wasapisink: pre-load the buffer with silence

This reduces the chances of startup glitches, and also reduces the
chances that we'll get garbled output due to driver bugs.

Recommended by the WASAPI documentation.

https://bugzilla.gnome.org/show_bug.cgi?id=793289

2 files changed, 80 insertions, 29 deletions

diff --git a/sys/wasapi/gstwasapisink.c b/sys/wasapi/gstwasapisink.c
index 81436cfa0..c8cfd729b 100644
--- a/sys/wasapi/gstwasapisink.c
+++ b/sys/wasapi/gstwasapisink.c
@@ -383,6 +383,35 @@ gst_wasapi_sink_close (GstAudioSink * asink)
   return TRUE;
 }
 
+/* Get the empty space in the buffer that we have to write to */
+static gint
+gst_wasapi_sink_get_can_frames (GstWasapiSink * self)
+{
+  HRESULT hr;
+  guint n_frames_padding;
+
+  /* There is no padding in exclusive mode since there is no ringbuffer */
+  if (self->sharemode == AUDCLNT_SHAREMODE_EXCLUSIVE) {
+    GST_DEBUG_OBJECT (self, "exclusive mode, can write: %i",
+        self->buffer_frame_count);
+    return self->buffer_frame_count;
+  }
+
+  /* Frames the card hasn't rendered yet */
+  hr = IAudioClient_GetCurrentPadding (self->client, &n_frames_padding);
+  if (hr != S_OK) {
+    gchar *msg = gst_wasapi_util_hresult_to_string (hr);
+    GST_ERROR_OBJECT (self, "IAudioClient::GetCurrentPadding failed: %s", msg);
+    g_free (msg);
+    return -1;
+  }
+
+  GST_DEBUG_OBJECT (self, "%i unread frames (padding)", n_frames_padding);
+
+  /* We can write out these many frames */
+  return self->buffer_frame_count - n_frames_padding;
+}
+
 static gboolean
 gst_wasapi_sink_prepare (GstAudioSink * asink, GstAudioRingBufferSpec * spec)
 {
@@ -483,6 +512,45 @@ gst_wasapi_sink_prepare (GstAudioSink * asink, GstAudioRingBufferSpec * spec)
 
   GST_INFO_OBJECT (self, "got render client");
 
+  /* To avoid start-up glitches, before starting the streaming, we fill the
+   * buffer with silence as recommended by the documentation:
+   * https://msdn.microsoft.com/en-us/library/windows/desktop/dd370879%28v=vs.85%29.aspx */
+  {
+    gint n_frames, len;
+    gint16 *dst = NULL;
+
+    n_frames = gst_wasapi_sink_get_can_frames (self);
+    if (n_frames < 1) {
+      GST_ELEMENT_ERROR (self, RESOURCE, WRITE, (NULL),
+          ("should have more than %i frames to write", n_frames));
+      goto beach;
+    }
+
+    len = n_frames * self->mix_format->nBlockAlign;
+
+    hr = IAudioRenderClient_GetBuffer (render_client, n_frames,
+        (BYTE **) & dst);
+    if (hr != S_OK) {
+      gchar *msg = gst_wasapi_util_hresult_to_string (hr);
+      GST_ELEMENT_ERROR (self, RESOURCE, WRITE, (NULL),
+          ("IAudioRenderClient::GetBuffer failed: %s", msg));
+      g_free (msg);
+      goto beach;
+    }
+
+    GST_DEBUG_OBJECT (self, "pre-wrote %i bytes of silence", len);
+
+    hr = IAudioRenderClient_ReleaseBuffer (render_client, n_frames,
+        AUDCLNT_BUFFERFLAGS_SILENT);
+    if (hr != S_OK) {
+      gchar *msg = gst_wasapi_util_hresult_to_string (hr);
+      GST_ERROR_OBJECT (self, "IAudioRenderClient::ReleaseBuffer failed: %s",
+          msg);
+      g_free (msg);
+      goto beach;
+    }
+  }
+
   hr = IAudioClient_Start (self->client);
   if (hr != S_OK) {
     GST_ERROR_OBJECT (self, "IAudioClient::Start failed");
@@ -549,39 +617,22 @@ gst_wasapi_sink_write (GstAudioSink * asink, gpointer data, guint length)
   guint pending = length;
 
   while (pending > 0) {
-    guint n_frames, write_len;
+    guint can_frames, have_frames, n_frames, write_len;
 
     WaitForSingleObject (self->event_handle, INFINITE);
 
-    if (self->sharemode == AUDCLNT_SHAREMODE_SHARED) {
-      guint have_frames, can_frames, n_frames_padding;
-
-      /* Frames the card hasn't rendered yet */
-      hr = IAudioClient_GetCurrentPadding (self->client, &n_frames_padding);
-      if (hr != S_OK) {
-        gchar *msg = gst_wasapi_util_hresult_to_string (hr);
-        GST_ERROR_OBJECT (self, "IAudioClient::GetCurrentPadding failed: %s",
-            msg);
-        g_free (msg);
-        length = 0;
-        goto beach;
-      }
-      /* We have N frames to be written out */
-      have_frames = pending / (self->mix_format->nBlockAlign);
-      /* We can write out these many frames */
-      can_frames = self->buffer_frame_count - n_frames_padding;
-      /* We will write out these many frames, and this much length */
-      n_frames = MIN (can_frames, have_frames);
-
-      GST_TRACE_OBJECT (self, "total: %i, unread: %i, have: %i (%i bytes), "
-          "will write: %i", self->buffer_frame_count, n_frames_padding,
-          have_frames, pending, n_frames);
-    } else {
-      n_frames = self->buffer_frame_count;
-    }
-
+    /* We have N frames to be written out */
+    have_frames = pending / (self->mix_format->nBlockAlign);
+    /* We have can_frames space in the output buffer */
+    can_frames = gst_wasapi_sink_get_can_frames (self);
+    /* We will write out these many frames, and this much length */
+    n_frames = MIN (can_frames, have_frames);
     write_len = n_frames * self->mix_format->nBlockAlign;
 
+    GST_DEBUG_OBJECT (self, "total: %i, have_frames: %i (%i bytes), "
+        "can_frames: %i, will write: %i (%i bytes)", self->buffer_frame_count,
+        have_frames, pending, can_frames, n_frames, write_len);
+
     hr = IAudioRenderClient_GetBuffer (self->render_client, n_frames,
         (BYTE **) & dst);
     if (hr != S_OK) {
diff --git a/sys/wasapi/gstwasapisrc.c b/sys/wasapi/gstwasapisrc.c
index 375400914..06dadd992 100644
--- a/sys/wasapi/gstwasapisrc.c
+++ b/sys/wasapi/gstwasapisrc.c
@@ -607,7 +607,7 @@ gst_wasapi_src_read (GstAudioSrc * asrc, gpointer data, guint length,
 
     {
       guint bpf = self->mix_format->nBlockAlign;
-      GST_TRACE_OBJECT (self, "have: %i (%i bytes), can read: %i (%i bytes), "
+      GST_DEBUG_OBJECT (self, "have: %i (%i bytes), can read: %i (%i bytes), "
           "will read: %i (%i bytes)", have_frames, have_frames * bpf,
           want_frames, wanted, n_frames, read_len);
     }