Synth and Software reached out to groundbreaking instrument designer Roger Linn to survey alternatives for MIDI Polyphonic Expression.
Most electronic music is played on a MIDI keyboard, which is essentially an array of on/off switches arranged like the keys on a piano. Unfortunately, MIDI keyboards alone aren’t very good at enabling expressive musical gestures such as vibrato, pitch bends and slides, timbral variations, or continuous loudness control.
I make an expressive MIDI controller called LinnStrument. My goal is to deliver the same kind of expressive gestural control to electronic music performers that acoustic instrument players enjoy. Similar professional-quality expressive controllers include the Continuum Fingerboard from Haken Audio, the Eigenharp from Eigenlabs, the Joué from Joué SAS, the K-Board Pro 4 from Keith McMillen Instruments, the Seaboard from Roli, and the Soundplane from Madrona Labs.
Because they all support MIDI Polyphonic Expression (MPE), these instruments are often called MPE controllers. MPE is a fairly recent addition to the MIDI Specification that makes it possible to independently control numerous parameters of individual notes in a polyphonic performance using only touch.
MPE controllers have been designed to overcome the limitations of traditional MIDI keyboards. Although they’re catching the attention of many musicians, I’ve noticed some confusion about what they are and what they do. In this article, I’d like to help clear that up by describing some of the differences in how they approach musical expression.
Introducing…The Dedicated Problem Solvers
Compared with acoustic instruments such as strings and woodwinds, a traditional MIDI keyboard allows only a very limited set of performance gestures. For example, almost all MIDI keyboards sense how fast you press a key (strike velocity). Many also sense limited pressure (aftertouch), but only after the key is fully depressed and the note is already sounding. That makes it more useful for adding an effect than for controlling a sound’s envelope. Most also provide a couple of sideways knobs (wheels) for controlling pitch bend and modulation with your left hand, but even those are not ideal for performing natural pitch gestures.
How do MPE controllers solve this problem? In addition to sensing strike velocity and release velocity, they all sense continuous finger movement in three dimensions, polyphonically:
- Finger pressure (the Z-axis) to control note loudness
- Side-to-side movement (the X-axis) to vary note pitch and thus control vibrato, pitch bend, and slides
- Front-to-back movement (the Y-axis), usually used to control note timbre variation such as guitar pick position or wind embouchure
The designer of each MPE controller took a unique and innovative approach to devising a new human interface for performing music. Every one of them had to make choices regarding how the pitches are arranged, the materials used in the touch surface, the tactile feedback used for finding notes, and other aspects. In my view, no instrument does everything perfectly, but each offers specific advantages. What follows is a brief overview of the design focus of each instrument, with the intent of helping a potential buyer understand which design fits his or her personal musical focus.
The Continuum Fingerboard was the first such instrument when it was released in 1999.
Its inventor, Lippold Haken, presented his goals and the underlying technology to the public in an academic paper way back in 1986. The Continuum’s playing surface consists of a large rectangular area made from neoprene—a soft, compressible material most often used to make wet suits—and covered in a fabric that makes finger slides easy. Pitch is determined by the left/right position of where you place your fingertips. You can simply slide from the starting pitch to the destination pitch and wiggle your finger for vibrato.
Unlike on a piano keyboard, the semitones are equally spaced and therefore isomorphic, meaning that 1) for a given chord or scale, the fingering is the same for all 12 keys, and 2) a pitch slide of a given interval will always be the same distance regardless of key. The Continuum provides a generous physical range for Y-axis (front-to-back) control, and finger pressure controls note loudness.
The natural compression of the soft surface makes it easy to control note loudness with finger pressure because as you press harder, the compressed surface offers greater resistance to force. The continuous flat surface is less helpful for finding the notes by feel, but it has printed lines to indicate where the black and white keys of a piano would be located.
Additionally, the Continuum contains a built-in sound engine called the EaganMatrix, which is highly versatile and optimized for expressive sound control. The EaganMatrix gives the Continuum a nice advantage over controllers with no dedicated sound engine.
The Roli Seaboard’s notes are arranged exactly as they are on a piano keyboard, making it much easier for keyboard players to adapt.
Instead of keys, the playing surface is a single molded silicone rubber sheet with raised, rounded areas called keywaves located where a piano’s keys would be. These soft rubber bumps make it easy to find the notes by feel and control note loudness. Because the keywaves are the approximate length of a piano key, the Seaboard has a long Y-axis range, but the raised portions of the “black keys” have a shorter physical Y-axis range than the “white keys.”
Vibrato is achieved by wiggling your finger left and right on the keywave. You can perform pitch slides by sliding your finger continuously from one keywave to another. Because sliding over the raised bumps is a little cumbersome, though, the Seaboard has two flat, continuous pitch strips, one above and one below the keywaves. To perform a pitch bend or slide, you play a starting keywave, slide up or down to the pitch strip and perform the pitch slide, and then slide back to the destination keywave. Overall, it’s an effective way to add pitch bends and slides to the familiar piano keyboard layout. For sounds, Roli bundles the Seaboard with two excellent soft synths, Equator and Strobe2, which are optimized for its use.
The Soundplane from Madrona Labs has a playing surface made from wood that deforms slightly when pressed.
This can be preferable to a soft surface like the Seaboard or Continuum when playing fast rhythmic parts. You can arrange the notes in a variety of ways, but by
As with the Continuum, this note arrangement is isomorphic and therefore has the same advantages. LinnStrument uses this same note arrangement, but the Soundplane has a unique advantage: while LinnStrument’s note pads are 3/4-of-an-inch square and therefore have a limited Y-axis physical range, Soundplane’s note regions are tall rectangles. This gives Soundplane a greater Y-axis range at the expense of note density (how many fit in a given space). That places it as an attractive compromise between the Continuum’s single row with a long Y-axis range and LinnStrument’s multiple rows with a short Y-axis range.
To help find notes by feel, the Soundplane has tactile indentations in the playing surface between the note regions. It comes with a versatile soft synth plug-in called Aalto that is optimized for its expressive control.
The Eigenharp from Eigenlabs uses a grid array of custom button switches that capture pressure and tilt along two axes like little joysticks.
They are easy to find by feel, like any physical buttons, and are highly sensitive and fast to play. Because separate buttons don’t permit you to perform a pitch bend by sliding your finger continuously from one button to another, of course, the Eigenharp has a pitch-control strip on the side of the instrument’s neck.
If the button tilt is assigned to pitch, it can be challenging to hold the button steady enough to maintain the desired pitch, but you can adjust the sensitivity to compensate. The note arrangement can be almost anything you want, but by default the rows are arranged in consecutive diatonic scales. The intended advantage is that you can develop fingerings in a major scale and then switch to a minor scale note layout and use the same fingerings for a minor mode. A built-in breath pipe controller provides a handy alternative to button pressure for controlling note loudness. Sound software for your computer is included, and of course it can play any other synth via MIDI.
The Joué from France’s Joué SAS has a similar look to my LinnStrument — with a rectangular playing surface and wood sides — except in a compact size and with a very attractive price.
The Joué’s note arrangement can be reconfigured by placing one of a number of flexible overlays onto the flat playing surface. You can purchase one for piano keys, one with an array of vertical note regions like on the Continuum, one with a grid for a stringed-instrument note layout like on LinnStrument, one for drum pads, one for knobs and sliders, plus some 1/3-width ones that you can mix and match as you please. These overlays include raised delimiters between the note regions for tactile feedback. Though the compact size limits the total playing pitch range, the Joué works well for playing expressive solos and simple chord progressions.
The K-Board Pro 4 from Keith McMillen Instruments is the closest thing you’ll find to the feel of MIDI keyboard.
Like the Seaboard, its notes are arranged as on a piano keyboard, but the keys are much closer to the shape and feel of real piano keys, with separate key elements and raised black keys made from molded silicone rubber. Each key responds to pressure and deforms slightly for tactile feedback. Vibrato can be performed by wiggling your finger left and right, and it uses the full height of the keys for Y-axis control.
Because the keys are separate, you can’t perform a pitch slide by sliding your finger directly from note key to another, but four continuous horizontal control strips are located behind the keys. The leftmost of these is used for pitch bend, like a bend wheel except more intuitive, because the horizontal pitch movements are in the same direction as the pitches of the keys.
LinnStrument, by my company Roger Linn Design, has a rectangular playing surface with finger-sized note pads arranged as on a stringed instrument.
There are eight rows of note pads with each row consisting of consecutive semitones. Like the Continuum and Soundplane, this note layout is isomorphic, giving it similar advantages. And like Soundplane, the overlapping pitches of the rows permit multiple instances of each pitch.
For tactile feedback, the note pads have tiny troughs between them, allowing you to feel where the notes are without impeding the performance of horizontal note-to-note pitch slides or bends. Each note pad has an RGB LED under it, used to display scale notes as well as the scale’s root note in different colors.
The playing surface is translucent silicone, which deforms only about 0.5 mm when played. That doesn’t give the pressure feedback of a soft surface like the Continuum or Seaboard, but it can be preferable for fast, rhythmic play. LinnStrument contains no built-in sounds but rather sends MIDI data through MIDI jacks or USB.
To Be Continuous…
Each of these instruments has unique advantages by virtue of its unique design focus. If you’re looking for an expressive instrument to play, your choice largely depends on which of those advantages apply to your particular style of playing.
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