GeoSWAM Instruments: GeoCello, GeoViolin, GeoViola, GeoTenorSax, GeoOboe, GeoFlute, GeoBassFlute, GeoClarinet, GeoTrumpet, GeoBassTrombone
Every GeoSWAM instrument has an Expression controller, which is the main dynamic level control (for playing louder or softer). This is more than an amplitude control! Acoustic instruments such as bowed strings and woodwinds exhibit many sonic phenomena as they are played louder or softer.
In GeoShred, any GeoShred UI control can be mapped to GeoSWAM Expression. However, the GeoSWAM Presets, included with GeoShred, control Expression from either pad expression via the Expression-Pad Y-Axis; or finger expression using KeyY vertical sliding or KeyZ pressure (on iPhones with 3D touch). To support this, there is a special Performance > Finger Expression Controller that toggles between these choices so that one Preset can support both cases. The difference between these two modalities is that pad expression is continuous across multiple notes, and finger expression is re-initialized for each new note.
Externally from MIDI, Expression can only be controlled by the MIDI/MPE controller Channel Pressure (Mono Aftertouch, NOT poly Aftertouch), which is typically KeyZ on MPE controllers
While the x and/or y axes of the GeoShred Expression Pad can be mapped to control GeoSWAM Expression, they are otherwise not related. The three occurrences of the word "Expression" on the Expression Pad for many GeoSWAM instrument presets may be understood as follows:
On any GeoSWAM instrument, Note-On Velocity controls
Legato and Portamento pitch glides are initiated by connected or overlapping notes (a common feature of mono-mode voices).
Note-On Velocity does not control Expression (dynamic level), which has its own Expression controller (as discussed above). A Legato note transition tries to preserve dynamic level from one note to the next as it smoothly changes the pitch or rearticulates the note.
Portamento is a slow, automatic pitch glide from one note to the next. Portamento pitch-glide is engaged when the second note is connected or overlapped with the first note (as in Legato) and the Note-On Velocity of the second note is below the Legato Velocity Threshold.
Portamento speed is proportional to the Note-On Velocity (which must be below the Legato Velocity Threshold).
In real life, not every instrument can bend continuously from one note to any another. For example, pitch-bend on a real flute involves tilting the embouchure, which can only bend so far. On a real saxophone, the player squeezes the reed up and down, which is also limited in bend range. On a clarinet, in addition to squeezing the reed, the player may slide fingering along the bore. On a bowed-string instrument, the player shifts the finger stopping the string, which is limited by the length of the stopped string over the fingerboard.Thus, the Continuous Portamento Range differs from instrument to instrument. Beyond the Continuous Portamento Range, Chromatic Portamento is used for woodwinds.
Chromatic Portamento means that the interval from the first note to the target note is performed automatically along a chromatic scale.
In GeoSWAM models, the Slide Range and Continuous and/or Chromatic Portamento Ranges are as follows (with some exceptions noted below):
The speed of the Portamento transition depends on the Note-On Velocity of the second note. In many of the GeoSWAM instrument factory presets, Note-On Velocity is controlled by Key Y position when the "Low V Porta Enable" Control Surface button (Portamento Enable controller) is on.
Since Flute behavior exploits overtones to produce higher notes, not all intervals can produce a portamento between notes. Intervals across register splits do not produce any portamento.
For the GeoFlute, register splits occur between:
Crossing a register break when playing a real flute requires a rapid change of both the pipe length and the overtone. This leads to a characteristic "knock" when playing legato, particularly audible when playing a trill. (More about flute physics can be found at the SWAM-W (Woodwinds) website.)
Audio Modeling has implemented an algorithm that tries to eliminate undesirable "trill knocks": If the trill crosses a register break, the register split-point is automatically shifted +/- 1 semitone to remain outside the trill range.
The GeoFlutes have a playable range of breath pressure, i.e., a range over which the fundamental frequency of the generated note matches the current pipe length, as selected by the player's fingering.
When the player blows harder, higher overtones are produced. The GeoFlute models have been calibrated so that the range of Expression (which is proportional to Breath Pressure) rarely engages overtones, unless the "Overblow" parameter is enabled.
When "Overblow" is enabled, "Overblow Threshold" controls the level of Expression at which the higher overtones get engaged. Note that overtones do not depend on the absolute value of Expression only, they depend also on sudden variations of Expression, for example during steep attacks.
Setting "Overblow Threshold" to a value above 0.5 means that overtones get engaged even with a very low value of Expression.
Setting "Overblow Threshold" to a value below 0.5 means that overtones get engaged for steep Expression variations or for very high values of Expression.