Prepraing/optimising/compressing digital audio files by Matt Ottewill

compressing audio

There are 3 primary situations in which you will need to compress audio ...

  1. To create network (internet etc) friendly audio file sizes
  2. To reduce the size of video files by compressing the soundtrack audio
  3. To compress audio for DVD authoring

NOTE: If you have not already read the article on Digital Audio File Formats you should do so before reading this page.

Compressing video soundtracks

Click here

Compressing audio/music for the web

MP3

As you are no doubt aware, MP3 is the current dominant file format for the web. Its popularity makes it the best choice and almost guarantees compatibility with all internet end-user systems.

Flac

You may also be interested in Flac (Free Lossless Audio Codec) which offers lossless high quality compression. However, whilst almost all end-users will have no trouble listening to MP3 files, many will not have the necessary device / software to replay Flac files. More at flac.sourceforge.net

MP3 compression programs

Many people use iTunes to convert uncompressed CD quality audio into MP3 files but almost all audio editing software will allow you to export to MP3. Programs include ...

  • iTunes
  • Cubase
  • Logic
  • Performer
  • Sound Forge
  • Peak
  • Audacity
  • etc

iTunes MP3 encoder settings.

MP3 parameters (settings) to alter

When creating MP3 file you can experiment with the following settings ...

  • Data Rate This will vary the "strength" of compression. How this achieved depends on the compression codec and it is rare to have access to more than just an overall bit rate (kilo bits per second) control.
  • Variable bit rate This constantly alters the bit rate to achieve the best file size to quality ratio for the material.
  • Sample rate Use 44.1kHz if you can. 32kHz is acceptable and you will notice a loss of quality at 20.050kKHz.
  • Bit-depth Use 16-bit if you can. 12-bit is acceptable.
  • Channels I use mono for previews and stereo for everything else

Suggested MP3 parameter settings

Experiment but here are a few suggestions ...

  • Previews = 96kbps, 32kHz, 12bit, mono, variable bit rate on/medium quality
  • Standard quality = 128kbps, 44.1kHz, 16bit, stereo, variable bit rate on/medium quality. Most internet MP3 file are encoded with these settings
  • High quality = 192 - 224kbps, 44.1kHz, 16bit, stereo, variable bit rate on/high quality

AAC compression

if you choose the AAC encoder in iTunes it chooses settings automatically for you. Try it out. If anyone knows more about this codec and alternative ways to encode files with it, please us know.

Adding audio files to web sites

Click here for advice

Audio compression for DVD

Click here to read more.