music_decoder.py

"""##Create Piano decoder"""

from IPython.display import Audio
import numpy as np
from scipy.io import wavfile

def notes_to_frequencies(notes):
    # Frequency doubles when you go up one octave; there are 12 semi-tones
    # per octave; Note A on octave 4 is 440 Hz, and it is note number 69.
    return 2 ** ((np.array(notes) - 69) / 12) * 440

def frequencies_to_samples(frequencies, tempo, sample_rate):
    note_duration = 60 / tempo # the tempo is measured in beats per minutes
    # To reduce click sound at every beat, we round the frequencies to try to
    # get the samples close to zero at the end of each note.
    frequencies = np.round(note_duration * frequencies) / note_duration
    n_samples = int(note_duration * sample_rate)
    time = np.linspace(0, note_duration, n_samples)
    sine_waves = np.sin(2 * np.pi * frequencies.reshape(-1, 1) * time)
    # Removing all notes with frequencies ≤ 9 Hz (includes note 0 = silence)
    sine_waves *= (frequencies > 9.).reshape(-1, 1)
    return sine_waves.reshape(-1)

def chords_to_samples(chords, tempo, sample_rate):
    freqs = notes_to_frequencies(chords)
    freqs = np.r_[freqs, freqs[-1:]] # make last note a bit longer
    merged = np.mean([frequencies_to_samples(melody, tempo, sample_rate)
                     for melody in freqs.T], axis=0)
    n_fade_out_samples = sample_rate * 60 // tempo # fade out last note
    fade_out = np.linspace(1., 0., n_fade_out_samples)**2
    merged[-n_fade_out_samples:] *= fade_out
    return merged

def play_chords(chords, tempo=160, amplitude=0.1, sample_rate=44100, filepath=None):
    samples = amplitude * chords_to_samples(chords, tempo, sample_rate)
    if filepath:
        samples = (2**15 * samples).astype(np.int16)
        wavfile.write(filepath, sample_rate, samples)
        return Audio(filepath)
    else:
        return Audio(samples, rate=sample_rate)