synthio.Note panning is reversed

[FoamyGuy]

CircuitPython version and board name

Adafruit CircuitPython 10.3.0-alpha.3-25-g50a1195c55-dirty on 2026-08-11; Adafruit Metro RP2350 with rp2350b

Code/REPL

# boot.py
import usb_audio

usb_audio.enable(
    sample_rate=22050, channel_count=2, microphone=True
)

# code.py

import time, usb_audio, synthio
import audiomixer

mixer = audiomixer.Mixer(voice_count=2, sample_rate=22050, channel_count=2,
                         bits_per_sample=16)
usb_audio.usb_microphone.play(mixer, loop=True)

mixer.voice[0].level = 0.5
mixer.voice[1].level = 0.5

synth1 = synthio.Synthesizer(sample_rate=22050, channel_count=2)
mixer.voice[0].play(synth1)


synth2 = synthio.Synthesizer(sample_rate=22050, channel_count=2)
mixer.voice[1].play(synth2)

scale = [60, 62, 64, 65, 67, 69, 71, 72]
while True:
    for up, down in zip(scale, reversed(scale)):
        print(f"up: {up}, down: {down}")
        left  = synthio.Note(synthio.midi_to_hz(up), panning=-1)
        right = synthio.Note(synthio.midi_to_hz(down), panning=1)
        synth1.press(left)
        # synth2.press(right)

        time.sleep(0.33)

        synth1.release(left)
        # synth2.release(right)

        time.sleep(0.1)
    time.sleep(1)

Behavior

In the code above an ascending scale should be playing on the left channel according the panning docs: https://docs.circuitpython.org/en/latest/shared-bindings/synthio/index.html#synthio.Note.panning but when recording this in audacity it is actually playing audio on the right channel only.

synthio.Note panning behaves oppositely from audiomixer.MixerVoice panning

Their code shows opposite functionality with regards to panning value.

synthio.Note:

circuitpython/shared-module/synthio/Note.c

Lines 200 to 206 in 2bb088e

	if (panning >= 0) {
	left_panning_scaled = 32768;
	right_panning_scaled = 32767 - panning;
	} else {
	right_panning_scaled = 32768;
	left_panning_scaled = 32767 + panning;
	}

audiomixer.MixerVoice:

circuitpython/shared-module/audiomixer/Mixer.c

Lines 232 to 239 in 2bb088e

	uint16_t left_panning_scaled = 32768, right_panning_scaled = 32768;
	if (MP_LIKELY(self->base.channel_count == 2)) {
	if (panning >= 0) {
	left_panning_scaled = 32767 - panning;
	} else {
	right_panning_scaled = 32767 + panning;
	}
	}

Description

No response

Additional information

LLM points to AudioMixer being correct, and synthio.Note being wrong.

I discovered this while testing #11102, but the issue is not related to the USB stereo functionality, it is a preexisting inconsistency between synthio and audiomixer.

[relic-se]

I haven't tested this yet, but your math seems absolutely correct. I vote to match the implementation within audiomixer.MixerVoice.