DD1 – Modulator / Low Pass Gate
The DD1 is a dual module providing a Modulator (BalMod) and a Low Pass Gate (LPG) in a compact
form. At its most basic, it can be seen as two separate VCA sections, but each has some very different
behaviours possible on top.
The Modulator section
is built around the AD633 Precision Multiplier chip. The main application is
Ring Modulation (RM) where two input signals are combined to generate the sum and difference
frequencies. The RM is DC coupled and can also be thought of as a VCA but with the ability to invert
the signal under voltage control. For example, feed a VCO signal into one side and then a DC control
into the other input - raising the DC control from 0V upwards will give typical VCA response while taking
the DC control below 0V will give a VCA response but with the polarity inverted. (eg. Use the CTL2
Joystick to achieve this)
The Amplitude Modulation (AM) mode can be thought of as a little bonus - it does not behave as a true
VCA in that it doesn't take the signal down to zero, but gives a very different sound to the RM mode. It
can be used for more subtle tremolo effects.
The Low Pass Gate
was created by Don Buchla and imparts great effects on audio signals as a
combined VC Filter / Amplifier. The key to the LPG is the Vactrol - a variable resistive element, driven
by an LED, which changes the intensity of the audio processing. It is the slightly slow response of the
Vactrol that gives the audio effects such a natural feel and is the wonderful strength of the module. The
LPG can be run in three different modes - VCF, VCA or a middle mode that combines the two.
Try adjusting the depth of modulation to achieve different timbres / responses. A traditional route may
be to use the ENV1 to modulate the input, but further variation can be achieved if the Env is mixed with
an LFO waveform via the bottom section of the DD2 module.
