[sdiy] Roland TB-303 filter responses graphed

[Richie Burnett]

For anyone interested, this graph shows the response of a real TB-303 filter 
set at full resonance for a range of cutoff frequencies:

http://www.richieburnett.co.uk/temp/303vcf.gif

There are lots of theoretical responses, mathematical transfer functions, 
and simulation results on the net, but i've never seen any real measured 
responses before.

These responses were generated at the same drive level that the sawtooth VCO 
drives the filter in the TB-303.  (The shape changes if the drive level is 
changed.)

Notice how the resonance is much stronger at higher frequencies, and runs 
out of steam at low cutoff frequencies.  The little peaks and dips to the 
right of the normal resonant peaks, are due to harmonic distortion in the 
filter and the VCA that follows it in the TB-303.  Also notice how the 
actual resonant peak of the TB-303 has a steeper slope on the right side, 
than the left side when the Q is high.  This "lop-sided" resonance is 
another artefact of the filter's non-linearity and occurs naturally in 
mechanical resonators as "hardening".

http://www.zainea.com/lowresonances.htm

Hopefully this is interesting to some TB-303 scrutinisers.

-Richie,