Odp: [sdiy] Getting back to my Midi->CV converter...

[Seb Francis]

P.S. After some reflection, I'm getting to like your idea Harry.  Once I've got the main beast working, I don't think I'm going to be able to resist giving it a try ;0)  Even in the context of my design I think some SPDT analog switches should be able to take care of the connections.


Seb Francis wrote:

> harrybissell wrote:
>
> > Hi Seb (et al)
> >
> > since you are doing ALL NEW research... why not use some
> > programmable current source to charge / discharge a cap to
> > do the LFO's... instead of setting the next voltage, you set the
> > next rate of change.  This would make the wave from line
> > segments instead of stairsteps...
> >
>
> Interesting idea.  I can think of a few problems with this (mostly related to my particular design though)
>
> (a) I have 15 CV outputs and the source of each must be freely assignable in software from the digital envelopes, LFOs, or from MIDI messages.  It would be quite complicated to switch the electrical source from the main voltage output DAC to an individual current output DAC per 'AC' S&H channel.
>
> (b) The expense and board space of added a few multiple current output DACs (and extra MUX to switch them in) is significant.  Perhaps I'm thinking the wrong way here though .. it's probably possible to make a current source in analog which is controlled by the voltage from the main DAC - but again extra board space and expense.
>
> (c) I don't just want to product triangle LFOs - I also want pseudo-sine, pulse, saw and random* shapes.  Ok, pulse and random aren't a problem - these are simple stepped voltages, but saw is tricky - this is a combination of straight slope and step.  Pseudo-sine could be realised by varying the current source in the shape of a triangle wave, but then you get steps in the gradient (not so bad as steps in the voltage though).  Overall, it gets complicated!
>
> * the random source is not strictly digital BTW - I'm going to use one of the A/D inputs of the PIC to sample a simple noise circuit.
>
> Overall, this project is getting so complicated already that I think I should stick to the single multiplexed DAC idea at least until I breadboard and find it's not good enough.  Remember that slow LFOs will be very smooth (if slow enough [> ~8s period @ 2KHz refresh rate], the max steps will be 1 LSB or 1mV).  And perhaps with fast LFOs the steps will be less obvious.
>
> Also, the S&H caps will cause some gradient in the steps.  I have the loan of a digital storage scope at the moment, so I will examine in detail what happens at different LFO frequencies.
>
> Still fighting with the ASM code at the moment though ...
>
> Seb