Add stereo support to usb_audio

[relic-se]

Updates USB Audio device descriptors to assume stereo operation. Mono operation (channel_count=1) is still supported although the device will enumerate as stereo.

Example Code

boot.py

import usb_audio
import usb_hid
import usb_midi

usb_hid.disable()
usb_midi.disable()

# Alter settings here to test different configurations
usb_audio.enable(
    sample_rate=48000,
    channel_count=2,
    microphone=True,
    speaker=True,
)

code.py

from synthio import Synthesizer, Note
from usb_audio import usb_microphone, usb_speaker

SAMPLE_RATE = 48000
CHANNEL_COUNT = 2

# This will need to be replaced with your hardware of choice
from relic_synthiota import Synthiota
synthiota = Synthiota(
    sample_rate=SAMPLE_RATE,
    channel_count=CHANNEL_COUNT,
)
audio = synthiota.audio

if usb_speaker is not None:
    audio.play(usb_speaker)

if usb_microphone is not None:
    synth = Synthesizer(
        sample_rate=SAMPLE_RATE,
        channel_count=CHANNEL_COUNT,
    )
    usb_microphone.play(synth)
    synth.press(Note(440))

if usb_microphone is not None or usb_speaker is not None:
    try:
        while True:
            pass
    except KeyboardInterrupt:
        pass

Notes

• The bits_per_sample argument is removed from usb_audio.enable(...) since it is always assumed to be 16. (open to input here)
• For operation on most devices, usb_hid (and potentially usb_midi) will need to be disabled to avoid reaching the limit of USB endpoint pairs when operating in bidirectional mode.

In Progress

• Mono output through usb_microphone results in random noise.
• Mono input through usb_speaker results in distorted audio.
• Bidirectional operation is somewhat functional, but an issue with the device descriptor is causing distortion with usb_speaker. Stereo usb_microphone operates correctly.

For interest: @FoamyGuy

[FoamyGuy]

Thanks for working on this! I will test out mono speaker, and try to resolve the issues with microphone.

[mikeysklar]

@relic-se

#11093 (tinyusb bump) just merged, which migrates usb_audio to the TUD_AUDIO20_/AUDIO20_ names and touches tusb_config.h + usb_audio_descriptors.h, so this branch will need a rebase onto main.

[relic-se]

@mikeysklar Thanks for your work here. I'll make sure to follow your guidelines closely.

[relic-se]

@mikeysklar I've updated my code accordingly, and it's looking good. There are still some bugs within this PR which prevents it from being ready to merge at this time.

[relic-se]

As an aside, Linus Torvalds appears to have some success with stereo headset operation on his RP2350 guitar pedal: https://github.com/torvalds/GuitarPedal/blob/main/Software/usb-device.c