Improve buffer timestamping

Make buffer timestamps more accurate and, more importantly, actually
representative of the MIDI events timing.

Previously, buffers were only sent with timetamps aligned at a 10ms
boundary which was just wrong, now the buffer timestamp represents the
real time of the MIDI event.

Conveniently, the ALSA sequence API supports scheduling events in the
future so the sequencing infrastructure can be used to have the tick
delivered at the right time, avoiding any custom scheduling mechanism.

The sequencer queue is started on the first transition to PLAYING, and
the delay is also calculated when the pipeline goes into PLAYING.

diff --git a/gst-aseq-appsrc.c b/gst-aseq-appsrc.c
index 1577ebf..e1b3418 100644 (file)
--- a/gst-aseq-appsrc.c
+++ b/gst-aseq-appsrc.c
@@ -36,7 +36,6 @@
 #define DEFAULT_BUFSIZE 65536
 #define DEFAULT_CLIENT_NAME "gst-aseq-appsrc"
 #define DEFAULT_TICK_PERIOD_MS 10
-#define DEFAULT_POLL_TIMEOUT_MS (DEFAULT_TICK_PERIOD_MS / 2)
 
 GST_DEBUG_CATEGORY (mysource_debug);
 #define GST_CAT_DEFAULT mysource_debug
@@ -61,6 +60,7 @@ struct _App
   int npfds;
 
   guint64 tick;
+  guint64 delay;
 };
 
 static App s_app;
@@ -161,6 +161,33 @@ start_queue_timer (snd_seq_t *seq, int queue)
   return ret;
 }
 
+static void
+schedule_tick (App * app)
+{
+  snd_seq_event_t ev;
+  snd_seq_real_time_t time;
+  int ret;
+
+  snd_seq_ev_clear (&ev);
+  snd_seq_ev_set_source (&ev, 0);
+  snd_seq_ev_set_dest (&ev, snd_seq_client_id (app->seq), 0);
+
+  ev.type = SND_SEQ_EVENT_TICK;
+
+  GST_TIME_TO_TIMESPEC (app->tick * DEFAULT_TICK_PERIOD_MS * GST_MSECOND, time);
+  app->tick += 1;
+
+  snd_seq_ev_schedule_real (&ev, app->queue, 0, &time);
+
+  ret = snd_seq_event_output (app->seq, &ev);
+  if (ret < 0)
+    GST_ERROR ("Event output error: %s\n", snd_strerror (ret));
+
+  ret = snd_seq_drain_output (app->seq);
+  if (ret < 0)
+    GST_ERROR ("Event drain error: %s\n", snd_strerror (ret));
+}
+
 static int
 create_port (App * app)
 {
@@ -188,8 +215,6 @@ create_port (App * app)
   if (ret < 0)
     GST_ERROR ("Cannot create port - %s", snd_strerror (ret));
 
-  ret = start_queue_timer (app->seq, app->queue);
-
   return ret;
 }
 
@@ -211,17 +236,14 @@ connect_ports (App * app)
 }
 
 static void
-push_buffer (App * app, gpointer data, guint size)
+push_buffer (App * app, gpointer data, guint size, GstClockTime time)
 {
-  GstClockTime time;
   gpointer local_data;
   GstBuffer *buffer;
   int ret;
 
   buffer = gst_buffer_new ();
 
-  time = app->tick * DEFAULT_TICK_PERIOD_MS * GST_MSECOND;
-
   GST_BUFFER_DTS (buffer) = time;
   GST_BUFFER_PTS (buffer) = time;
 
@@ -237,15 +259,13 @@ push_buffer (App * app, gpointer data, guint size)
       (gpointer) buffer, app->tick, size);
   g_signal_emit_by_name (app->appsrc, "push-buffer", buffer, &ret);
   gst_buffer_unref (buffer);
-
-  app->tick += 1;
 }
 
 static void
-push_tick_buffer (App * app)
+push_tick_buffer (App * app, GstClockTime time)
 {
   app->buffer[0] = 0xf9;
-  push_buffer (app, app->buffer, 1);
+  push_buffer (app, app->buffer, 1, time);
 }
 
 /* This method is called by the need-data signal callback, we feed data into the
@@ -254,17 +274,17 @@ push_tick_buffer (App * app)
 static void
 feed_data (GstElement * appsrc, guint size, App * app)
 {
-  long size_ev = 0;
+  GstClockTime time;
+  long size_ev;
   int err;
   int ret;
 
+poll:
   snd_seq_poll_descriptors (app->seq, app->pfds, app->npfds, POLLIN);
-  ret = poll (app->pfds, app->npfds, DEFAULT_POLL_TIMEOUT_MS);
-  if (ret < 0) {
+  ret = poll (app->pfds, app->npfds, -1);
+  if (ret <= 0) {
     GST_ERROR ("ERROR in poll: %s", strerror (errno));
     gst_app_src_end_of_stream (GST_APP_SRC (appsrc));
-  } else if (ret == 0) {
-    push_tick_buffer (app);
   } else {
     do {
       snd_seq_event_t *event;
@@ -275,6 +295,18 @@ feed_data (GstElement * appsrc, guint size, App * app)
         break;
       }
       if (event) {
+        time = GST_TIMESPEC_TO_TIME (event->time.time) + app->delay;
+
+        /*
+         * Special handling is needed because decoding SND_SEQ_EVENT_TICK is
+         * not supported by the ALSA sequencer.
+         */
+        if (event->type == SND_SEQ_EVENT_TICK) {
+          push_tick_buffer (app, time);
+          schedule_tick (app);
+          break;
+        }
+
         size_ev =
             snd_midi_event_decode (app->parser, app->buffer, DEFAULT_BUFSIZE,
             event);
@@ -282,7 +314,7 @@ feed_data (GstElement * appsrc, guint size, App * app)
           /* ENOENT indicates an event that is not a MIDI message, silently skip it */
           if (-ENOENT == size_ev) {
             GST_WARNING ("Warning: Received non-MIDI message");
-            push_tick_buffer (app);
+            goto poll;
           } else {
             GST_ERROR ("Error decoding event from ALSA to output: %s",
                 strerror (-size_ev));
@@ -290,7 +322,7 @@ feed_data (GstElement * appsrc, guint size, App * app)
             break;
           }
         } else {
-          push_buffer (app, app->buffer, size_ev);
+          push_buffer (app, app->buffer, size_ev, time);
         }
       }
     } while (err > 0);
@@ -336,6 +368,39 @@ bus_message (GstBus * bus, GstMessage * message, App * app)
       g_main_loop_quit (app->loop);
       break;
     }
+    case GST_MESSAGE_STATE_CHANGED:
+    {
+      GstState old_state, new_state;
+
+      gst_message_parse_state_changed (message, &old_state, &new_state, NULL);
+      if (new_state == GST_STATE_PLAYING) {
+        GstClockTime gst_time;
+        GstClockTime base_time;
+        GstClockTime running_time;
+        GstClockTime queue_time;
+        GstClock *clock;
+        snd_seq_queue_status_t *status;
+
+        if (app->tick == 0) {
+          start_queue_timer (app->seq, app->queue);
+          schedule_tick (app);
+        }
+
+        clock = gst_element_get_clock (GST_ELEMENT (app->appsrc));
+        gst_time = gst_clock_get_time (clock);
+        gst_object_unref (clock);
+        base_time = gst_element_get_base_time (GST_ELEMENT (app->appsrc));
+        running_time = gst_time - base_time;
+
+        snd_seq_queue_status_malloc (&status);
+        snd_seq_get_queue_status (app->seq, 0, status);
+        queue_time = GST_TIMESPEC_TO_TIME (*snd_seq_queue_status_get_real_time (status));
+        snd_seq_queue_status_free (status);
+
+        app->delay = running_time - queue_time;
+      }
+      break;
+    }
     case GST_MESSAGE_EOS:
       g_main_loop_quit (app->loop);
       break;