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Free Lossless Audio Codec (FLAC) is a file format for lossless audio data compression. During compression, FLAC does not lose quality from the audio stream, as lossy compression formats such as MP3, AAC, and Vorbis do. Josh Coalson is the primary author of FLAC.

FLAC reduces bandwidth and storage requirements without sacrificing the integrity of the audio source. A digital audio recording (such as a CD track) encoded to FLAC can be decompressed into an identical copy of the audio data. Audio sources encoded to FLAC are typically reduced in size 40 to 50 percent (46% according to their own comparison).

FLAC is suitable for everyday audio playback and archival, with support for tagging, cover art and fast seeking. FLAC's free and open source royalty-free nature makes it well-supported by many software applications, but FLAC playback support in portable audio devices and dedicated audio systems is limited at this time.

On January 29, 2003, Xiphophorus (now called the Xiph.Org Foundation) announced the incorporation of FLAC under their banner, alongside Vorbis, Theora, Speex, and others.

The project

The FLAC project consists of:

''Free'' means that the specification of the stream format can be implemented by anyone without prior permission (Xiph.org reserves the right to set the FLAC specification and certify compliance), and that neither the FLAC format nor any of the implemented encoding/decoding methods are covered by any patent. It also means that the reference implementation is free software. The sources for libFLAC and libFLAC++ are available under Xiph.org's BSD license, and the sources for flac, metaflac, and the plugins are available under the GPL.

In its stated goals, the FLAC project encourages its developers not to implement copy prevention features of any kind.

Comparisons

FLAC is specifically designed for efficient packing of audio data, unlike general lossless algorithms such as DEFLATE which is used in ZIP and gzip. While ZIP may compress a CD-quality audio file by 10-20%, FLAC achieves compression rates of 30-50% for most music, with significantly greater compression for voice recordings. By contrast, lossy codecs can achieve ratios of 80% or more by discarding data from the original stream.

FLAC uses linear prediction to convert the audio samples to a series of small, uncorrelated numbers (known as the residual), which are stored efficiently using Golomb-Rice coding. It also uses run-length encoding for blocks of identical samples, such as silent passages. The technical strengths of FLAC compared to other lossless codecs lie in its ability to be streamed and in a fast decode time, which is independent of compression level.

As a lossless scheme, FLAC is also a popular archive format for owners of CDs and other media who wish to preserve their audio collections. If the original media is lost, damaged, or worn out, a FLAC copy of the audio tracks ensures that an exact duplicate of the original data can be recovered at any time. An exact restoration from a lossy archive (e.g., MP3) of the same data is impossible. A CUE file can optionally be created when ripping a CD. If a CD is read and ripped perfectly to FLAC files, the CUE file allows later burning of an audio CD that is identical in audio data to the original CD, including track order, pregaps, and CD-Text. However, additional data present on some audio CDs such as lyrics and CD+G graphics are beyond the scope of a CUE file and most ripping software, so that data will not be archived.

The European Broadcasting Union (EBU) has adopted the FLAC format over its Euroradio network for the distribution of high quality audio.

The Hydrogenaudio Wiki features a of lossless codecs, including FLAC.

Technical details

FLAC supports only fixed-point samples, not floating-point. It can handle any PCM bit resolution from 4 to 32 bits per sample, any sampling rate from 1 Hz to 655,350Hz in 1 Hz increments, and any number of channels from 1 to 8. Channels can be grouped in cases like stereo and 5.1 channel surround to take advantage of interchannel correlations to increase compression. FLAC uses CRC checksums for identifying corrupted frames when used in a streaming protocol, and also has a complete MD5 hash of the ''raw PCM'' audio stored in its ''STREAMINFO'' metadata header. FLAC allows for a Rice parameter between 0-16. FLAC supports Replay Gain.

FLAC is implemented as the libFLAC core encoder & decoder library with the main distributable program ''flac'' being the reference program utilizing the libFLAC API. This codec API is also available in C++ as libFLAC++.

The reference implementation of FLAC compiles on many platforms, including most Unix (such as Solaris) and Unix-like (including Linux, BSD and Mac OS X), Windows, BeOS, and OS/2 operating systems. There are build systems for autoconf/automake, MSVC, Watcom C, and Xcode.

For tagging, FLAC uses the same system as Vorbis comments.

API organization

libFLAC API is organized into streams, seekable streams, and files (listed in the order of increasing abstraction from the base FLAC bitstream). Most FLAC applications will generally restrict themselves to encoding/decoding using libFLAC at the file level interface.

Software support

Encoding

Decoding