2.2.
Filtered Noise Generator
The noise generator produces white noise, that is, noise that is equally loud at all frequencies.
This noise passes through a voltage-controlled filter which removes some of the noise
frequencies. The filter has three different modes, selected with panel switch. The modes are as
usual for a VCF: low-pass, which removes high frequencies; high-pass, which removes low
frequencies; and band-pass, which removes low and high frequencies but preserves a range of
frequencies in the middle. The range of frequencies is nominally controlled by a panel knob but
can also be affected by an external control voltage and one of several internal signals, as
described below.
The filter also sports a resonance knob which causes the filter to emphasize the sound at the
corner or center frequency. If you turn this knob up far enough, the filter will oscillate, creating a
tone. This sounds kind of rough since there is still white noise entering the filter. However, if you
plug one end of a loose cable into the Filter Input jack, the noise will be removed and you’ll hear
a steady tone whose timbre varies slightly depending on the position of the Filter Mode switch.
2.2.1.
Filter Modulation
Setting the filter’s frequency with its panel knob allows you to change the timbre of
BoomTschak’s noise source (or that of an external signal). That frequency can be changed, or
modulated, by one of several signals while BoomTschak plays, imparting some motion to the
timbre. There are three internal sources of modulation available, selectable with the MOD
SOURCE switch: the noise-amplitude envelope generator, the oscillator pitch-bend envelope
generator, and the oscillator’s audio signal (post-VCA and wave shaper). The MOD DEPTH
knob controls how much the chosen signal modulates the filter’s frequency. It is a bidirectional
control; if it’s set to its center position, the filter won’t be modulated. Turn the MOD DEPTH knob
clockwise to make the filter frequency increase in response to the modulation signal, and turn it
anti-clockwise to make the filter frequency decrease. For example, if you’re looking for the
classic noise-with-downward-sweeping-filter effect. select one of the EGs with the MOD
SOURCE switches and rotate the MOD DEPTH knob clockwise from center.
The OSC position of the MOD SOURCE switch presents some interesting possibilities. If you
turn the oscillator frequency way down to a sub-audio rate, you can use it as an LFO to
modulate the filter, creating rapid flutters or slow undulations. If you turn the FILTER Q knob all
the way up to make the filter oscillate, you can modulate its pitch with the oscillator, creating
video-game laser noises, FM-synth pings, and so on.
Finally, the filter has a jack labeled FREQ. Any signal applied to this input will alter the filter’s
frequency, adding to whatever modulation you’ve created with the MOD SOURCE and MOD
DEPTH knob. Positive voltages increase the frequency; negative voltages decrease it. Audio-
rate signals will create FM effects, particularly if the filter is oscillating.
