opus: fix FEC

FEC may only be used when PLC is enabled on the audio decoder,
as it relies on empty buffers to generate audio from the next
buffer. Hooking to the gap events doesn't work as the audio
decoder does not like more buffers output than it sends.

The length of data to generate using FEC from the next packet
is determined by rounding the gap duration to nearest. This
ensures that duration imprecision does not cause quantization
to 2.5 milliseconds less than available. Doing so causes the
Opus API to fail decoding. Such duration imprecision is common
in live cases.

The buffer to consider when determining the length of audio
to be decoded is the previous buffer when using FEC, and the
new buffer otherwise. In the FEC case, this means we determine
the amount of audio from the previous buffer, whether it was
missing or not (and get the data either from this buffer, or
the current one if the previous one was missing).

1 files changed, 11 insertions, 7 deletions

diff --git a/ext/opus/gstopusdec.c b/ext/opus/gstopusdec.c
index 649cc0c61..1470ea321 100644
--- a/ext/opus/gstopusdec.c
+++ b/ext/opus/gstopusdec.c
@@ -133,8 +133,8 @@ gst_opus_dec_class_init (GstOpusDecClass * klass)
       "Vincent Penquerc'h <vincent.penquerch@collabora.co.uk>");
   g_object_class_install_property (gobject_class, PROP_USE_INBAND_FEC,
       g_param_spec_boolean ("use-inband-fec", "Use in-band FEC",
-          "Use forward error correction if available", DEFAULT_USE_INBAND_FEC,
-          G_PARAM_READWRITE | G_PARAM_STATIC_STRINGS));
+          "Use forward error correction if available (needs PLC enabled)",
+          DEFAULT_USE_INBAND_FEC, G_PARAM_READWRITE | G_PARAM_STATIC_STRINGS));
 
   g_object_class_install_property (gobject_class, PROP_APPLY_GAIN,
       g_param_spec_boolean ("apply-gain", "Apply gain",
@@ -367,7 +367,7 @@ opus_dec_chain_parse_data (GstOpusDec * dec, GstBuffer * buffer)
   GstFlowReturn res = GST_FLOW_OK;
   gsize size;
   guint8 *data;
-  GstBuffer *outbuf;
+  GstBuffer *outbuf, *bufd;
   gint16 *out_data;
   int n, err;
   int samples;
@@ -429,6 +429,9 @@ opus_dec_chain_parse_data (GstOpusDec * dec, GstBuffer * buffer)
       && gst_buffer_get_size (dec->last_buffer) >
       0) ? dec->last_buffer : buffer;
 
+  /* That's the buffer we get duration from */
+  bufd = dec->use_inband_fec ? dec->last_buffer : buffer;
+
   if (buf && gst_buffer_get_size (buf) > 0) {
     gst_buffer_map (buf, &map, GST_MAP_READ);
     data = map.data;
@@ -441,10 +444,10 @@ opus_dec_chain_parse_data (GstOpusDec * dec, GstBuffer * buffer)
     size = 0;
   }
 
-  if (gst_buffer_get_size (buf) == 0) {
+  if (gst_buffer_get_size (bufd) == 0) {
     GstClockTime const opus_plc_alignment = 2500 * GST_USECOND;
     GstClockTime aligned_missing_duration;
-    GstClockTime missing_duration = GST_BUFFER_DURATION (buf);
+    GstClockTime missing_duration = GST_BUFFER_DURATION (bufd);
 
     GST_DEBUG_OBJECT (dec,
         "missing buffer, doing PLC duration %" GST_TIME_FORMAT
@@ -455,9 +458,10 @@ opus_dec_chain_parse_data (GstOpusDec * dec, GstBuffer * buffer)
     missing_duration += dec->leftover_plc_duration;
 
     /* align the combined buffer and leftover PLC duration to multiples
-     * of 2.5ms, always rounding down, and store excess duration for later */
+     * of 2.5ms, rounding to nearest, and store excess duration for later */
     aligned_missing_duration =
-        (missing_duration / opus_plc_alignment) * opus_plc_alignment;
+        ((missing_duration +
+            opus_plc_alignment / 2) / opus_plc_alignment) * opus_plc_alignment;
     dec->leftover_plc_duration = missing_duration - aligned_missing_duration;
 
     /* Opus' PLC cannot operate with less than 2.5ms; skip PLC