The Oberheim Xpander was an analog synthesiser launched by Oberheim in 1984 and discontinued in 1988. It was essentially a keyboardless, six-voice version of the Matrix-12. Utilizing Oberheim's Matrix Modulation technology, the Xpander combined analog audio generation (VCOs, VCF and VCAs) with the flexibility of digital controls logic.
The Xpander "Owner's Manual, First Edition" describes the technology as this:
"An analogy to the Matrix Modulation system might be all of those millions of wires that existed on the first modular synthesizers. As cumbersome as all of that wiring was, it alllowed the user to connect any input to any output, resulting in sophistication and flexibility unmatched by any programmable synthesizer...until now."
Architecture
Analog Components
Each of the six voices of the Xpander is completely independent. That is to say, each could be configured to create a different timbre - this is accomplished via the multi-patch mode which will be described below.
Starting at waveform generation, each voice has two voltage controlled oscillators (VCOs). Each of which is capable of generating sawtooth, triangle, pulse or noise waveforms. Furthermore, the "duty cycle" of the pulse width can be modulated as well. Although perhaps better known for subractive synthesis, the Xpander is also capable of frequency modulation (FM) synthesis by modulating VCO 1 with VCO2.
Moving on from the VCOs, the signal then passes through a multi-mode voltage controlled filter (VCF). The available modes on the filter are:
one-, two-, three- and four-pole low pass
one-, two and three-pole high pass
two- and four-pole band pass
two-pole notch
three-pole phase shift
two- and three-pole high pass plus one-pole low pass
two-pole notch plus one-pole low pass
three-pole phase shift plus one-pole low pass
From the filter, there are two sequential voltage controlled amplifiers (VCAs) through which the signal must pass. And finally the audio is delivered to a variety of outputs: mono, stereo and six independent outputs (corresponding to the six voices).
Digital Controls
Of those analog audio components (VCOs, VCF and VCAs), each can be modulated by several different digital controls.
ADSR Envelopes - each voice can have up to five envelope generators. Each envelope is of the standard Attack-Decay-Sustain-Release model (ADSR), with the addition of an initial Delay phase, thus making them DADSR envelopes, to be exact.
Low Frequency Oscillators (LFOs) - each voice can have up to five LFOs applied. Each LFO can have a different waveform: triangle, square, up-saw, down-saw, random or noise. Additionally, a sampling mode is provided, whereby an independent source (e.g. a different LFO) is sampled at a set frequency.
Lag Generator - the lag function is similar to portamento on traditional sythesizers. However, the lag modulation in the Xpander can be applied to any control or audio signal.
Ramp Generators - each voice can have up to four ramp generators. Similar to the attack portion of an ADSR envelope, the ramp generates a linear signal from zero to the user-defined ramp height.
Tracking Generators - there are three tracking generators available for each voice. The tracking generator provides a mapping from a control source (e.g. key range on the keyboard, or volume pedal, or mod wheel) to a modified output, based on the user-defined settings of the generator.
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