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Architecture.md

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Architecture

The Max Handpan architecture consists of three parts:

Frontend

The frontend interfaces with the control surface. Currently the only control supported control face is the Ableton Push 2, but support for other controllers such as the Ableton Push 1, Novation Launchpad or the Akai Pro Force could in principle be added as well.

The frontend has the following responsiblities:

  1. Configure the layout and colors of the controller. For example, on the Push 2 it currently looks something like this:

    The layout here is an attempt to mimick a real-life handpan as close as possible: tone fields around the outside, doum and tak in the middle, slaps (red) on the very outside1, and a bass note at the bottom. (The bass note is normally played on the shoulder of the instrument, but the placement on the bottom of the button matrix of the Push means we can play the base with a fist, as it is normally done.)

  2. Since a handpan's sensitivity to force is non-uniform, the frontend provides multiple velocity curves so that different areas of the handpan can be configured differently.

  3. Generate MIDI data to interface with the next stage in the pipeline, the backend. The MIDI data here reflects the physical layout of the handpan rather than refer to specific notes; it is reminiscent of the MIDI data for a drum controller. See MidiLayout.md for a specification of the MIDI layout used.

See Frontend: Ableton Push 2 for a technical description of the frontend.

Backend

The backend has the following responsibilities:

  1. As mentioned, the MIDI data coming from the frontend reflects the physical layout of the handpan; for example, it might record something like "tone field 3, tone field 1, doum". A real-life handpan is of course tuned to a specific scale, so on particular drum this sequence might sound like "C4, A4, D3". One advantage of the virtual handpan over a real one is that we can change the scale on the fly: the first responsibility of the backend is to take the MIDI data from the front end and interpret it with respect to a choice of scale.

    The backend allows a choice of scale (Kurd 9, Hijaz, etc.; see the full list of scales). The root of the scale and the note of the doum can be chosen independently; for example, there are some handpans with a D Hijaz scale over a doum of G, and some handpans with a D Hijaz scale over a doum of A.

  2. While the scale selection part is instrument independent, the backend's second main responsibility is not: it must translate the MIDI data from the frontend, as interpreted through the scale options, into MIDI data that can then be turned into actual audio by a choice of Ableton instrument. Currently the only instrument supported is the Soniccouture Pan Drum, in particular, the Pan Drum Ableton Live pack.

    This Live pack actually contains two sampled hand pans, a first generation hang tuned in A Pelog with a doum of F, and a second generation hang tuned in A Integral with a doum of D. The Max Handpan backend supports both (select mk1 or mk2).

    The backend translates the incoming MIDI data into MIDI data for the instrument, translating tone fields not just into notes, but also into key-switch commands that set parameters such as articulation; for example, when playing a tone field, the "Mid" articulation is chosen, but when playing ghost notes, typically played in between tone fields on the shoulder of the instrument, the "Between" articulation is chosen.

    Many other instruments could in principle be supported; see list of instruments.

  3. Since the sampled handpan is tuned to a specific scale, any deviation from this scale will require transposition. The slides shown by the backend show how much transposition is required to emulate the selected scale; the first slider represents the doum, the other slides the tone fields. It also shows the minimum and maximum transition used, in semi-tones. (It is useful to remember that samples sound better when transposed down then when transposed up.)

See Backend: Soniccouture Pan Drum for a technical description of the backend.

Instrument

The instrument takes the MIDI data created by the backend and turns it into actual sound. Ideally, the instrument is not part of the Max Handpan infrastructure at all, but is an off-the-shelf Ableton Instrument or VST.

Unfortunately, unlike many VSTs, Ableton Instruments do not typically support key-switching. Specifically, the Soniccouture Pan Drum is essentially just a collection of configured Sampler instances. To choose a particular articulation, we must set an instrument rack Chain slider instead of sending key-switch commands.

While that works fine for MIDI programming melodies, it does not work for the "just in time" articulation that we need to do when playing the handpan: we don't want to first chose an articulation, and then play it; we just want to strike the corresponding part of the instrument and have the articulation chosen correspondingly. While it is possible to control parameters such as the Chain Selector from within a Max for Live device, this unfortunately has a fundamental limitation: while we can change a parameter, we do not know when the parameter change takes effect, and so when we send a MIDI note, we have no guarantee if new parameter setting will be applied to that note or not. Max for Live devices that emulate key-switching such as the excellent KeySwitch & Expression Map for Ableton Live suffer from the same limitation.

Consequently, for the specific case of the Soniccouture Pan Drum, Max Handpan additionally includes a custom instrument that uses the sample data from the Soniccouture Ableton Live pack, but does not use Sampler. See Instrument: Soniccouture Pan Drum for a technical description of this instrument.

Footnotes

  1. Currently we only have two areas for slaps, one on the left and one on the right. I posted the following question in the Handpan Dojo group:

    For percussive slaps, hitting the handpan near different zone fields (in between different tonefields) results in a different sound. Do you use this to your advantage when playing, or do you just play what is convenient? If the former, how do you think about it? Do you associate a particular slap with a particular tone field (like, "this is a D slap" or whatever), or do you just gradually learn how the different slaps sound, and you don't really think of them as notes as such?

    and David Kuckhermann answered:

    I check out the various spots on an instrument for slaps and usually choose the ones that give me the strongest accent with as little "tone" from the neighbouring tone fields as possible.

    So having two slaps available will probably suffice. As an potential improvement, we could use the additional sample data for further round robin sample selection, or make it configurable which slaps to use.