[sdiy] Digital pots for polyphonic portamento..

[Jean-Pierre Desrochers]

Hi Colin,
Here are some calculations I made..

Assuming I want an adjustable range of 2msec to 5sec portamento
from note A to note B, for CV's I use 12bits DAC's but keep the 7 MSB'S
for MIDI notes (128 notes). That leaves 5 bits for glide between
2 halftones. OK. (no staircase effect) so I need to keep these 5 bits
for each halftone glides.. no matter the distance between
note A to note B. So this brings me to the following point:
Suppose I want to glide between 2 notes 7 octaves apart
that means 7 X 12 halftones X 32 steps= 2688 DAC steps.. right?
i need these 2688 steps in a 5 sec portamento to hear
a glide without staircase effect..
But doing so,  the worst case in my example is min portamento which is
2 msec overall glide time --> 2msec. / 2688 steps = 744nanosec. per DAC steps!!!!

Do you have a solution on that?
I probably mistake myself in my calculations..(?)

JP


 On Wed 04/03/09 20:36 , "Colin f" colin at colinfraser.com sent:
> 
> > Software method.. Good idea! But I don't want to
> load> the 'analysis' time for each received MIDI bytes
> to CV outs.. > wondering..
> > I'm using a PIC16F877 at 20mHz.
> 
> Analysis time isn't an issue if you're receiving MIDI data in an
> interruptroutine which then updates a state table for the code generating the
> glide.The glide code must run at a fixed sample rate - easily done in a timer
> interrupt, with a lower priority than the serial interrupts.
> 
> On a current project, I'm using a PIC 18F4520 with a 16-bit DAC.
> The DAC is demultiplexed between 8 outputs.
> The PIC is calculating an RC curve at 16-bit resolution for each
> output,with a total refresh time for all channels of 1ms.
> There's plenty CPU time left over for MIDI handling, voice allocation and
> soon.
> A PIC 16F877 only manages half the speed of the 18F part, but for
> 12-bitvalues, and fewer outputs, it should manage the same sort of refresh
> speed.
> Not only could you save on the hardware glide circuits, you could save
> onthe DACs by swapping all but one of them for a 4051 and some op-amp
> buffers.
> Cheers,
> Colin f
> 
> 
> 
> 
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