Free Lossless Audio Codec

FLAC (; Free Lossless Audio Codec) is an audio coding format for lossless compression of digital audio, developed by the Xiph.Org Foundation, and is also the name of the free software project producing the FLAC tools, the reference software package that includes a codec implementation. Digital audio compressed by FLAC's

algorithm

can typically be reduced to between 50 and 70 percent of its original size and decompress to an identical copy of the original audio data.

FLAC is an open format with royalty-free licensing and a reference implementation which is

free software

. FLAC has support for

metadata

tagging, album cover art, and fast seeking.

History

Development was started in 2000 by Josh Coalson. The bit-stream format was frozen when FLAC entered beta stage with the release of version 0.5 of the reference implementation on 15 January 2001. Version 1.0 was released on 20 July 2001.

On 29 January 2003, the Xiph.Org Foundation and the FLAC project announced the incorporation of FLAC under the Xiph.org banner. Xiph.org is home to other free compression formats such as Vorbis, Theora, Speex and Opus.

Version 1.3.0 was released on 26 May 2013, at which point development was moved to the Xiph.org git repository.

Composition

The FLAC project consists of:
* The stream formats
* A simple container format for the stream, also called FLAC (or ''Native FLAC'')
* libFLAC, a library of reference encoders and decoders, and a metadata interface
* libFLAC++, an object-oriented wrapper around libFLAC
* flac, a command-line program based on libFLAC to encode and decode FLAC streams
* metaflac, a command-line metadata editor for .flac files and for applying ReplayGain
* Input plugins for various music players (Winamp,

XMMS

, foobar2000, musikCube, and many more)
* With Xiph.org incorporation, the Ogg container format, suitable for streaming (also called ''Ogg FLAC'')

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 neither the FLAC format nor any of the implemented encoding or decoding methods are covered by any patent. The reference implementation is

free software

. The

source code

for libFLAC and libFLAC++ is available under the BSD license, 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 (DRM) or lossy compression of any kind.

Design

Audio sources encoded to FLAC are typically reduced to 50–70% of their original size, similar to other lossless formats, though the final size depends on the density and amplitude of the music being compressed, and, with some music, file size can be reduced by as much as 80%.

Source encoder

* The FLAC format supports only integer 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 65,535 Hz in 1 Hz increments or from 10 Hz to 655,350 Hz in 10 Hz increments, and any number of channels from 1 to 8. To date (version 1.3.3 of the reference encoder), FLAC encoding is limited to 24 bits per sample since no encoder for 32 bits per sample exists, but decoders are present in libFLAC and in ffmpeg.
* Channels can be grouped in some cases, for example stereo and 5.1 channel surround, to take advantage of interchannel correlations to increase compression.
* CRC checksums are used for identifying corrupted frames when used in a streaming protocol. The file also includes a complete

MD5

hash of the raw PCM audio in its ''STREAMINFO'' metadata header. FLAC allows for a Rice parameter between 0 and 16.
* FLAC uses linear prediction to convert the audio samples. There are two steps, the predictor and the error coding. The predictor can be one of four types (Zero, Verbatim, Fixed Linear and Finite Impulse Response (FIR) Linear). The difference between the predictor and the actual sample data is calculated and is known as the residual. The residual is stored efficiently using Golomb-Rice coding. It also uses run-length encoding for blocks of identical samples, such as silent passages.

Metadata

* FLAC supports ReplayGain.
* For tagging, FLAC uses the same system as Vorbis comments.
* The 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.

Compression levels

libFLAC uses a compression level parameter that varies from 0 (fastest) to 8 (slowest). The compressed files are always perfect, lossless representations of the original data. Although the compression process involves a tradeoff between speed and size, the decoding process is always quite fast and not dependent on the level of compression.

According to a .WAV benchmark running with an Athlon XP 2400+, using higher rates above default level −5 takes considerably more time to encode without real gains in space savings.

Comparison to other formats

FLAC is specifically designed for efficient packing of audio data, unlike general-purpose lossless algorithms such as DEFLATE, which are used in ZIP and gzip. While ZIP may reduce the size of a CD-quality audio file by 10–20%, FLAC is able to reduce the size of audio data by 40–50% by taking advantage of the characteristics of audio.

The technical strengths of FLAC compared to other lossless formats lie in its ability to be streamed and decoded quickly, independent of compression level.

Since FLAC is a lossless scheme, it is suitable as an archive format for owners of CDs and other media who wish to preserve their audio collections. If the original media are 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 copy (e.g.,

MP3

) of the same data is impossible. FLAC being lossless means it is highly suitable for transcoding e.g. to MP3, without the normally associated transcoding quality loss between one lossy format and another. 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 and pregap, but excluding CD-Text and other additional data such as lyrics and CD+G graphics.

Adoption and implementations

The reference implementation of FLAC is implemented as the ''libFLAC'' core encoder & decoder library, with the main distributable program flac being the reference implementation of 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,

BSD

) and Unix-like (including

Linux

), Microsoft Windows