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Section 0

Use of the Pressure Sensor

Thanks to Martin Straw [21010093+] for providing this document.

Preliminary Information
April 25, 1983

This is about the Pressure Sensor: what it is and some ways to use it. If this is used with the Chroma Computer Interface Kit, the interface must be Rev. 4 [or later, presumably; see Rev 5 Sequencer Manual].

The Pressure Sensor is a fully programmable, polyphonic, after-touch, pressure sensor. With it, the performer can effect one or more notes, independently of each other, in a number of programmable ways. The degree of the effect is controlled by the amount of finger pressure applied to one or more keys after they have been struck. The Pressure Sensor can be used to bend the pitch, add vibrato, control sweep speeds, accelerate attack times, accelerate decay times, modulate wave shape width, add or subtract brilliance, and/or control loudness. Any one effect or a combination of several effects may be used in a program. The Pressure Sensor is truly polyphonic; only the note sounding from each pressed key will be affected. Also, the effect can be latched if the key or keys are released rapidly. For example, if a note is bent sharp and the sustain pedal is depressed and the key is quickly release; the sustaining note will stay at the bent pitch.

Here are the parameters the Pressure Sensor can effect and some ways to use them. There are modified programs from "Factory Set 1" that illustrate these effects on the accompanying program sheets. To demonstrate an effect, change the program according to the instructions in the "COMMENTS" section on the program sheet. [I don't have a copy of these sheets--Chris]


The Pressure Sensor can modulate two parameters of the SWEEP section: 10 RATE MOD Value 1 and 12 AMPL MOD Value 1. In the RATE MOD parameter, the rate will increase in speed as the pressure increases. The maximum degree at which this happens is preset at the factory. This is useful when changing the rotating speaker effect speed in an organ program.

In the AMPL MOD parameter, the size of 11 WAVE SHAPE increases as the pressure increases. This could be used to add vibrato to notes.

Envelope 1 and 2

The Pressure Sensor can speed up the Attack and Decay times via 15 ATTACK MOD Value 1; 17 DECAY MOD Value 1; 22 ATTACK MOD Value 1; and/or 24 DECAY MOD Value 1. The ATTACK time will speed up as the pressure increases. The DECAY time will also speed up when the pressure increases. In both cases, the maximum amount of change is preset at the factory. These can be used to control attack timing or decay timing. For example, individual notes of a struck chord can be heard at different times by pressing on each note at different times.


The Pressure Sensor can be used to bend notes and there are two ways it can respond: PRESSURE and THRESH PRESS. PRESSURE uses a full range of pressure to control depth; THRESH PRESS (threshold pressure) needs a higher initial pressure before the pitch bends. PRESSURE Value 14 and THRESH PRESS Value 15 are in the chart for 27 MOD 1 SELECT, 29 MOD 2 SELECT, AND 31 MOD 3 SELECT. PRESSURE should be used for maximum control of a bend; THRESH PRESS should be used on programs that have a lot of velocity control and the keys get struck hard. Using THRESH PRESS helps prevent accidental bends on key strike. The maximum depth of a bend is controlled by the value of the appropriate MOD DEPTH. For example, MOD 1 SELECT's size is set by MOD 1 DEPTH. Remember, the range of MOD 1 DEPTH is small, MOD 2 DEPTH is medium, and MOD 3 DEPTH is full. In MOD 3 DEPTH, each value of depth equals a half step. A MOD 3 DEPTH Value 2 would enable enable a maximum bend of a whole step.

Wave Shape

The PRESSURE SENSOR can be used to change the width of the wave shape. To use it, select Parameter 35 MOD SELECT and change the Value to 14 or 15. The PRESSURE and THRESH PRESS work the same way as they do in the PITCH section except that the width of the WAVE SHAPE is changed. The depth is adjusted with Parameter 36 MOD DEPTH. It uses the Value in Parameter 34 WIDTH as a starting point of the change.


The brilliance of a sound can be modulated with the PRESSURE SENSOR by using Value 14 or 15 in Parameter 40 MOD SELECT 1, 42 MOD SELECT 2, or 44 MOD SELECT 3 along with a non-zero Value in the appropriate MOD DEPTH. (Use Value 15 in the MOD SELECT to prevent accidental brilliance changes with velocity controlled patches). When using a positive Value in the MOD DEPTH, the brilliance will increase as the pressure increases. A negative value will do the opposite. The starting point is controlled by Parameter 39 TUNE and any controls in the other MOD SELECTs affecting CUTOFF.


The volume in a program can be changed with the PRESSURE SENSOR by using Parameter 50 MOD SELECT 3 Value 1. The maximum depth of the effect is preset in software.