/// <summary>
        ///    Reads an audio packet, assigning the audio header and
        ///    advancing the position to the next packet position.
        /// </summary>
        /// <param name="position">
        ///    A <see cref="long" /> value reference specifying the
        ///    position at which to start reading the packet. This value
        ///    is updated to the position of the next packet.
        /// </param>
        void ReadAudioPacket(ref long position)
        {
            Seek(position + 4);
            int length = ReadBlock(2).ToUShort();

            if (!audio_found)
            {
                audio_found = AudioHeader.Find(
                    out audio_header, this, position + 15,
                    length - 9);
            }
            position += length;
        }
 /// <summary>
 ///    Reads format specific information at the start of the
 ///    file.
 /// </summary>
 /// <param name="start">
 ///    A <see cref="long" /> value containing the seek position
 ///    at which the tags end and the media data begins.
 /// </param>
 /// <param name="propertiesStyle">
 ///    A <see cref="ReadStyle" /> value specifying at what level
 ///    of accuracy to read the media properties, or <see
 ///    cref="ReadStyle.None" /> to ignore the properties.
 /// </param>
 /// <remarks>
 ///    This method only searches for an audio header in the
 ///    first 16384 bytes of code to avoid searching forever in
 ///    corrupt files.
 /// </remarks>
 protected override void ReadStart(long start,
                                   ReadStyle propertiesStyle)
 {
     // Only check the first 16 bytes so we're not stuck
     // reading a bad file forever.
     if (propertiesStyle != ReadStyle.None &&
         !AudioHeader.Find(out first_header, this,
                           start, 0x4000))
     {
         throw new CorruptFileException(
                   "MPEG audio header not found.");
     }
 }