Add Scientific Pitch Notation converter helper to synthio

[CedarGroveStudios]

In addition to the MIDI note value to frequency converter helper already in synthio, it would be very handy to include a converter for a note's Scientific Pitch Notation (SPN) string to a note frequency.

Here's an example of a helper in CedarGroveStudios/CircuitPython_MIDI_Tools (also in the community bundle) that converts SPN to a MIDI note value.

NOTE_BASE = ["C", "C#", "D", "D#", "E", "F", "F#", "G", "G#", "A", "A#", "B"]

def name_to_note(name):
    """Translates a note name to a MIDI sequential note value. Note names are
    character strings expressed in Scienfic Pitch Notation (NoteOctave) format
    such as 'C4' or 'G#7' with middle C defined as 'C4'. Note names range from
    'C-1' (note value 0) to 'G9' (note value 127). Note values are of integer
    type in the range of 0 to 127 (inclusive). If the input value is outside
    that range, the value of `None` is returned.

    :param str name: The note name input in SPN format. No default value.
    """
    name = name.upper()  # Convert lower to uppercase
    if "-" in name:
        octave = int(name[-2:])
        name = name[:-2]
    else:
        octave = int(name[-1:])
        name = name[:-1]

    if name in NOTE_BASE:
        # Note name is valid
        note = NOTE_BASE.index(name)
        midi_note = note + (12 * (octave + 1))  # MIDI note value
        if 0 <= midi_note <= 127:
            return midi_note
    return None  # Name is invalid or outside MIDI value range

[tannewt]

Why should this be in the core instead of a library?

[CedarGroveStudios]

For the same reason that a MIDI note value to frequency conversion method is included in the core. SPN is a fairly common notation that has more intrinsic value to musicians than note values. No need to memorize MIDI notes or have frequency tables at-hand.

My typical approach is to use a library method for both so I'm not personally hung up on it being in the core. Having it in a library also makes it easy to change the tuning from A=440 to A=432 ("nature" tuning) or A=441 (harmonica tuning).

[jepler]

FWIW I'd say that midi note numbering is supported just as a sort of historical accident: the original synthio was a midi player and couldn't play notes with arbitrary frequency in Hz.

Another reason to use something outside of the core for conversion from musical notation to frequency is to support different kinds of Musical Temperament, rather than the equal temperament that is the one that you get with MIDI note numbers. we wouldn't want to add all sorts of temperaments in the core.

[CedarGroveStudios]

Good point and interesting historical reference. The SPN conversion and frequency to MIDI note value methods are already in the Community Bundle, BTW.