title: How to confirm opus encode buffer size?
body: <p>Function opus_encode need frame size as parameter. in api  doc it says buffer size is number of samples per channel.
  But how to determine which size should i use?</p>
  
  <p>I use opus in android. sample rate 16k, buffer size 1280. when i set frame size to 640 in encode and decode, the length of decoded file is half of raw pcm. when i set to 960, decoded file is 2/3 of raw pcm. but set to 1280, encode will return -1 as arg error.</p>
  
  <p>When i use cool edit to play decoded, it is faster than raw pcm.  </p>
  
  <p>there must be something about my parameters. 
  Is anyone using opus can help me.
  Thanks a lot.</p>