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Audio File Formats for Podcasts

Many different audio file formats exist for storing recorded audio data on a computer system. This post compares various types and gives suggestions on which format one should use, especially when producing podcasts or other online audio.

If you just want to see some practical tips, skip the description and read the conclusion.

Types of audio formats

There are three main types of audio file formats:
  • Uncompressed audio formats:
    Uncompressed audio formats store the audio information as it is recorded. This results in big files, but no information is lost, therefore they are suitable for archiving original recordings. The most common uncompressed audio format is PCM, which is usually stored in a WAV or AIFF file.
  • Lossless compression:
    Lossless audio compression formats need less space then uncompressed formats, without any loss in quality. They work similar to ZIP, but the compression algorithms are specifically designed for audio data. Some example formats are FLAC (Free Lossless Audio Codec) or ALAC (Apple Lossless Audio Codec), for a comparison of various codecs see Lossless comparison.
  • Lossy compression:
    Lossy compression formats significantly reduce the file size, by throwing away information imperceptible to humans. This gives very small files, but some information is lost and cannot be reconstructed. The best-known example is MP3, others are Ogg Vorbis, AAC, WMA, ...

A great source of information about audio codecs and listening tests is the hydrogenaudio.org wiki and forum.

Relevant audio formats for podcasts

When distributing a podcast or other audio over the internet, you want to have the smallest possible filesize, the best possible quality and everyone should be able to play it (on all operating systems, on mobile phones, portable audio players, car audio players etc.).
Because of the much smaller filesize, lossy formats are the only real option. Additionally one may archive the produced podcast in an uncompressed or lossless compressed audio file.

The characteristics of a few important formats are listed below.

MP3: MPEG-1 Audio Layer 3 (description)
  • the most widespread and the de facto standard of online audio file formats
  • most, if not all, hardware players support MP3
  • acceptable quality, but already quite old
  • patented: a license is required to "distribute and/or sell decoders and/or encoders" (see e.g. MP3 licensing and patent issues)
Ogg Vorbis: (description, official page)
  • an open source and patent free audio codec
  • performs very well from low to high bitrates (more advanced than MP3)
  • widespread in the open source community
  • some implementations are more computationally intensive than MP3
  • some portable players support Ogg Vorbis out of the box (see Vorbis Portable Players)
AAC: Advanced Audio Codec (description)
  • the latest industry standard and the official successor to MP3 (also called MP4 audio, most common file extension is M4A)
  • different types of AAC exist (LC AAC, HE AAC, HE AAC v2)
  • the AAC format is on par with Ogg Vorbis and other modern codecs, HE AAC should provide higher quality at low bitrates
  • HE AAC uses spectral band replication (high frequencies are removed and calculated from lower frequencies, see SBR), HE AAC v2 adds a parametric stereo method (stereo audio is created out of a mono signal, see PS) - don't use HE AAC for higher bitrates!
  • heavily patented: a patent license is required for all manufacturers or developers of AAC codecs
  • widespread in the apple community (iTunes, iPod, iPhones etc.), many other portable players don't support AAC
FLAC: Free Lossless Audio Codec (description, official page)
  • lossless compression, no information is lost
  • flac files are typically reduced to 40-60% of their original size
  • very fast encoding and decoding
  • open source and patent free
For a detailed comparison of various codecs at different bitrates see List of Codec Listening Tests.

Conclusion

If you want to reach everyone and maintain only one file format, you have only one choice: MP3. It is not the most advanced codec, but everything supports it and the quality is sufficient.

Additionally it may be advantageous to offer your audio in Ogg Vorbis or AAC. Ogg Vorbis is widespread in the open source and creative commons movement and should be used when patent problems are important for you. AAC is the new standard on all "i"-Devices (Apple) and shows very good performance especially at low bitrates (HE AAC).

Finally, if you also want to archive an original version of your audio, use FLAC or just PCM (WAV, AIFF).

UPDATE:
Opus is the new audio codec in town, especially interesting for podcasters! For more information see Opus, the revolutionary open audio codec for podcasts and internet audio.

Further reading: Podcast Comparison, Part 1: File Formats and Bitrates.


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