title: Changing time base on an ffmpeg encode (libffmpeg)
body: <p>I'm using libffmpeg to decode an RTSP video stream and write it to a file.</p>
  
  <p>The time_base reported by the codec when I open the stream is 1 / 180000. When I create my output AVStream, I'm copying this time_base over to the output. It works, but I get this message when I call avformat_write_header:</p>
  
  <p>"WARNING codec timebase is very high. If duration is too long,file may not be playable by quicktime. Specify a shorter timebase"</p>
  
  <p>I tried specifying a shorter timebase (say, 1/30) but when I do this, the video plays back at the wrong speed.</p>
  
  <p>What's the right way to adjust the time_base on my output stream without modifying the playback time?</p>
  
  <p>Thanks.</p>