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FLAC (; Free Lossless Audio Codec) is an
audio coding format An audio coding format (or sometimes audio compression format) is a content representation format for storage or transmission of digital audio Digital audio is a representation of sound In physics Physics is the natural s ...
for
lossless compression Lossless compression is a class of data compression In information theory Information theory is the scientific study of the quantification, storage, and communication of information. The field was originally established by the works o ...
of
digital audio Digital audio is a representation of sound In physics Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy ...
, 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 A codec is a device or computer program A computer program is a sequence or set of instructions in a programming language A programming language is a system of notation for writing computer programs. Most programming languages are text-b ...
implementation. Digital audio compressed by FLAC's
algorithm In mathematics Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represen ...
can typically be reduced to between 50 and 70 percent of its original size and decompresses to an identical copy of the original audio data. FLAC is an open format with royalty-free licensing and a
reference implementation In the software development process In software engineering Software engineering is a systematic engineering approach to software development. A software engineer is a person who applies the principles of software engineering to design, d ...
which is
free software Free software or libre software is computer software distributed under terms that allow users to run the software for any purpose as well as to study, change, and distribute it and any adapted versions. Free software is a matter of liberty, ...
. FLAC has support for
metadata Metadata is " data that provides information about other data", but not the content of the data, such as the text of a message or the image itself. There are many distinct types of metadata, including: * Descriptive metadata – the descriptive ...
tagging,
album cover An album cover (also referred to as album art) is the front packaging art of a commercially released studio album An album is a collection of audio recordings issued on compact disc (CD), vinyl, audio tape, or another medium such as ...
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 Vorbis is a free and open-source software Free and open-source software (FOSS) is a term used to refer to groups of software consisting of both free software Free software or libre software is computer software distributed under terms ...
, Theora,
Speex Speex is an audio compression codec specifically tuned for the reproduction of human speech and also a free software Free software or libre software is computer software distributed under terms that allow users to run the software for an ...
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 Object-oriented programming (OOP) is a programming paradigm Programming paradigms are a way to classify programming language A programming language is a system of notation for writing computer programs. Most programming languages a ...
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 Winamp is a media player for Microsoft Windows Windows is a group of several Proprietary software, proprietary graphical user interface, graphical operating system families developed and marketed by Microsoft. Each family caters to a cer ...
, XMMS,
foobar2000 foobar2000 (often abbreviated as fb2k or f2k) is a freeware Freeware is software Software is a set of computer programs and associated documentation and data. This is in contrast to hardware, from which the system is built and whi ...
, 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 In communication Communication (from la, communicare, meaning "to share" or "to be in relation with") is usually defined as the transmission of information. The term may also refer to the message communicated through such transmissi ...
or decoding methods are covered by any patent. The
reference implementation In the software development process In software engineering Software engineering is a systematic engineering approach to software development. A software engineer is a person who applies the principles of software engineering to design, d ...
is
free software Free software or libre software is computer software distributed under terms that allow users to run the software for any purpose as well as to study, change, and distribute it and any adapted versions. Free software is a matter of liberty, ...
. The
source code In computing, source code, or simply code, is any collection of code, with or without comments, written using a human-readable programming language A programming language is a system of notation for writing computer programs. Most progr ...
for libFLAC and libFLAC++ is available under the
BSD license BSD licenses are a family of permissive free software licenses, imposing minimal restrictions on the use and distribution of covered software. This is in contrast to copyleft licenses, which have share-alike requirements. The original BSD l ...
, and the sources for flac, metaflac, and the plugins are available under the
GNU General Public License The GNU General Public License (GNU GPL or simply GPL) is a series of widely used free software licenses that guarantee end users the four freedoms to run, study, share, and modify the software. The license was the first copyleft for genera ...
. In its stated goals, the FLAC project encourages its developers not to implement copy prevention features ( DRM) or lossy compression of any kind.


Design


File structure

A FLAC file consists of the magic number fLaC,
metadata Metadata is " data that provides information about other data", but not the content of the data, such as the text of a message or the image itself. There are many distinct types of metadata, including: * Descriptive metadata – the descriptive ...
, and encoded audio. The encoded audio is divided into frames, which consists of a header, a data block, and a CRC16 checksum. Each frame is encoded independent of each other. A frame header begins with a
sync word In computer network A computer network is a set of computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations ( computation) automatically. Modern digital electronic comp ...
, used to identify the beginning of a valid frame. The rest of the header contains the number of samples, position of the frame, channel assignment, and optionally the
sample rate In signal processing Signal processing is an electrical engineering subfield that focuses on analyzing, modifying and synthesizing '' signals'', such as sound In physics Physics is the natural science that studies matte ...
and bit depth. The data block contains the audio information. Metadata in FLAC precedes the audio. Properties like the
sample rate In signal processing Signal processing is an electrical engineering subfield that focuses on analyzing, modifying and synthesizing '' signals'', such as sound In physics Physics is the natural science that studies matte ...
and the number of channels are always contained in the metadata. It may also contain other information, the
album cover An album cover (also referred to as album art) is the front packaging art of a commercially released studio album An album is a collection of audio recordings issued on compact disc (CD), vinyl, audio tape, or another medium such as ...
for example. FLAC uses Vorbis comments for some types of metadata, like the title and artist name.


Encoding and decoding

The FLAC encoding algorithm consists of multiple stages. In the first stage, the input audio is split into blocks. If the audio contains multiple channels, each channel is encoded separately as a subblock. The encoder then tries to find a good mathematical approximation of the block, either by fitting a simple
polynomial In mathematics Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in mo ...
, or through general
linear predictive coding Linear predictive coding (LPC) is a method used mostly in audio signal processing and speech processing for representing the spectral envelope of a digital signal of speech Speech is a human vocal communication Communication (fro ...
. A description of the approximation, which is only a few bytes in length, is then written. Finally, the difference between the approximation and the input, called residual, is encoded using Rice coding. In many cases, a description of the approximation and the encoded residual takes up less space than using
pulse-code modulation Pulse-code modulation (PCM) is a method used to digitally represent sampled analog signals. It is the standard form of digital audio in computers, compact disc The compact disc (CD) is a digital optical disc data storage format th ...
. The decoding process is the reverse of encoding. The compressed residual is first decoded. The description of the mathematical approximation is then used to calculate a waveform. The result is formed by adding the residual and the calculated waveform. As FLAC compresses losslessly, the decoded waveform is identical to the waveform before encoding. For two-channel stereo, the encoder may choose to joint-encode the audio. The channels are transformed into a side channel, which is the difference between the two input channels, and a mid channel, the sum of the two input channels. In place of a mid channel, the left channel or the right channel may be encoded instead, which is sometimes more space-efficient. Even though the reference encoder uses a single block size for the whole stream, FLAC allows the block size in samples to vary per block.


Compression

The amount of compression is determined by various parameters, including the order of the linear prediction model and the block size. Regardless of the amount of compression, the original data can always be reconstructed perfectly. For user's convenience, the reference implementation defines 9 compression levels, which are presets of the more technical parameters to the encoding algorithm. The levels are labeled from 0 to 8, with higher numbers resulting in a higher compression ratio, at the cost of compression speed. The meaning of each compression level varies by implementation. FLAC is optimized for decoding speed at the expense of encoding speed. A benchmark has shown that, while there is little variation in decoding speed as compression level increases, beyond the default compression level 5, the encoding process takes up considerably more time with little space saved compared to level 5.


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 gzip is a file format A file format is a standard way that information is encoded for storage in a computer file. It specifies how bits are used to encode information in a digital storage medium. File formats may be either proprietar ...
. 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 CD-Text is an extension of the Red Book Compact Disc The compact disc (CD) is a digital optical disc data storage format that was co-developed by Philips and Sony to store and play digital audio recordings. In August 1982, the fir ...
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 A codec is a device or computer program A computer program is a sequence or set of instructions in a programming language A programming language is a system of notation for writing computer programs. Most programming languages are text-b ...
API is also available in C++ as libFLAC++. The reference implementation of FLAC compiles on many platforms, including most
Unix Unix (; trademarked as UNIX) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, ...
(such as Solaris, BSD) and
Unix-like A Unix-like (sometimes referred to as UN*X or *nix) operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs. Time-sharing ope ...
(including
Linux Linux ( or ) is a family of open-source Unix-like operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs. Time-sharing ...
),
Microsoft Windows Windows is a group of several Proprietary software, proprietary graphical user interface, graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, W ...
,
BeOS BeOS is an operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs. Time-sharing operating systems schedule tasks for efficien ...
, and
OS/2 OS/2 (Operating System/2) is a series of computer operating systems, initially created by Microsoft and IBM under the leadership of IBM software designer Ed Iacobucci. As a result of a feud between the two companies over how to position OS/2 re ...
operating systems. There are build-systems for
autoconf GNU Autoconf is a tool for producing configure scripts for building, installing, and packaging software on computer systems where a Bourne shell is available. Autoconf is agnostic about the programming languages used, but it is often used fo ...
/
automake In software development, GNU Automake is a programming tool to automate parts of the compilation process. It eases usual compilation problems. For example, it points to needed dependencies. It automatically generates one or more ''Makefile.in ...
, MSVC, Watcom C, and
Xcode Xcode is Apple An apple is an edible fruit In botany, a fruit is the seed-bearing structure in flowering plants that is formed from the ovary after flowering. Fruits are the means by which flowering plants (also known as an ...
. There is currently no
multicore A multi-core processor is a microprocessor A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit An integrated circuit or monolithic integrated circuit ...
support in libFLAC, but utilities such as GNU parallel and various graphical frontends can be used to spin up multiple instances of the encoder. FLAC playback support in portable audio devices and dedicated audio systems is limited compared to formats such as MP3 or uncompressed PCM. FLAC support is included by default in
Windows 10 Windows 10 is a major release of Microsoft Microsoft Corporation is an American multinational corporation, multinational technology company, technology corporation producing Software, computer software, consumer electronics, person ...
,
Android Android may refer to: Science and technology * Android (robot) An android is a humanoid robot or other artificial being often made from a flesh-like material. Historically, androids were completely within the domain of science fiction ...
, BlackBerry 10 and Jolla devices. In 2014, several aftermarket mobile electronics companies introduced multimedia solutions that include support for FLAC. These include the NEX series from Pioneer Electronics and the VX404 and NX404 from Clarion. The
European Broadcasting Union The European Broadcasting Union (EBU; french: Union européenne de radio-télévision, links=no, UER) is an alliance of Public broadcasting, public service media organisations whose countries are within the European Broadcasting Area or who ar ...
(EBU) has adopted the FLAC format for the distribution of high quality audio over its Euroradio network. The
Windows Windows is a group of several Proprietary software, proprietary graphical user interface, graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, W ...
operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs. Time-sharing operating systems schedule tasks for efficient use of the system and may al ...
has supported native FLAC integration since the introduction of Windows 10. The Android operating system has supported native FLAC playback since version 3.1. macOS High Sierra and iOS 11 add native FLAC playback support. Among others the Pono music player and streaming service used the FLAC format.
Bandcamp Bandcamp is an American online audio distribution platform founded in 2007 by Oddpost co-founder Ethan Diamond and programmers Shawn Grunberger, Joe Holt and Neal Tucker, with headquarters in Oakland, California, US. On March 2, 2022, Bandcam ...
insists on a lossless format for uploading, and has FLAC as a download option. The
Wikimedia Foundation The Wikimedia Foundation, Inc., or Wikimedia for short and abbreviated as WMF, is an American 501(c)(3) nonprofit organization A nonprofit organization (NPO) or non-profit organisation, also known as a non-business entity, not-for-p ...
sponsored a free and open-source online
ECMAScript ECMAScript (; ES) is a JavaScript JavaScript (), often abbreviated as JS, is a programming language A programming language is a system of notation for writing computer programs. Most programming languages are text-based formal lan ...
FLAC tool for browsers supporting the required
HTML5 HTML5 is a markup language Markup language refers to a text-encoding system consisting of a set of symbols inserted in a text document to control its structure, formatting, or the relationship between its parts. Markup is often used to con ...
features. Various other containers are supported, independently from used operating system, depending on used playback software.


See also

*
Comparison of audio coding formats The following tables compare general and technical information for a variety of audio coding formats. For listening tests comparing the perceived audio quality of audio formats and codecs, see the article Codec listening test. General informat ...


References


External links

*
Lossless audio formats comparison
measuring FLAC against five other lossless audio formats
Lossless comparison
FLAC against seven other lossless audio formats on Hydrogenaudio
GSMArena Phone Finder
all phones & tablets with FLAC support {{Xiph.org 2001 software Cross-platform software Free audio codecs Lossless audio codecs Xiph.Org projects Open formats