audiobasesink: Improve clock skew corrections.

The external time should be moved only as much as needed
to get back to the ideal center point, so that the clock
is still allowed to drift both directions after the correction.
This reduces excessive back and forth corrections that were
caused by the assumption of a linear drift.

https://bugzilla.gnome.org/show_bug.cgi?id=788006

diff --git a/gst-libs/gst/audio/gstaudiobasesink.c b/gst-libs/gst/audio/gstaudiobasesink.c
index ff4e4e2..275d2a5 100644 (file)
--- a/gst-libs/gst/audio/gstaudiobasesink.c
+++ b/gst-libs/gst/audio/gstaudiobasesink.c
@@ -1410,7 +1410,7 @@ gst_audio_base_sink_skew_slaving (GstAudioBaseSink * sink,
 {
   GstClockTime cinternal, cexternal, crate_num, crate_denom;
   GstClockTime etime, itime;
-  GstClockTimeDiff skew, mdrift, mdrift2;
+  GstClockTimeDiff skew, drift, mdrift2;
   gint driftsamples;
   gint64 last_align;
 
@@ -1452,25 +1452,27 @@ gst_audio_base_sink_skew_slaving (GstAudioBaseSink * sink,
       GST_STIME_ARGS (sink->priv->avg_skew));
 
   /* the max drift we allow */
-  mdrift = sink->priv->drift_tolerance * 1000;
-  mdrift2 = mdrift / 2;
+  mdrift2 = (sink->priv->drift_tolerance * 1000) / 2;
 
   /* adjust playout pointer based on skew */
   if (sink->priv->avg_skew > mdrift2) {
-    /* master is running slower, move internal time forward */
+    /* master is running slower, move external time backwards */
     GST_WARNING_OBJECT (sink,
         "correct clock skew %" GST_STIME_FORMAT " > %" GST_STIME_FORMAT,
         GST_STIME_ARGS (sink->priv->avg_skew), GST_STIME_ARGS (mdrift2));
 
-    if (sink->priv->avg_skew > (2 * mdrift)) {
-      cexternal -= sink->priv->avg_skew;
-      sink->priv->avg_skew = 0;
-    } else {
-      cexternal = cexternal > mdrift ? cexternal - mdrift : 0;
-      sink->priv->avg_skew -= mdrift;
-    }
+    /* Move the external time backward by the average skew, but don't ever
+     * go negative.  Moving the average skew by the same distance defines
+     * the new clock skew window center point.  This allows the clock to
+     * drift equally into either direction after the correction. */
+    if (G_LIKELY (cexternal > sink->priv->avg_skew))
+      drift = sink->priv->avg_skew;
+    else
+      drift = cexternal;
+    cexternal -= drift;
+    sink->priv->avg_skew -= drift;
 
-    driftsamples = (sink->ringbuffer->spec.info.rate * mdrift) / GST_SECOND;
+    driftsamples = (sink->ringbuffer->spec.info.rate * drift) / GST_SECOND;
     last_align = sink->priv->last_align;
 
     /* if we were aligning in the wrong direction or we aligned more than what
@@ -1490,15 +1492,15 @@ gst_audio_base_sink_skew_slaving (GstAudioBaseSink * sink,
         "correct clock skew %" GST_STIME_FORMAT " < -%" GST_STIME_FORMAT,
         GST_STIME_ARGS (sink->priv->avg_skew), GST_STIME_ARGS (mdrift2));
 
-    if (sink->priv->avg_skew < (2 * -mdrift)) {
-      cexternal -= sink->priv->avg_skew;
-      sink->priv->avg_skew = 0;
-    } else {
-      cexternal += mdrift;
-      sink->priv->avg_skew += mdrift;
-    }
+    /* Move the external time forward by the average skew, and move the
+     * average skew by the same distance (which equals a reset to 0). This
+     * defines the new clock skew window center point.  This allows the
+     * clock to drift equally into either direction after the correction. */
+    drift = -sink->priv->avg_skew;
+    cexternal += drift;
+    sink->priv->avg_skew = 0;
 
-    driftsamples = (sink->ringbuffer->spec.info.rate * mdrift) / GST_SECOND;
+    driftsamples = (sink->ringbuffer->spec.info.rate * drift) / GST_SECOND;
     last_align = sink->priv->last_align;
 
     /* if we were aligning in the wrong direction or we aligned more than what