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Encoding (compressing) audio files for DVD by Matt Ottewill

The DVD video disc format allows for both Dolby Digital / AC3 compressed and uncompressed audio formats. To understand more about audio files and audio compression you should read our comprehensive article on audio file formats and codecs before reading this page.

DVD-video audio

DVD-video allows for uncompressed PCM audio in 48kHz or 96kHz 16-bit format and compressed AC3 (Dolby Digital) format. DVD-video players are required to be able to replay PCM (uncompressed) and AC3 (compressed) audio files. This is why all DVD-video players can play audio CDs.

Uncompressed stereo 48kHz 16-bit audio files will take 1.5Mbps of the available DVD bandwidth (about 15%) and 96kHz 16-bit files will take 3Mbps (about 30%).

Audio left in uncompressed format will take more bandwidth than if its compressed. AC3 compression can reduce the size of audio files by a factor of 12:1 with very little audible side effects.

AC3 (Dolby Digital) audio compression

Encoding (compressing) audio files to AC3 has 2 advantages ...

  • Reduced file size means more video, audio, menus etc will fit on a disc.
  • Reduced data rate means more is available for video streams.

An AC3 file can contain between 1 and 6 channels of audio, allowing 5.1 surround sound.

Preparing files for encoding to AC3

Before you encode to AC3 you should ensure your audio files are ...

  • Mono or stereo
  • 16 or 24 bit depth
  • 48 or 96 kHz sample rate (if you are using A.Pack, use 48kHz)
  • AIFF, WAV or SDII file format

What do I need to encode audio to AC3?

Video files (such as DV) can be encoded with several programs ...

Apple's A.Pack encoder (included with DVD Studio Pro)

Apple's A.Pack encoder does a good job of creating multi-channel AC3 files. Comes with DVD Studio Pro.

Sony / Sonic Foundry 5.1 Surround plug-in pack

Sony recently brought Sonic Foundry who developed an AC3 encoder for Vegas and Acid.

DVD-Video authoring applications

Most DVD-video disc authoring programs come with their own AC3 encoder programs.

Choosing AC3 bit rates

Among other options, AC3 encoding programs allow you to set the bit rate.

The bit rate you choose will determine ...

  • Audio quality
  • File size
  • Percentage of overall DVD-video bandwidth
  • How much strain will be put on a computer's CPU when it is using a "soft MPEG codec" to play DVD-video files on a desktop computer

What bit rate settings should I use?

You can experiment and test, but here are some good guidelines ...

  • 5.1 surround sound ... between 224 and 448 kbps (kilo bits per second)
  • Stereo ... between 192 and 224 kbps
  • Mono ... between 64 and 128 kbps