[sdiy] Deceptively simple question: Panels

[Oakley Sound]

> How does an 8 bit dac resolution degrade polyphonic operation?

Standard 8-bit DACs are pretty awful for any synth use, monophonic or 
polyphonic. However, with polyphonic synths you have the oportunity to 
play more than one note at once and this means that the instrument will 
be noticeably out of tune with itself.

So if you play two notes an octave apart and you will find they will 
beat. Any interval on keyboard is simply not correct, except for the one 
that you have calibrated your V/octave trimmers. The degree of accuracy 
will vary from note to note.

This of course happens on a monosynth, but it is simply less obvious to 
the listener.

All DACs will produce this error but at anything less than 10-bit it 
becomes more noticeable. Anything above 12-bit and other sources of error 
are more likely to be noticeable. However, even with a 12-bit DAC you 
still need to choose your Vreference carefully so as to allow the DAC to 
grab the correct 1V/octave chunks. ie. 5.333V, 10.667V etc.

The nice thing about 16-bit DACs is that you can chose any reference 
voltage since the DAC and its driving logic can produce any voltage with 
sufficient accuracy. You [but not me, I'm not clever enough] can even do 
glides, scaling trimming and other fancy stuff.

There are some 8-bit DAC with 9-bit [+/-0.5LSB] accuracy, eg DAC08HP from 
AD. This is a drop in replacement for the one used in the Paia unit and 
makes it slightly more usuable.

Tony

www.oakleysound.com

Oakley Sound Systems
Penrith, Cumbria, England