Distortion on 8-bit audio sample caused by audiomixer.MixerVoice.level #10574
Description
CircuitPython version and board name
Adafruit CircuitPython 10.0.0-beta.2-7-g117b5d1ed9 on 2025-08-12; Adafruit Fruit Jam with rp2350bCode/REPL
import audiobusio
import audiocore
import audiomixer
import board
import time
import adafruit_tlv320
# setup audio
i2c = board.I2C()
dac = adafruit_tlv320.TLV320DAC3100(i2c)
# load wave file
wave = audiocore.WaveFile("test16.wav")
# set sample rate & bit depth
dac.configure_clocks(
sample_rate=wave.sample_rate,
bit_depth=wave.bits_per_sample,
)
# use headphones
dac.headphone_output = True
dac.headphone_volume = -15 # dB
# setup audio output
audio_config = {
"channel_count": wave.channel_count,
"sample_rate": wave.sample_rate,
"bits_per_sample": wave.bits_per_sample,
"samples_signed": wave.bits_per_sample >= 16,
}
print(audio_config)
audio = audiobusio.I2SOut(board.I2S_BCLK, board.I2S_WS, board.I2S_DIN)
mixer = audiomixer.Mixer(**audio_config)
audio.play(mixer)
mixer.play(wave, loop=True)
while True:
for i in range(11):
mixer.voice[0].level = (10 - i) / 10
print(mixer.voice[0].level)
time.sleep(2)Behavior
When playing an unsigned 8-bit WAV file, significant distortion is heard when the audio level approaches lower values. This effect is not heard when playing 16-bit WAV files.
REPL Output:
{'bits_per_sample': 8, 'samples_signed': False, 'channel_count': 1, 'sample_rate': 11025}
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
Description
This is likely due to an error in converting unsigned to signed samples somewhere within
circuitpython/shared-module/audiomixer/Mixer.c
Lines 211 to 216 in 117b5d1
Additional information
The audio files were generated using the following commands:
$ ffmpeg -i test.wav -f wav -ar 11025 -ac 1 -acodec pcm_u8 test8.wav
$ ffmpeg -i test.wav -f wav -ar 11025 -ac 1 -acodec pcm_s16le test16.wav