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

[Tom Wiltshire]

On 4 Feb 2010, at 00:38, karl dalen wrote:

> Dave Manley <dlmanley at sonic.net>
>> 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 part is obsolete and no longer available:
>
> It's actually a Motorola part, now ON Semi.
>
>> 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.
>
> Kinda slow when Dave S had claimed, quote something:
> *Analog process blocks are slower then the digitally
> *generated function generators.*

That's not so bad. Gives him a theoretical top end of about 500Hz for  
modulation frequencies. But the LFOs on his stuff go only go up to  
Middle C at 261Hz, so he's keeping it all on the safe side.
He might mean that the modulations within the dsPIC voice processor  
(e.g. the ones that never leave the digital domain) are calculated at  
a higher rate than the DAC runs at.
For example, I did some code for a dsPIC dual oscillator. It only  
read its CV inputs at about 5KHz, which puts a limit on the  
modulation frequency, but it could still do FM effects because you  
could send one oscillator to the other's phase input digitally at the  
full sample rate.

T.