Appendix B - Filter & Vca Behavior - Abstrakt Instruments Avalon Reference Manual

APPENDIX B - FILTER & VCA BEHAVIOR
1 FILTER CONTROL INPUT BEHAVIOR
Because the Avalon filter control input is based on the TB-303, and the TB-303 is a battery powered 'single supply' instrument, the resting state of
the filter control input is set to a positive voltage in order to allow for negative modulation. This value is 3.28V. Because of this, a control voltage
input less than 3.28V will decrease the filter cutoff frequency and an input greater than 3.28V will increase the cutoff frequency. The control input
can be viewed as a standard 1V/octave input simply offset by 3.28V.
1.1 EFFECTS ON FILTER ENVELOPE
If you pay particular attention to the way the filter envelope effects the filter throughout the range of the ENV MOD control you may notice some
peculiar behavior. If you have a more experimental approach and prefer to find good sounds and settings without particular attention to what's
going on behind the scenes you may not want to read this. If you are more analytical and wanted to understand the machine behavior and what the
adjustments are doing you may be interested in what follows:
You will notice that for increasing ENV MOD amount the filter decays to a lower frequency, so for increasing ENV MOD the control voltage has a
greater range but also has a progressively negative offset. The graph below shows what's happening. As the ENV MOD amount is reduced the
control voltage converges towards the filter resting voltage of 3.28V. The reason for this is that the filter envelope is a 0-10V control voltage, but the
ENV MOD control is referenced to the filter resting voltage, so the less ENV MOD the more the filter envelope converges to the reference.
1.2 EFFECTS ON MODULATION ENVELOPE
The control voltage from the MODULATION ENVELOPE to the filter is progressively offset in the same way as ENV MOD above. This allows for a
wider range of complex modulation. As the VCF DEPTH amount is decreased from (+) modulation towards the center off position the control
voltage converges to 3.28V. Likewise, as the VCF DEPTH amount is decreased from (-) modulation towards the center off position the control
voltage also converges to 3.28V.
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AVALON - Reference Manual