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Section 0

Programming Manual: Structure

Inside the Chroma

The Chroma's central computer controls all aspects of the instrument. Keyboard information, the control panel, the cassette player, pedals and levers all send their information to the central computer. There are eight dual channel synthesizer circuit boards which produce all of the Chroma's sounds. They also connect to the central computer.

The Chroma's sixteen synthesizer channels each consist of an oscillator, waveshaper, filter and amplifier. The channels are grouped into eight pairs so that they may be reconfigured, or "repatched," thus providing a wide variety of sounds. For ease of programming, one of the channels in each pair is labelled the "A" channel, the other the "B" channel.

The central computer controls the oscillators, filters, and amplifiers directly. The computer digitally generates 32 envelopes (two per channel) and 16 low frequency sweep signals. The control of the synthesizer channels is completely digital. Signals from the levers, pedals, control panel or the keyboard are all encoded digitally, processed by the central computer, and then sent to the synthesizer channels.

All of the parameters which determine a sound (including features like the keyboard split, transposition lever functions, etc.) are stored digitally in the Chroma's memory; therefore, programs may be recalled in their entirety. This same information may also be sent out to a cassette, or to an external computer or even another Chroma.

Synthesizer Channels

The synthesizer channels inside the Chroma have a structure that determines the kinds of sounds they create. One of the important things in determining the power of a synthesizer is the degree to which the structure can be varied. In other word, what can be patched into what. The Chroma has better patching capabilities than most modular systems, and it's fully programmable. Also important is the degree of control over critical adjustments. Not only does the Chroma have plenty of resolution on all its parameters, the sounds it creates are perfectly repeatable from channel to channel, from day to day, and from Chroma to Chroma. This is because all control signals are generated digitally by the computer, and all audio circuits are kept precisely tuned by the computer against digital standards.

This suffices for simple sounds that require only one oscillator and a two-pole filter. The two envelope generators allow a variety of shapes. Either is capable of generating ARs and ADRs by itself. Using two modulation inputs, say to the filter, the mix of AR and ADR envelopes yields the traditional ADSR shape, as long as the two attacks are the same. But note that the ADR and AR signals are still available separately. The AR might have a different touch sensitivity setting from the ADR (which is useful, not whimsical). Combining a fast percussive envelope with a slow percussive envelope yields a realistic "piano" envelope, with a rapid initial decay and a long final decay. Combining a short envelope with a delayed low attack envelope yields a sforzando envelope. And envelope 2 may be used for auto-repeat while the other creates a long decay, for a realistic echo effect.

The structure is easy to remember:

Basic Building Blocks

Each channel in the synthesizer consists of the following sections.


The pitch of the oscillator can be tuned from one octave below concert pitch (two with the transpose switches) to over four octaves above (over five with transpose). Each oscillator has three modulation inputs.


The waveshape can be selected to be either a pulse or a shape called "saws" which is a combination of a pulse and a sawtooth, simulating the sound of two sawteeth. The pulse width (and the saws shape) can be adjusted from 0% to almost 100% and can be modulated.


The filter can be set up as a low-pass or high-pass filter. Its resonance is adjustable from 0 up to self-oscillate. The tuning of the filter can be adjusted over the entire audio spectrum, and there are three modulation inputs.


The volume of the channel is controlled by an amplifier with linear control. The amplifier has two inputs for envelopes, which are fully adjustable, and has a third input for selecting special modulations, such as tremolo or pedal control.


The pitch information generated by the keyboard passes through the glide processor, which is capable of slowing down the transitions from pitch to pitch, either in a smooth portamento or a chromatic glissando. A wide range of rates are selectable. In addition, certain keyboard algorithms automatically enable and disable the glide according to how the notes are played.


The sweep generator generates low-frequency repetitive control signals. It has a basic rate that can be adjusted over a wide range. Its rate can be modulated by one of 15 other control signals. It has 16 waveshapes available, including sine, triangle, saw, square and random. Its amplitude can be modulated by one of 15 other control signals, including its own internal delay envelope generator. And lastly, it can be synchronized to key-depressions, and all sweeps can be locked together as one.

Envelope 1

The envelopes generate AR (attack, release) shapes or ADR (attack, decay, release) shapes. More complex shapes are created by combining envelopes. The Attack, Decay and Release times are variable from instantaneous to very long and can be modulated by one of 7 control signals. The release time can be made to respond to the rate the key is released. And the peak value can be made to respond to the force of attack in 7 different ways.

Envelope 2

This envelope is just like envelope 1, except that an adjustable delay is provided. Also, a special setting allows the envelope to be triggered off the sweep.

Performance Controls

There are six control signals available that come from various performance controls.

The two levers by the keyboard generate bipolar control signals. The two pedals on the floor generate unipolar control signals. The key strike velocity is available as a control signal, and key pressure is available as a hardware option. The lever and pedal signals differ from all other controls signal in that they are common to all channels.

Pairing Channels

Much more synthesizer power is available when the channels are paired together. This yields two glides, two sweeps, four envelopes, two oscillators, two waveshapers, two filters, and two amplifiers, in addition to the performance controls. The range of shapes available with four envelopes is vast. Having two sweeps is extremely useful, with one usually being reserved for vibrato. When the channels are paired, there are fifteen different ways that the audio building blocks can be patched together, including three forms of non-linear crossmodulation, ring-mod, sync and filter FM. The choice is controlled by the Patch parameter, which is parameter number 1 in each program. And, of course, either channel has access to the control signals generated by the opposite channel, so the patching possibilities are limitless.

Paired Channel Examples

Modulation Selections

The oscillator has three modulation inputs, the waveshaper one, and the filter three more. These seven modulation inputs each use two prarmeters, one to select the control signal to be used and the other to adjust the amount, or depth of modulation. The sixteen selections of modulation sources are the same for all thes inputs, and are thus called the general modulation selections. This list of selections is probably the most important list for the programmer to memorize. It can be found in the appropriate seven places in the Table of Parameters in the back of this manual.