Types of Audio File Formats  (March 2018)

 

This article is intended for people who want to use audio files to back up their music CDs, to play audio files on their computers, smartphone, or portable players, or to retain track-level metadata like Artist Name that are not reflected on the CD.  (CD metadata reflect artist information only at the album level, regardless of the metadata within the individual track files; see Labeling Music CDs:  CD-Text and Altering Metadata for additional background.)  In pursuing any of these goals, it’s helpful to understand something about audio formats.  I’ll mention only five of the 40 or so in existence, but the important thing is that all audio file formats are one of three general types:  lossless, compressed lossless, and lossy.

 

Lossless Formats

The main characteristic of lossless files is what the term implies.  When a sound recording is digitized, the analogue waves are converted to digital samples, and none of the digital information is discarded or doctored.  What you hear comes as close as possible to matching the fidelity of the original recording, given the resolution of the conversion processed (resolution is discussed in Digital Audio Resolution).  Lossless formats that you may be familiar with include WAV (Waveform Audio File Format, developed by Microsoft and IBM) and AIFF (Audio Interchange File Format, developed by Apple).

 

The major drawback of lossless formats is the size of the files.  A lossless file with CD-standard resolution will require roughly 10 MB per minute.  With the decreasing cost of disc capacity, storage has become less of an issue than it was 15 to 20 years ago, at least for computers.  Still, a music library can eat up hundreds of gigabytes of disc space.  File size remains a factor for smartphones and portable players, as well as for downloading and streaming.  Depending on the size of one’s download “pipe,” listening to or downloading a lossless file could be subject to buffering interruptions.

 

Compressed Lossless Formats

Compressed lossless formats address the file size issue.  Like zipped files, the audio information is packed into a smaller sized lossless file and then expanded when played.  (I’ll refer to this as “file compression” to differentiate it from “audio compression,” the subject of a future article.)  Two major formats are FLAC (Free Lossless Audio Codec) and ALAC (Apple Lossless Audio Codec).  (A codec is a program that encodes digital audio data during recording and decodes digital audio data during playback.)

 

Be advised that the ability to play lossless and compressed lossless audio files may depend on the device.  Some Microsoft devices may not be able to play AIFF files or decompress ALAC files, and some Apple devices may not be able to play WAV files or decompress FLAC files.  Also, your computer’s player may not be able to process FLAC or ALAC files unless you download and install the appropriate codec.

 

Lossy Formats

With lossy formats, audio information is removed from the file using algorithms based on the study of sound perception—psychoacoustics.  For example, we generally perceive only the louder of two sounds with similar frequency profiles, and so the softer of the two is discarded in creating a lossy audio file, thereby reducing the file size.  The file is then compressed, which saves more space and facilitates downloading and streaming.  When a lossy music file is played, the file is first decompressed, and then what’s played is the music minus what was discarded during the file’s creation.

 

The most common lossy format by far is MP3, which has bit rates ranging from 32 to 320 Kbps.  (“Bit rate” is a measure of how much information is processed per second in creating a continuous signal from a digital audio file during playback).  The lower end of the range is perhaps acceptable for speech but not for music, and the higher end is reported to be close to CD quality.  The most prevalent bit rate is 128 Kbps, and you are likely to run into that in music you listen to from the Internet, as well as with some audio format converters.  A 128 Kbps MP3 file is about 9% the size of a 1,411 Kbps WAV file, so the savings on storage space is substantial (see Figure 1).  Some streaming and downloading services provide the option to upgrade to a higher, better quality bit rate.  But regardless of the bit rate, MP3 or other lossy formats will never quite sound like CD quality, and you will never be able to reconstitute the original lossless audio from a lossy file.