune a Hammered Dulcimer. 31 x 3 = 93 separate strings.  It 
had never been tuned, and the strings are stretching like crazy.  Three 
passes through, and it is still not staying in tune, even with 
stretching the strings between each tuning pass - you think calibrating 
a VCO is a pain?

Here's some info on the Mopho DAC used for envelope, modulation, etc.

The DAC is a 16-bit serial TI part: DAC8531, power is +5V/GND

More info:  http://focus.ti.com/docs/prod/folders/print/dac8531.html

The output of the DAC is buffered by a TL34072 Op Amp, in non-inverting 
mode with a gain of 2. The power is +12/-5.  Interestingly the TI web 
page indicates this part is obsolete and no longer available:

http://focus.ti.com/docs/prod/folders/print/tl34072.html

The buffered output is connected to two HCT4051 Analog Demultiplexers, 
power is +/-5V.  Each 4051 provides 8 S/H outputs in the standard way, 
for a total of 16 S/H outputs.

Each S/H is refreshed at 1.25kHz, or every 800uS.

The DAC inputs are driven by the dsPIC:

Pin 5, Sync -> dsPIC pin 40, U2CTS/RF12
Pin 6, SCLK -> dsPIC pin 55, SCK1
Pin 7, DIN -> dsPIC pin 53 SDO1

SCLK is running at 10MHz.

-Dave