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bluez5: media-source: fix off-by-one cycle in rate matching

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The rate matching filter assumes buffer level for cycle j+1 is

    buffer(j+1) = buffer(j) + recv(j) - corr(j+1) * duration

but what we are actually doing is instead

    buffer(j+1) = buffer(j) + recv(j) - corr(j-1) * duration

because the correction factor that is computed is not used for the next
cycle, but the one following that. Although the filter is still stable
in theory the extra lag causes oscillations to be damped less.

Fix by using the computed correction factor for the next cycle, as
there's no reason why we'd like to have more lag in rate matching.

This then changes c(j-1) -> c(j) in the assumptions, which turns out to
fix the situation. Fix the filter derivation to match.  The filter
coefficients stay as they were, and they are actually exactly correct
also for short averaging times.

In practice, it is observed that ISO RX with quantum 4096 converges to
stable rate, whereas previously the matching retained small
oscillations.
This commit is contained in:
Pauli Virtanen 2025-12-22 15:29:16 +02:00 • committed by Wim Taymans
parent c4812af436
commit de34ce606f
3 changed files with 68 additions and 41 deletions
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30
spa/plugins/bluez5/media-source.c
View file

@ -872,6 +872,13 @@ static void media_on_timeout(struct spa_source *source)
spa_log_trace(this->log, "%p: io:%d->%d status:%d", this, io_status, port->io->status, status);
}
/* Use any updated rate correction already for the next cycle */
this->next_time = (uint64_t)(now_time + duration * SPA_NSEC_PER_SEC / port->buffer.corr / rate);
if (SPA_LIKELY(this->clock)) {
this->clock->rate_diff = port->buffer.corr;
this->clock->next_nsec = this->next_time;
}
spa_node_call_ready(&this->callbacks, SPA_STATUS_HAVE_DATA);
set_timeout(this, this->next_time);
@ -1807,13 +1814,9 @@ static void process_buffering(struct impl *this)
if (plc)
spa_bt_decode_buffer_recover(&port->buffer);
setup_matching(this);
spa_bt_decode_buffer_process(&port->buffer, samples, duration_ns);
spa_bt_decode_buffer_process(&port->buffer, samples, duration_ns,
this->position ? this->position->clock.rate_diff : 1.0,
this->position ? this->position->clock.next_nsec : 0,
this->resampling ? this->port.rate_match->delay : 0,
this->resampling ? this->port.rate_match->delay_frac : 0);
setup_matching(this);
/* copy data to buffers */
if (!spa_list_is_empty(&port->free)) {
@ -1935,6 +1938,7 @@ static int impl_node_process(void *object)
struct impl *this = object;
struct port *port;
struct spa_io_buffers *io;
int ret;
spa_return_val_if_fail(this != NULL, -EINVAL);
@ -1959,9 +1963,19 @@ static int impl_node_process(void *object)
/* Follower produces buffers here, driver in timeout */
if (this->following)
return produce_buffer(this);
ret = produce_buffer(this);
else
return SPA_STATUS_OK;
ret = SPA_STATUS_OK;
/* Update decode buffer vs. next wakeup timing */
spa_bt_decode_buffer_set_next(&port->buffer,
this->position ? this->position->clock.rate_diff : 1.0,
this->position ? this->position->clock.next_nsec : 0,
this->resampling ? this->port.rate_match->delay : 0,
this->resampling ? this->port.rate_match->delay_frac : 0,
this->following);
return ret;
}
static const struct spa_node_methods impl_node = {