[pulseaudio-discuss] [PATCH v6 03/25] loopback: Improve latency estimation

[Georg Chini]

To improve the overall latency estimation, the delay between the two snapshots
is taken into account. To minimize the snapshot delay, the order of the snapshots
is reverted. Additionally the latency at the base rate is calculated. It will be
used later as the input to the latency controller.

---
 src/modules/module-loopback.c | 27 +++++++++++++++++++--------
 1 file changed, 19 insertions(+), 8 deletions(-)

diff --git a/src/modules/module-loopback.c b/src/modules/module-loopback.c
index aa559aa..6539a95 100644
--- a/src/modules/module-loopback.c
+++ b/src/modules/module-loopback.c
@@ -172,36 +172,47 @@ static void teardown(struct userdata *u) {
 static void adjust_rates(struct userdata *u) {
     size_t buffer, fs;
     uint32_t old_rate, base_rate, new_rate;
-    pa_usec_t buffer_latency;
+    pa_usec_t current_buffer_latency, snapshot_delay, current_source_sink_latency, current_latency, latency_at_optimum_rate;
 
     pa_assert(u);
     pa_assert_ctl_context();
 
-    pa_asyncmsgq_send(u->source_output->source->asyncmsgq, PA_MSGOBJECT(u->source_output), SOURCE_OUTPUT_MESSAGE_LATENCY_SNAPSHOT, NULL, 0, NULL);
     pa_asyncmsgq_send(u->sink_input->sink->asyncmsgq, PA_MSGOBJECT(u->sink_input), SINK_INPUT_MESSAGE_LATENCY_SNAPSHOT, NULL, 0, NULL);
+    pa_asyncmsgq_send(u->source_output->source->asyncmsgq, PA_MSGOBJECT(u->source_output), SOURCE_OUTPUT_MESSAGE_LATENCY_SNAPSHOT, NULL, 0, NULL);
 
-    buffer = u->latency_snapshot.sink_input_buffer;
+    /* Rates and latencies*/
+    old_rate = u->sink_input->sample_spec.rate;
+    base_rate = u->source_output->sample_spec.rate;
 
+    buffer = u->latency_snapshot.sink_input_buffer;
     if (u->latency_snapshot.recv_counter <= u->latency_snapshot.send_counter)
         buffer += (size_t) (u->latency_snapshot.send_counter - u->latency_snapshot.recv_counter);
     else
         buffer = PA_CLIP_SUB(buffer, (size_t) (u->latency_snapshot.recv_counter - u->latency_snapshot.send_counter));
 
-    buffer_latency = pa_bytes_to_usec(buffer, &u->sink_input->sample_spec);
+    current_buffer_latency = pa_bytes_to_usec(buffer, &u->sink_input->sample_spec);
+    snapshot_delay = u->latency_snapshot.source_timestamp - u->latency_snapshot.sink_timestamp;
+    current_source_sink_latency = u->latency_snapshot.sink_latency + u->latency_snapshot.source_latency - snapshot_delay;
+
+    /* Current latency */
+    current_latency = current_source_sink_latency + current_buffer_latency;
+
+    /* Latency at base rate */
+    latency_at_optimum_rate = current_source_sink_latency + current_buffer_latency * old_rate / base_rate;
 
     pa_log_debug("Loopback overall latency is %0.2f ms + %0.2f ms + %0.2f ms = %0.2f ms",
                 (double) u->latency_snapshot.sink_latency / PA_USEC_PER_MSEC,
-                (double) buffer_latency / PA_USEC_PER_MSEC,
+                (double) current_buffer_latency / PA_USEC_PER_MSEC,
                 (double) u->latency_snapshot.source_latency / PA_USEC_PER_MSEC,
-                ((double) u->latency_snapshot.sink_latency + buffer_latency + u->latency_snapshot.source_latency) / PA_USEC_PER_MSEC);
+                (double) current_latency / PA_USEC_PER_MSEC);
+
+    pa_log_debug("Loopback latency at base rate is %0.2f ms", (double)latency_at_optimum_rate / PA_USEC_PER_MSEC);
 
     pa_log_debug("Should buffer %zu bytes, buffered at minimum %zu bytes",
                 u->latency_snapshot.max_request*2,
                 u->latency_snapshot.min_memblockq_length);
 
     fs = pa_frame_size(&u->sink_input->sample_spec);
-    old_rate = u->sink_input->sample_spec.rate;
-    base_rate = u->source_output->sample_spec.rate;
 
     if (u->latency_snapshot.min_memblockq_length < u->latency_snapshot.max_request*2)
         new_rate = base_rate - (((u->latency_snapshot.max_request*2 - u->latency_snapshot.min_memblockq_length) / fs) *PA_USEC_PER_SEC)/u->adjust_time;
-- 
2.8.1