[sdiy] Digital pots as the gain element in a filter (Roland Gaia)

[Richie Burnett]

> I was in a music store a couple of months ago, and spent some quality time
> with some Roland or Korg keyboard or other.  I can't remember which one it
> was, but I think it might have been the Roland GAIA.  Anyway, it had a
> decidated filter cutoff knob on the panel.  If you swept this knob while
> holding down a note, it swept the cutoff in digital steps.

The Roland Gaia is odd.  I was lucky enough to get a loan of one for a few 
nights just after it came out, and in my opinion it's a pretty naff synth. 
Some of the presets sound quite impressive with all of the effects on, but 
if you listen to the raw VCO waveforms, or slowly sweep the filter, there 
are loads of weird things going on that wreak of bad digital modelling. 
Stair-stepping (quantisation) or simply slow updating of the filter 
parameters was one obvious thing.  The filter resonance also sounds very 
boomy and out of control, compared to how a real analogue filter smoothly 
transitions into self-oscillation.  It also undoubtedly uses wavetables for 
the raw oscillator waveforms because if you experiment playing up and down 
the keyboard and pitch-bending notes you can hear abrupt transitions and 
discontinuities in the brightness (harmonics) that plague multi-sampled 
waveforms.  So really I think Roland are just wrapping up another "wavetable 
playback through digital filter" synth as "virtual analogue"  Whilst you get 
a lot of spec on paper for your money, and it's not a bad synth to learn 
about subtractive synthesis, the raw oscillators and filter certainly didn't 
have the smooth analogue feel of a Nord Lead.  Anyway, just my initial 
impressions.

-Richie,