We cannot calculate the duration of vorbis packets even if we do know the

"minimal" frame_size.

Originally committed as revision 13969 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/utils.c

@@ -538,12 +538,12 @@ static int get_audio_frame_size(AVCodecContext *enc, int size)
 {
     int frame_size;
 
+    if(enc->codec_id == CODEC_ID_VORBIS)
+        return -1;
+
     if (enc->frame_size <= 1) {
         int bits_per_sample = av_get_bits_per_sample(enc->codec_id);
 
-        if(enc->codec_id == CODEC_ID_VORBIS)
-            return -1;
-
         if (bits_per_sample) {
             if (enc->channels == 0)
                 return -1;