Free Lossless Audio Codec
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 data compression formats such as MP3, Advanced Audio Coding, 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 compact disc 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, and fast seeking. FLAC's 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. 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 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 licenses, and the sources for flac, metaflac, and the plugins are available under the GNU General Public License. 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 file format 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 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 sheet 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.


Technical Details
FLAC supports only Fixed-point arithmetic samples, not floating-point. It can handle any PCMaudio bit depth from 4 to 32 bits per sample, any sampling rate from 1 Hertz 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 sound to take advantage of interchannel correlations to increase compression. FLAC uses Cyclic redundancy check 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 Golomb coding parameter between 0 and 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 (operating system) and Mac OS X) and Unix-like (including Linux and BSD), Microsoft Windows, BeOS, and OS/2 operating systems. For tagging, FLAC uses the same system as Vorbis comments.