title: ffmpeg modify audio length/size ( stretch or shrink)
body: <p>I am developing a web app, where people can record videos. I have been able to send chunks of audio n video to server successfully, where I am trying to combine them and return as single proper file.</p>
  
  <p>my problem is if the recording is for one hour, after merging the chunks</p>
  
  <p>video length : 1:00:00 , audio length : 00:59:30,</p>
  
  <p>now, this is not a issue of audio not getting recorded( I have checked that), the problem is, somehow, when i merge the chunks of audio, it shrinks, </p>
  
  <blockquote>
    <p>I find that it is progressive sync issue where it gets worse and worse as time increases.</p>
  </blockquote>
  
  <p>I have searched the net for the solution, most places say <code>async</code>, I have tried using it, but to no avail, is the below usage correct?</p>
  
  <p><code>ffmpeg  -i audio.wav -async 1 -i video.webm -y -strict -2 v.mp4</code></p>
  
  <p>(v.mp4 is the final file that I provide to the users.)</p>