Remapping Audio

By Isak - July 8, 2015

Using the transcode preset associated with the shape tag, it is really easy to map audio to different channels/streams.

Media have the audio grouped in streams and channels. With the Vidispine Transcoder, you can choose how the map the input channels into the output format.

Example: how to take input channel 3/4 from a source and use those as the input audio tracks for the shape on channels 1/2.

For this, we started with the __mov shape tag, see APIdoc. The result:

<TranscodePresetDocument xmlns="http://xml.vidispine.com/schema/vidispine">
  <name>myshape</name>
  <format>mov</format>
  <audio>
    <codec>aac</codec>
    <bitrate>128000</bitrate>
    <channel>2</channel>
    <channel>3</channel>
    <stream>2</stream>
  </audio>
  <video>
    <scaling>
      <width>512</width>
    </scaling>
    <codec>h264</codec>
    <bitrate>2000000</bitrate>
    <framerate>
      <numerator>1</numerator>
      <denominator>25</denominator>
    </framerate>
    <preset>high</preset>
  </video>
</TranscodePresetDocument>

What does channel and stream mean? The stream element with value 2 means create a stream with two audio channels. The channel elements specifies which channels to use from the source material (and in which order). The stream configuration in the source does not matter, Vidispine will automatically find the right channel by enumerating all channels in all streams, starting with 0. So, what if you want multiple streams?

  <audio>
    <codec>aac</codec>
    <bitrate>128000</bitrate>
    <channel>2</channel>
    <channel>3</channel>
    <channel>0</channel>
    <channel>1</channel>
    <channel>4</channel>
    <stream>2</stream>
    <stream>2</stream>
    <stream>1</stream>
  </audio>

This will create three streams. The first stream will contain the third and fourth input channels. The second stream will contain the first and second input channels. The third stream is a mono channel stream, containing the fifth input channel. So, the number of channel elements in the definition should equal the sum of all stream element values.

Mapping source channels to output streams and channels.

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