General Set-Up Functions Cutoff Resonance
Contents
1. General The 24dB VCF module Voltage Controlled Filter is a low pass filter using the famous filter cascade invented by Bob Moog However this filter is not meant to be a 100 copy but offers its individual sonic fingerprint Set Up 24dB VCF is fully compatible to Doepfer s A 100 modular system in size bus power and CV Gate voltage Connect the 16 pin cable to a corresponded 16 pin jack on the Doepfer mainframe bus or the MIDI CV circuit board Supply voltage needs to be 9 12 volts 5 volt connections are not required The wattage is 30 mA the module size 8 TE Teileinheiten or 40 mm ATTENTION Please check for correct polarity The colored side of the connector cable needs to point downwards so that the cable is not twisted Functions A few words on the basic principle of a filter The filter receives audio from oscillators a noise generator or any other signal at its audio input The cutoff control will now manipulate the signal s harmonics content Since 24 dB VCF is a low pass design turning the cutoff control counterclockwise will reduce the amount of harmonics present in the output signal Resonance pronounces the filter s working point resulting in the typical analog filter sound Control voltages CV can modulate both parameters The signal then reaches the audio output where it is routed to additional sound design tools or directly to the amplifier VCA The 24dB VCF module has two audio inputs Input In1 o2. ffers a corresponding attenuator while Input In2 has fixed sensitivity that matches the summed output of OSC 01 and OSC 02 modules Input In1 is internally prewired to the filter output With In1 not being patched its attenuator controls filter feedback to supply additional bass punch Cutoff The filter s cutoff frequency can be controlled manually or via CV inputs Input CV1 with attenuator is meant for standard modulations by an envelope generator CV2 has two functions e It carries the mainframe system Key CV bus information as applied from a MIDI CV interface or a bus access module This allows direct keyboard tracking of the cutoff frequency Tracking intensity is set with the trim control CV2 Key e The front panel input CV2 allows a second control voltage to be used for cutoff modulation e g from a LFO Here the modulation intensity is set with trim control CV2 Key A jumper on the module s back sets or disables internal wiring Disconnection from the mainframe bus will allows independent use of the CV2 input Resonance In reminiscence to Moog s filter cascade the resonance control is named Emphasis Emphasis can be either controlled manually or by CV applied to input CV Emp with corresponding attenuator With Emphasis set higher than six this value could slightly differ between modules due to component tolerance the filter starts self oscillating Without any signal applied to the input the filter generates a sound
3. that is close to a sine wave This sound s pitch is controlled by the cutoff frequency With cutoff under Key CV control this sound is even playable to a certain extent However the filter is not as precisely controllable as an oscillator Hint Create nice analog percussions using a self oscillating filter Many classic analog drum machines use self oscillating filters instead of VCOs For this the cutoff frequency needs to be controlled by a fast envelope generator short decay low sustain level Additional coloring can be applied by mixing in filter FM modulate the filter with an oscillator or a fast LFO pr 7 Operating Manual 24dB VCF Module
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