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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
Download patch file Download diff file Expand all files Collapse all files

8
spa/plugins/alsa/alsa-pcm.c
View file

@ -20,6 +20,7 @@
static struct spa_list cards = SPA_LIST_INIT(&cards);
static struct spa_list states = SPA_LIST_INIT(&states);
#define SPA_ALSA_DLL_BW_MIN 0.001
static struct card *find_card(uint32_t index)
{
@ -198,6 +199,9 @@ static int alsa_set_param(struct state *state, const char *k, const char *s)
state->disable_batch = spa_atob(s);
} else if (spa_streq(k, "api.alsa.disable-tsched")) {
state->disable_tsched = spa_atob(s);
} else if (spa_streq(k, "api.alsa.dll-bandwidth-max")) {
state->dll_bw_max = SPA_CLAMPD(spa_strtod(s, NULL),
SPA_ALSA_DLL_BW_MIN, SPA_DLL_BW_MAX);
} else if (spa_streq(k, "api.alsa.use-chmap")) {
state->props.use_chmap = spa_atob(s);
} else if (spa_streq(k, "api.alsa.multi-rate")) {
@ -2799,7 +2803,7 @@ static int update_time(struct state *state, uint64_t current_time, snd_pcm_sfram
int32_t diff;
if (SPA_UNLIKELY(state->dll.bw == 0.0)) {
spa_dll_set_bw(&state->dll, SPA_DLL_BW_MAX, state->threshold, state->rate);
spa_dll_set_bw(&state->dll, state->dll_bw_max, state->threshold, state->rate);
state->next_time = current_time;
state->base_time = current_time;
}
@ -2862,7 +2866,7 @@ static int update_time(struct state *state, uint64_t current_time, snd_pcm_sfram
err, state->max_error, state->max_resync, state->err_avg, state->err_var, bw);
spa_dll_set_bw(&state->dll,
SPA_CLAMPD(bw, 0.001, SPA_DLL_BW_MAX),
SPA_CLAMPD(bw, SPA_ALSA_DLL_BW_MIN, state->dll_bw_max),
state->threshold, state->rate);
}