Audio Codecs

1
Audio Codecs
Mike Bell
An overview of the audio format war and a brief
guide to properly maintaining your music
collection
2
Overview

• Uncompressed
• PCM (.wav, .aiff)?
• Lossless
• ALAC, FLAC, etc
• Lossy
• MP3, WMA, Vorbis,
• AAC

• Budget audiophilia

3
Uncompressed Audio

• Analog signals compressed using Pulse-code
  Modulation (PCM)?
• Digital signals decompressed using a reverse
  modulation to remove/smooth out auditory
  artifacts (represented by the gaps of the signal)
  
• Special filters and error-correction techniques
  help accomplish this

4
WAV and AIFF

• Both are file-formats that store PCM-encoded
  uncompressed audio data
• WAV v.s. AIFF gt Intel v.s. Mac gt Little v.s.
  Big (endian)?
• Pros
• First-generation encoding (i.e. Audio CDs)?
• Useful in synchronization
• Cons
• Uncompressed LOTS of space used
• Size limit of 4GB (around 6.6 hours of CD-quality
  audio)?
• An open-source 64-bit format called W64 is
  available on Sourceforge that gets around this
  limitation

5
Compressed Audio

• Lossless Compression
• Pros
• Preservation of prior-generation data
• Specialized compression for audio data achieves
  greater results than standard data compression
  (zip, etc)?
• Cons
• Still unsatisfactory size compression
• Decompression speed unusable on many platforms
• Lossy Compression
• Pros
• Optimized for speedy decompression and for what
  humans can hear
• In wide-spread use in many applications
• Cons
• Incurs generational data loss
• The Format War

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Lossless Codecs

• The most commonly used ones
• Apple Lossless Audio Codec (ALAC)?
• Quick decompression allows for use on mobile
  platforms (iPods, etc)?
• Container format hints at possible DRM
  implementation (oh no!!)?
• Free Lossless Audio Codec (FLAC)?
• Uses fixed-point samples, linear prediction, and
  run-length encoding to increase speed.
• Available on Windows/Mac/Linux and several
  portable platforms (possible on iPod with custom
  firmware)?
• Monkey Audio (APE)?
• Ambiguous licensing, slow seeking time,
  unpredictable file quality v.s. Other lossless
  codecs
• Currently only available on Windows. Too slow
  for portable music.
• Windows Media Audio Lossless ( WMA Lossless)?
• Windows Media Player, XBOX 360, Zune (version
  2.2 firmware), and a few other portable music
  players

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Lossy Codecs

• MP3
• Wide-spread commercial and public use
• On the verge of obsoletion
• AAC
• Many improvements over MP3
• Only slightly-licensed
• MANY platforms for use

• Vorbis
• It's like the Linux of codecs
• VERY little mobile support
• WMA
• Different versions support higher-quality audio
• DRM is a big deal here
• Many platforms for use
• Ranks lower than the prior 3

8
Budget Audiophilia I

• Existing collections
• Fidelity is damaged on each successive lossy
  encode.
• If you have a lot of lossy files, don't bother
  re-encoding.
• Adding high-quality files to collections
• Have the original media.
• Rip the media into a lossless format (for
  archiving).
• Encode the lossless-encoded media into a lossy
  format.
• If ever your lossy files are corrupted, just
  re-encode the lossless set.