H.264 is by default variable bitrate (VBR), as opposed to constant bitrate (CBR). 

VBR files vary the amount of output data per time segment. VBR allows a higher bitrate (and therefore requires more storage space) to be allocated to the more complex segments of media files while less space is allocated to less complex segments. The average of these rates can be calculated to produce an average bitrate for the file.


Constant bit rate encoding (CBR) means that the rate at which a codec's output data should be consumed is constant. CBR is useful for streaming multimedia content on limited capacity channels since it is the maximum bit rate that matters, not the average, so CBR would be used to take advantage of all of the capacity.


To configure constant bitrate (CBR) for H.264, insert the lines below;

   <setting>
     <key>bit_rate_mode</key>
     <value>H264_CBR</value>
   </setting>



Into your transcode preset, like this XML-example;

<TranscodePresetDocument xmlns="http://xml.vidispine.com/schema/vidispine">
  <format>mp4</format>
  <audio>
    <codec>mp3</codec>
    <framerate>
      <numerator>1</numerator>
      <denominator>44100</denominator>
    </framerate>
    <channel>0</channel>
    <channel>1</channel>
    <stream>2</stream>
  </audio>
  <video>
  <setting>
    <key>bit_rate_mode</key>
    <value>H264_CBR</value>
  </setting>
    <scaling>
      <width>1280</width>
      <height>720</height>
    </scaling>
    <codec>h264</codec>
    <bitrate>3000000</bitrate>
    <framerate>
      <numerator>1</numerator>
      <denominator>25</denominator>
    </framerate>
    <preset>main</preset>
  </video>
</TranscodePresetDocument>