synthio: add midi_to_hz

to convert notes in the MIDI 1-127 note scale to floating point Hz

Author: jepler

shared-bindings/synthio/__init__.c

@@ -262,12 +262,19 @@ STATIC mp_obj_t synthio_from_file(size_t n_args, const mp_obj_t *pos_args, mp_ma
 }
 MP_DEFINE_CONST_FUN_OBJ_KW(synthio_from_file_obj, 1, synthio_from_file);
 
+STATIC mp_obj_t midi_to_hz(mp_obj_t arg) {
+    mp_int_t note = mp_arg_validate_int_range(mp_obj_get_int(arg), 1, 127, MP_QSTR_note);
+    return mp_obj_new_float(common_hal_synthio_midi_to_hz_float(note));
+}
+MP_DEFINE_CONST_FUN_OBJ_1(synthio_midi_to_hz_obj, midi_to_hz);
+
 STATIC const mp_rom_map_elem_t synthio_module_globals_table[] = {
     { MP_ROM_QSTR(MP_QSTR___name__), MP_ROM_QSTR(MP_QSTR_synthio) },
     { MP_ROM_QSTR(MP_QSTR_MidiTrack), MP_ROM_PTR(&synthio_miditrack_type) },
     { MP_ROM_QSTR(MP_QSTR_Synthesizer), MP_ROM_PTR(&synthio_synthesizer_type) },
     { MP_ROM_QSTR(MP_QSTR_from_file), MP_ROM_PTR(&synthio_from_file_obj) },
     { MP_ROM_QSTR(MP_QSTR_Envelope), MP_ROM_PTR(&synthio_envelope_type_obj) },
+    { MP_ROM_QSTR(MP_QSTR_midi_to_hz), MP_ROM_PTR(&synthio_midi_to_hz_obj) },
 };
 
 STATIC MP_DEFINE_CONST_DICT(synthio_module_globals, synthio_module_globals_table);

shared-bindings/synthio/__init__.h

@@ -33,3 +33,4 @@ extern int16_t shared_bindings_synthio_square_wave[];
 extern const mp_obj_namedtuple_type_t synthio_envelope_type_obj;
 void synthio_synth_envelope_set(synthio_synth_t *synth, mp_obj_t envelope_obj);
 mp_obj_t synthio_synth_envelope_get(synthio_synth_t *synth);
+mp_float_t common_hal_synthio_midi_to_hz_float(mp_int_t note);

shared-module/synthio/__init__.c

@@ -36,6 +36,12 @@ STATIC const int16_t square_wave[] = {-32768, 32767};
 STATIC const uint16_t notes[] = {8372, 8870, 9397, 9956, 10548, 11175, 11840,
                                  12544, 13290, 14080, 14917, 15804}; // 9th octave
 
+mp_float_t common_hal_synthio_midi_to_hz_float(mp_int_t arg) {
+    uint8_t octave = arg / 12;
+    uint16_t base_freq = notes[arg % 12];
+    return MICROPY_FLOAT_C_FUN(ldexp)(base_freq, octave - 10);
+}
+
 STATIC int16_t convert_time_to_rate(uint32_t sample_rate, mp_obj_t time_in, int16_t difference) {
     mp_float_t time = mp_obj_get_float(time_in);
     int num_samples = (int)MICROPY_FLOAT_C_FUN(round)(time * sample_rate);