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Chapter 4: Andromeda A6 Overview
98 ANDROMEDA A6 REFERENCE MANUAL
Tone Modifiers
Filters
Referring back to Chapter 3 again, the second component in the signal flow is the
synthesizer’s Filters – the circuitry that controls the harmonic content of the sound
wave. Like the A6’s VCOs, its filters are true analog Voltage-Controlled Filters. We’ll
refer to them throughout the rest of this manual as VCFs or by their panel labels
FILTER 1 and FILTER 2.
The A6’s VCFs operate by removing unwanted frequencies from the sound waves
generated by the VCOs, the Noise Generator or audio signals input to the A6 via the
EXT FILTER INPUTS on the unit’s rear panel. This is commonly referred to as subtractive
synthesis whereby the VCOs, for example, generate a full spectrum of harmonics, and
the VCFs are used to “filter out” unwanted overtones. If the sound you are creating
calls for the full range of harmonics to be heard, you can either “open up” the VCFs
by turning their
FREQ controls fully clockwise or press the FILTER BYPASS button.
Although both VCFs appear to have identical controls, they are quite different.
FILTER 1 is a multi-mode filter – it filters harmonics in several ways. FILTER 2 is a single
mode filter – only one type of harmonic filtering is accomplished by this circuit. We’ll
cover these modes in detail in Chapter 5.
Last, the A6’s VCFs are designed such that the output of
FILTER 1 can be routed into
FILTER 2. This allows additional control over the harmonic content of the sound wave
by providing a means of “filtering a filter”.
Ring Modulator
The A6’s Ring Modulator, labeled on the front panel as RING MOD, is another classic
analog tone modifier. It generates a series of sum-and-difference overtones from the
input signals, resulting often in a metallic, distorted character that is frequency-
dependent. For more on Ring Modulation, see Chapter 8.
Modulation Sources
The term modulation means “to change”. Musically, we use the term modulation
when referring to a key change in a composition. In the world of synthesis, the
concept of change applies to any control that causes variation to the original sound
wave over a period of time.
The A6 offers the common sources of modulation in a synthesizer: Low-Frequency
Oscillators, Envelopes, and the Tracking Generator. (Strictly speaking, however, any
change to the sound wave’s components would be considered a mod source. The
keyboard, for example – and for that matter incoming MIDI notes – would be
considered a modulation source because it changes the frequency of the VCOs.)
The A6’s Low-Frequency Oscillators – hereinafter referred to as LFOs – are similar to
the A6’s VCOs in that they produce periodic waveforms. The difference is in the
“low-frequency” designation: LFOs produce waveforms whose frequency or pitch is
usually below our threshold of hearing. Because of this, LFOs aren’t considered a
source of sound in the A6 and are not part of the audio path. They’re only used for
modulation.
Because of their low-frequency nature, LFOs are not only perfect for the familiar
vibrato and trills, they provide a virtually unlimited source of special effects. In
addition to the periodic waves, a fourth LFO in the A6 provides Sample-and-Hold
modulation –
S & H on the front panel – a classic analog mod that produces a random
modulation wave.
The A6’s Envelopes are the time-dependent modulators we introduced in Chapter 2.
Commonly used to shape the harmonics (
ENV 2 (FILTER)) and overall loudness (ENV 3