code: func (self *Transcoder) Do(pkt av.Packet) (out []av.Packet, err error) {
  	stream := self.streams[pkt.Idx]
  	if stream.aenc != nil && stream.adec != nil {
  		if out, err = stream.audioDecodeAndEncode(pkt); err != nil {
  			return
  		}
  	} else {
  		out = append(out, pkt)
  	}
  	return
  }
docs: // Do the transcode.
  // 
  // In audio transcoding one Packet may transcode into many Packets
  // packet time will be adjusted automatically.