TrimUI Brick Pro (tg4040): idle speaker amp not muted → constant switching noise through speaker

[bindike]

Device

TrimUI Brick Pro (tg4040)

Tested on

Reproduced on BOTH stable v4.3.4 AND nightly v4.3.6-20260826 (not nightly-only).
Root cause below confirmed on nightly v4.3.6-20260906.

Problem

With nothing playing (menus/idle, and especially under load: box-art scraping, SD
writes, WiFi) a constant electrical noise is audible through the speaker. It scales
with system load and with the volume setting (volume 0 = completely silent), so it
is amplified analog noise on the speaker amp, not a digital/sample-rate issue.
In-game audio itself is clean.

Root cause (confirmed)

The speaker amplifier is left enabled at idle. In ALSA this is the
HpSpeaker Switch (numid=15), which stays on in the menu, so the board's
switching noise (CPU governor / SD / WiFi) couples into the always-on amp and is
amplified. It is only audible outside games because in-game the amp is doing real
work and the noise is masked.

Confirmed directly:

amixer cset numid=15 off   # idle noise gone completely
amixer cset numid=15 on    # noise returns; needed for speaker audio

Ruled out

• Volume 0 silences it entirely (amp path).
• Not the mic path: disabling ADCL/ADCR MIC1/MIC2 Boost (numid 12/13), MIC1/MIC2
  gain (numid 5/6) and ADC volume (numid 9) via amixer made no difference.

Suggested fix

Gate the speaker amp (HpSpeaker Switch / numid=15) to playback: mute it when
nothing is playing (menu/idle) and unmute it when a game/stream is active.
(Not observed on stock TrimUI OS, which appears to keep the amp gated.)

Working stopgap on my device — a small background watchdog:

# off when no emulator/RetroArch is running, on when one is
if pgrep -f 'ra32|ra64|retroarch|/Emu/' >/dev/null; then
    amixer -q cset numid=15 on
else
    amixer -q cset numid=15 off
fi

Silent menu, normal in-game speaker audio.

Codec

Card 0 = sun50iw10codec (SUNXI-CODEC), playback S16_LE / 48000 Hz / stereo.
Mixer: HpSpeaker Switch = numid=15, Headphone Switch = numid=14
(both were on simultaneously at idle).### Device
TrimUI Brick Pro (tg4040)

Tested on

Reproduced on BOTH stable v4.3.4 AND nightly v4.3.6-20260826 (not nightly-only).

Problem

With nothing playing (menus/idle, and especially under load: box-art scraping, SD
writes, WiFi) a constant electrical noise is audible through the speaker. It scales
with system load and with the volume setting (volume 0 = completely silent), so it
is amplified analog noise on the speaker amp, not a digital/sample-rate issue.
In-game audio itself is clean.

Ruled out

• Volume 0 silences it entirely (amp path).
• Not the mic path: disabling ADCL/ADCR MIC1/MIC2 Boost (numid 12/13), MIC1/MIC2
  gain (numid 5/6) and ADC volume (numid 9) via amixer made no difference.

Hypothesis / suggested fix

Likely the speaker amp (and/or DAC output) is left enabled while idle, so board
switching noise (WiFi/SD/CPU) couples in and is amplified. Muting/disabling the amp
when no stream is active would likely fix it. (Not observed on the stock TrimUI OS.)

Codec

Card 0 = sun50iw10codec (SUNXI-CODEC), playback S16_LE / 48000 Hz / stereo.