code.py

from math import sin
import sdcardio
import storage
import audiobusio
import audiocore
import audiomixer
import audiomp3
import board
import time
import math

# Mount SD card
bus = board.SPI()
sd_cs = board.SD_CS
sdcard = sdcardio.SDCard(bus, sd_cs)
vfs = storage.VfsFat(sdcard)
storage.mount(vfs, "/sd")

# Open a couple of MP3s
sample1 = audiomp3.MP3Decoder("/sd/bartlebeats - Frequency - 01 Idea3.mp3")
sample2 = audiomp3.MP3Decoder("/sd/bartlebeats - Frequency - 14 Tall Bird.mp3")

# Engage the sound hardware
with audiobusio.I2SOut(bit_clock=board.D12, word_select=board.D13, data=board.D11) as i2s:

    # Start up a mixer
    m = audiomixer.Mixer(channel_count=2, sample_rate=44100, buffer_size=8192)
    i2s.play(m)

    # And associate an MP3 file with each voice
    v0, v1 = m.voice
    v0.level = 1
    v1.level = 0
    v0.play(sample1, loop=True)
    v1.play(sample2, loop=True)

    # Now, show off the mixer by switching back and forth between the two songs
    # .. in a sinusoidal fashion
    t = 0
    ov = .2
    while True:
        t = t + .01
        l = sin(t) * .5 + .5
        v0.level = l * ov
        v1.level = (1-l)  * ov
        time.sleep(.01)
    # At some point (end of track?) I get static & USB disconnect so there's a bug somewhere :-/
    # maybe looping MP3s is broken