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module-rtp: Account for in-flight samples in RTP receive

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When not using PTP as the driver, it is possible that packet receive and
the process() callback are out of sync, meaning that the target buffer
fill level might be off by upto one ptime's worth of samples
occasionally. This would make the DLL hunt for the target rate, and
cause a constantly varying delay.

Accounting for the delta between the packet receive time and the
process() time allows us to eliminate this jitter, resulting in much
more consistent rate matching.
This commit is contained in:
Arun Raghavan 2024-11-26 13:18:24 -05:00 • committed by Wim Taymans
parent e76e057038
commit 45eee02a99
2 changed files with 16 additions and 1 deletions
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16
src/modules/module-rtp/audio.c
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@ -63,7 +63,20 @@ static void rtp_audio_process_playback(void *data)
/* when not using direct timestamp and clocks are not
* in sync, try to adjust our playback rate to keep the
* requested target_buffer bytes in the ringbuffer */
error = (double)target_buffer - (double)avail;
double in_flight = 0;
struct spa_io_position *pos = impl->io_position;
if (SPA_LIKELY(pos && impl->last_recv_timestamp)) {
/* Account for samples that might be in flight but not yet received, and possibly
* samples that were received _after_ the process() tick and therefore should not
* yet be accounted for */
int64_t in_flight_ns = pos->clock.nsec - impl->last_recv_timestamp;
/* Use the best relative rate we know */
double relative_rate = impl->io_rate_match ? impl->io_rate_match->rate : pos->clock.rate_diff;
in_flight = (double)(in_flight_ns * impl->rate) * relative_rate / SPA_NSEC_PER_SEC;
}
error = (double)target_buffer - (double)avail - in_flight;
error = SPA_CLAMPD(error, -impl->max_error, impl->max_error);
corr = spa_dll_update(&impl->dll, error);
@ -127,6 +140,7 @@ static int rtp_audio_receive(struct impl *impl, uint8_t *buffer, ssize_t len)
timestamp = ntohl(hdr->timestamp) - impl->ts_offset;
impl->receiving = true;
impl->last_recv_timestamp = pw_stream_get_nsec(impl->stream);
plen = len - hlen;
samples = plen / stride;