Polaris Service Manual: System Overview
The Chroma Polaris is a hybrid, combining the rich sounds of an analog synthesizer with the nearly limitless control capabilities of a digital computer. The latest microchips are evident in both domains: Curtis CEM 3374 dual VCO's and 3372 VCF/VCA'S in the analog channels and the very powerful Intel 80186, 16-bit microprocessor in the digital computer.
It features a 61 note velocity sensitive keyboard, storage for 132 different programmed sounds and an onboard polyphonic single track sequencer with a minimum note capacity of 650, internally expandable to more than 9000. In addition to a host of other features, the Polaris provides a parallel Chroma port and a serial MIDI interface.
There are 6 voices in the synthesizer. Each voice may be divided into 2 major parts: signal processing and modulation. The signal processing elements, which generate and shape the sound of each note, are implemented in the hardware. The modulation sources generate the signals that controls the signal processing elements and are implemented in the software.
Each voice consists of the following signal processing elements:
Each oscillator can be transposed in semitones over a wide range, and oscillator 2 can be detuned up to a semitone sharp. Vibratos and trills can be applied to the oscillators via the Mod Lever, with an adjustable depth. Delayed vibratos and trills can also be directly applied to the oscillators. The depth or this type of modulation is adjustable independently for the two oscillators, and can have positive or negative polarity. Oscillator 2 can be synchronized to oscillator 1, and oscillator 2 can be modulated by the main envelope, especially useful with sync.
Two basic shapes are available, Pulse and Saws. The pulse width is adjustable from 0% to 100%, and can be modulated by either the sweep or the main envelope. A simple sawtooth shape can be obtained by selecting the Saws shape and setting the pulse width to either extreme. An oscillator can be turned off by selecting the Pulse shape and setting the width to either extreme.
When this function is selected, the signals from the two oscillators are replaced by the digital cross-product of the two pulse signals. In this mode, the Saws/Pulse selections have no effect, but the pulse width controls do.
This is a pink noise source that feeds into the filter. It can be turned on or off.
This is a four-pole low-pass filter with resonance that can be adjustable from none at all to self-oscillation. It is modulatable by the sweep (delayed vibrato or trill), the main envelope, the foot pedal and the keyboard. All modulations may be positive or negative.
The volume is controlled by its own dedicated envelope and its own volume control parameter. In addition, the foot pedal can modulate the volume with an adjustable depth.
These signal processing elements are controlled be a number of modulation signals:
This generates either sine waves or square waves over a wide range of frequencies. In sine wave mode, all generators free-run asynchronously. In square wave mode, all generators run at the same rate, and each is synchronized to key depressions. The square wave shape is unipolar. The sweep rate can be modulated by the foot pedal to any degree in either direction. In addition, there is an adjustable delay that can be applied to the sweep.
The main envelope is an ADSDR type that can be used to modulate the pitch of oscillator 2, the pulse width of either oscillator, or filter cutoff. A switch allows it to be made touch-sensitive or not. The envelope operates in four phases. During the attack phase the envelope rises to its peak value and switches to its decay phase. It decays at an adjustable rate to the sustain level which is a percentage of its peak value. It then switches to the sustain decay phase during which it decays toward zero. When the note is released, the envelope is forced into its release phase. The two decays and the release can be set to infinity, if desired. A conventional ADSR shape can be approximated by setting the sustain decay time to infinity.
This is an ADR type envelope that controls the volume only. A switch allows it to be made touch-sensitive or not. The decay and release can be set to infinity, which is useful for drone effects.
The left lever is dedicated to controlling the amount of sweep modulation (vibrato or trill) applied to the oscillators. Its depth is adjustable in both directions. Note that the sweep signal applied through the mod lever to the oscillators is not the delayed sweep.
The right lever is dedicated to the pitch bend function. Both depth and polarity are adjustable.
The foot pedal puts out a unipolar signal that can modulate pitch, filter cutoff, volume, sweep rate and sweep depth into the oscillators, all with adjustable depths. As with the mod lever, the sweep signal applied through the pedals to the oscillators is not the delayed sweep.
The keyboard is of course the main source of pitch information to the oscillators. Changes from note to note can be instantaneous or can be slurred by the glide control. The glide time is adjustable from 0 to 10 seconds. The keyboard pitch can also be used to modulate the filter cutoff. The depth of this modulation is adjustable over a wide range and allows no tracking, unison tracking, undertracking or reverse tracking. A special control transposes the keyboard an octave up without affecting notes already sounding. This effectively makes the keyboard an octave longer.