Same wave file sounds different played on different Mixer voices

[FoamyGuy]

I first noticed this issue with a larger project when I upgraded a NeoTrellis M4 from 4.1.2 to cp 5 beta-5. With that project I found that I hear a pop sound when the file starts on voice 0 but no other voices. I eventually shifted all voices up by 1 skipping 0 and hear no artifacts.

I've narrowed it down to this code which runs on the CLUE:

import board
from audiopwmio import PWMAudioOut as AudioOut
import audiomixer
import audiocore
import digitalio
import time

note = audiocore.WaveFile(open("c4.wav", "rb"))

audio = AudioOut(board.SPEAKER)
mixer = audiomixer.Mixer(voice_count=7, sample_rate=8000,
    channel_count=2, bits_per_sample=16, samples_signed=True)

audio.play(mixer)

print("playing")
mixer.play(note, voice=0, loop=True)
time.sleep(1.0)
print("stopping")
mixer.stop_voice(0)

time.sleep(1.0)
mixer.play(note, voice=1, loop=True)
time.sleep(1.0)
print("stopping")
mixer.stop_voice(1)

time.sleep(1.0)
mixer.play(note, voice=0, loop=True)
time.sleep(1.0)
print("stopping")
mixer.stop_voice(0)

The c4.wav file used is here.

This code plays the same file 3 times in succession on voices 0 then 1 then 0. When I run in on the CLUE I hear 3 one with "choppiness" at the beginning, one with "choppiness" at the end, and one clean playback, in this order consistently.

I've also tested slight variation of this code (using AudioOut instead of PWMAudioOut) on SAMD51 based devices and found similar issues. Though no two devices I've tested so far produce the exact same sound artifacts. I've tested on PyGamer and NeoTrellis M4.

[ladyada]

Clue uses PWM not analog output - it will never sound as good or clean as the SAMD51 or SAMD21! :)

[FoamyGuy]

Ah I see, thank you. I will close this issue and post one with code and outcome for the SAMD51 based devices.

[kevinjwalters]

@jepler has mentioned voice 0 is processed differently to rest.

I'm about to file a bug ticket for voice 0 being noticeably louder than the rest and was looking to see if that was already covered by an existing issue ticket.