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alsa-pcm: add support for api.alsa.dll-bandwidth-max

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In USB Audio Class 2 (UAC2) setups, pitch control is handled by
feedback endpoints. The host adjusts its data rate accordingly.

When pitch control is active (pitch_elem), applying the default
delay-locked loop (DLL) bandwidth can lead to instability and
oscillations around the target rate.

This patch adds a new parameter, api.alsa.dll-bandwidth-max, to
configure the maximum DLL bandwidth. It introduces a new field
in the ALSA state to store this value.

By default, it uses SPA_DLL_BW_MAX, but when pitch control is in
use, setting it to a lower value (e.g. 0.02) helps ensure better
stability, based on empirical testing.
This commit is contained in:
Julien Massot 2025-07-04 14:51:15 +02:00 • committed by Wim Taymans
parent deb7dddbef
commit 8aa836d588
3 changed files with 13 additions and 2 deletions
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6
doc/dox/config/pipewire-props.7.md
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@ -778,6 +778,12 @@ Setting this to 0 makes htimestamp never get disabled.
Disable timer-based scheduling, and use IRQ for scheduling instead.
The "Pro Audio" profile will usually enable this setting, if it is expected it works on the hardware.
@PAR@ node-prop api.alsa.dll-bandwidth-max # double
Sets the maximum bandwidth of the DLL (delay-locked loop) filter used to smooth out rate adjustments.
The default value may be too responsive in some scenarios.
For example, with UAC2 pitch control, the host reacts more slowly compared to local resampling,
so using a lower bandwidth helps avoid oscillations or instability.
@PAR@ node-prop api.alsa.auto-link = false # boolean
Link follower PCM devices to the driver PCM device when using IRQ-based scheduling.
The "Pro Audio" profile will usually enable this setting, if it is expected it works on the hardware.