mpegaudioparse: Use a constant bit rate to convert between time and bytes if possible.

This should result in no worse accuracy than the base parse element, and may
result in better accuracy. In particular, the number of bytes processed at any
given point, as accumulated by baseparse, can be only accurate to
(1 / # of frames) bytes per second, and if we try to seek immediately after
pausing the pipeline to a large offset, this small inaccuracy can propagate to
something noticeable.

The use case that prompted this patch is a 45-minute MPEG-1 layer 3 file, which
has a constant bit rate but no seek tables. Trying to seek the pipeline
immediately after pauisng it, without the ACCURATE flag, to a location 41
minutes in, yields a location that is, even with <https://gitlab.freedesktop.org/gstreamer/gstreamer/merge_requests/374>,
still audibly incorrect. This patch yields a much closer position, no longer
audibly incorrect, and likely within a frame of the most correct position.

gst/audioparsers/gstmpegaudioparse.c

@@ -199,6 +199,7 @@ gst_mpeg_audio_parse_reset (GstMpegAudioParse * mp3parse)
   mp3parse->freerate = 0;
 
   mp3parse->hdr_bitrate = 0;
+  mp3parse->bitrate_is_constant = TRUE;
 
   mp3parse->xing_flags = 0;
   mp3parse->xing_bitrate = 0;
@@ -755,6 +756,9 @@ gst_mpeg_audio_parse_handle_frame (GstBaseParse * parse,
         (version == 1) ? 10 : 30, 2);
   }
 
+  if (mp3parse->hdr_bitrate && mp3parse->hdr_bitrate != bitrate) {
+    mp3parse->bitrate_is_constant = FALSE;
+  }
   mp3parse->hdr_bitrate = bitrate;
 
   /* For first frame; check for seek tables and output a codec tag */
@@ -1227,6 +1231,14 @@ gst_mpeg_audio_parse_time_to_bytepos (GstMpegAudioParse * mp3parse,
     return TRUE;
   }
 
+  /* If we have had a constant bit rate (so far), use it directly, as it
+   * may give slightly more accurate results than the base class. */
+  if (mp3parse->bitrate_is_constant && mp3parse->hdr_bitrate) {
+    *bytepos = gst_util_uint64_scale (ts, mp3parse->hdr_bitrate,
+        8 * GST_SECOND);
+    return TRUE;
+  }
+
   return FALSE;
 }
 
@@ -1292,6 +1304,14 @@ gst_mpeg_audio_parse_bytepos_to_time (GstMpegAudioParse * mp3parse,
     return TRUE;
   }
 
+  /* If we have had a constant bit rate (so far), use it directly, as it
+   * may give slightly more accurate results than the base class. */
+  if (mp3parse->bitrate_is_constant && mp3parse->hdr_bitrate) {
+    *ts = gst_util_uint64_scale (bytepos, 8 * GST_SECOND,
+        mp3parse->hdr_bitrate);
+    return TRUE;
+  }
+
   return FALSE;
 }
 

gst/audioparsers/gstmpegaudioparse.h

@@ -69,6 +69,7 @@ struct _GstMpegAudioParse {
 
   /* Bitrate from non-vbr headers */
   guint32      hdr_bitrate;
+  gboolean     bitrate_is_constant;
 
   /* Xing info */
   guint32      xing_flags;