Editing FLAC

{{Short description|Lossless digital audio coding format}}
{{Other uses}}
{{Redirect|Flac|flak anti-aircraft fire|Anti-aircraft warfare}}
{{Use dmy dates|date=May 2022}}

{{Infobox software
| name = Free Lossless Audio Codec
| logo = FLAC logo vector.svg
| screenshot = 
| caption = 
| developer = [[Xiph.Org Foundation]], Josh Coalson, Erik de Castro Lopo
| released = {{Start date and age|2001|07|20|df=yes}}
| latest release version = {{wikidata|property|reference|P348}}
| latest release date = {{start date and age|{{wikidata|qualifier|P348|P577}}}}
| operating_system = [[Cross-platform]]
| programming language = [[C (programming language)|C]], [[C++]]
| genre = [[Codec]]<!--File format has a separate infobox-->
| license = Command-line tools: [[GNU GPL]]<br />Libraries: [[BSD licenses|BSD]]
| website = {{url|https://xiph.org/flac}}
}}
{{Infobox file format
| name          = Free Lossless Audio Codec
| icon          = 
| logo          = 
| extension     = .flac
| mime          = audio/flac
| type code     = 
| uniform type  = org.xiph.flac
| magic         = fLaC<ref>{{cite web |url=https://xiph.org/flac/format.html#stream |access-date=4 April 2013 |title=FLAC – format |first=Josh |last=Coalson |quote="fLaC", the FLAC stream marker in ASCII, meaning byte 0 of the stream is 0x66, followed by 0x4C 0x61 0x43 |publisher=[[Xiph.Org Foundation]]}}</ref>
| owner         = 
| type          = [[Lossless]] [[Audio file format|audio]]
| container for = 
| contained by  = 
| extended from = 
| extended to   = 
| standard      = {{IETF RFC|9639}}
| open          = Yes<ref>{{cite web |url=http://www.fsf.org/campaigns/playogg/en/ |title=PlayOgg! |website=Free Software Foundation |access-date=1 October 2013 |date=17 March 2010}}</ref>
| free          = Yes
}}

'''FLAC''' ({{IPAc-en|f|l|æ|k}}; '''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<ref name="looking">{{cite web |url=https://hbfs.wordpress.com/2012/02/07/looking-at-flac-compression-ratios/ |title=Looking at Flac Compression Ratios |date=7 February 2012 |access-date=10 May 2018 |publisher=Steven Pigeon}}</ref> and decompresses 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 supports [[metadata]] tagging, [[album cover]] art, and fast seeking.

== History ==
Development was started in 2000 by Josh Coalson. The [[bitstream]] format was frozen with the release of version 0.9 of the reference implementation on 31 March 2001. Version 1.0 was released on 20 July 2001.<ref name="news">{{cite web |url=https://xiph.org/flac/news.html |title=Features |access-date=2023-11-23 |publisher=[[Xiph.Org Foundation]]}}</ref>

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 (codec)|Opus]].<ref name="news" /><ref name="xiph-press-flac">{{cite web |website=Xiph.org Foundation |url=https://www.xiph.org/press/2003/flac/ |title=FLAC Joins Xiph.org |date=29 January 2003 |access-date=31 August 2009}}</ref><ref name="xiph-flac">{{cite web |first=Emmett |last=Plant |website=Xiph.org Foundation |url=http://xiph.org/ogg/flac.html |title=FLAC Joins Xiph! |access-date=31 August 2009 |archive-url=https://web.archive.org/web/20080529055542/http://xiph.org/ogg/flac.html |archive-date=29 May 2008 |url-status=dead }}</ref>

Version 1.3.0 was released on 26 May 2013, at which point development was moved to the Xiph.org git repository.<ref name="xiph-change">{{cite web |website=Xiph.org Foundation  |url=https://xiph.org/flac/changelog.html |title=FLAC – changelog |access-date=15 October 2013}}</ref>

In 2019, FLAC was proposed as an IETF standard.<ref>{{cite IETF|first1=Martijn|last1=van Beurden|first2=Andrew|last2=Weaver|title=Free Lossless Audio Codec|draft=draft-ietf-cellar-flac|access-date=2023-05-14}}</ref> 

In December 2024, FLAC was formally specified in and published as {{IETF RFC|9639}}.<ref>{{Cite web |last=Coalson |first=Josh |title=FLAC - FLAC is now formally specified in RFC 9639 |url=https://xiph.org/flac/2024/12/19/rfc-9639-published.html |access-date=2025-02-12 |website=xiph.org |language=en}}</ref>

== Design ==
FLAC is a lossless encoding of [[linear pulse-code modulation]] data.<ref name="format-about">{{Cite web|last=Coalson|first=Josh|title=About the FLAC Format|url=https://xiph.org/flac/documentation_format_overview.html|access-date=13 February 2022}}</ref>

=== File structure ===
A FLAC file consists of the [[File format#Magic number|magic number]] <code>fLaC</code>, [[metadata]], and encoded audio.<ref name="format-about"/>

The encoded audio is divided into frames, each of 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]], 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]] and [[audio bit depth|bit depth]]. The data block contains the audio information.<ref name="format-about"/>

Metadata in FLAC precedes the audio. Properties like the [[sample rate]] and the number of channels are always contained in the metadata. It may also contain other information, the [[album cover]] for example.<ref name="format-about"/> FLAC uses [[Vorbis comment]]s for textual metadata like track title and artist name.

=== Encoding and decoding ===
[[File:FLAC_1.5.0_screenshot.webp|thumb|250x250px|Screenshot of <code>flac</code> command-line software]]
The FLAC encoding algorithm consists of multiple stages. In the first stage, the input audio is split into blocks. If the audio [[stereo|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]], or through general [[linear predictive coding]]. 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]].<ref name="format-about"/>

The decoding process is the reverse of encoding. The compressed residual is first decoded.<ref>{{cite web |title=stream_decoder.c |url=https://gitlab.xiph.org/xiph/flac/-/blob/a2fe43f64e7ce8057fb274c64996568d69b301b8/src/libFLAC/stream_decoder.c |website=libFLAC |access-date=19 February 2022 |ref=libflac_stream_decoder}}</ref><ref>[[#libflac_stream_decoder|libFLAC/stream_decoder.c]], line 2734.</ref> 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.<ref>{{cite web |title=lpc.c |url=https://gitlab.xiph.org/xiph/flac/-/blob/a2fe43f64e7ce8057fb274c64996568d69b301b8/src/libFLAC/lpc.c |website=libFLAC |access-date=19 February 2022 |ref=libflac_lpc}}</ref><ref>[[#libflac_lpc|libFLAC/lpc.c]], lines 813–820.</ref> As FLAC [[lossless compression|compresses losslessly]], the decoded waveform is identical to the waveform before encoding.

For two-channel stereo, the encoder may choose to [[joint stereo|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.<ref>{{Cite web |title=FLAC Format Specification |url=https://xiph.org/flac/format.html |access-date=19 February 2022 |website=FLAC}}</ref>

Even though the [[reference implementation|reference encoder]] uses a single block size for the whole stream,<ref name="format-about"/> 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 the user's convenience, the reference implementation defines 9 compression levels, which are [[default (computer science)|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.<ref>{{Cite web |url=http://www.cuetools.net/wiki/CUETools_FLAC_encoders_comparison |title=CUETools FLAC encoders comparison |website=CUETools Wiki |access-date=27 May 2013}}</ref><ref>{{cite web |url=http://wiki.jriver.com/index.php/Encoding_Settings |title=Encoding Settings |website=JRiver Media Centre |access-date=27 May 2013}}</ref>

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.<ref>{{cite web |title=Lossless Codec Comparison |url=http://synthetic-soul.co.uk/comparison/lossless/index.asp |website=Synthetic-soul.co.uk |access-date=26 November 2016 |archive-url=https://web.archive.org/web/20090202063734/http://synthetic-soul.co.uk/comparison/lossless/index.asp |archive-date=2 February 2009 |url-status=dead }}</ref>

=== Implementation ===
Alongside the format, the FLAC project also contains a [[free and open-source]] reference implementation of FLAC called libFLAC. libFLAC contains facilities to encode and decode FLAC data and to manipulate the metadata of FLAC files. libFLAC++, an [[object-oriented]] wrapper around libFLAC for [[C++]], and the [[command-line program]]s <code>flac</code> and <code>metaflac</code>, are also part of the reference implementation.

The FLAC format, along with libFLAC, are not known to be covered by any [[software patent|patents]], and anyone is free to write their own implementations of FLAC.

== 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 file format|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.{{cn|date=July 2024}}

The technical strengths of FLAC compared to other lossless formats lie in its ability to be streamed and decoded quickly, independent of compression level.{{cn|date=July 2024}}

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's being lossless means it is highly suitable for [[transcoding]] e.g. to MP3, without the normally associated [[digital generation loss|transcoding quality loss]] between one lossy format and another. A [[Cue sheet (music software)|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 additional data such as lyrics and [[CD+G]] graphics.<ref name="flacfaq">{{cite web |title=FAQ |url=https://xiph.org/flac/faq.html |access-date=23 September 2014 |website=FLAC}}</ref> But depending on the burning program used, [[CD-Text]] may be recovered from the metadata stored in the CUE sheet and burned back to a new copy on blank CD-R media.

== Adoption and implementations ==
{{see also|List of hardware and software that supports FLAC|Comparison of video container formats#Audio coding formats support}}

=== Reference implementation ===
The reference implementation of FLAC is implemented as the ''libFLAC'' core encoder & decoder library, with the main distributable program <code>flac</code> 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 (operating system)|Solaris]], [[BSD]]) and [[Unix-like]] (including [[Linux]]), [[Windows]], [[BeOS]], and [[OS/2]] operating systems. There are build-systems for [[autoconf]]/[[automake]], [[MSVC]], [[Watcom C]], and [[Xcode]]. There is currently no [[multicore]] support in libFLAC, but utilities such as [[GNU parallel]] and various graphical frontends can be used to spin up multiple instances of the encoder.

=== Hardware and software ===
FLAC playback support in portable audio devices and dedicated audio systems is limited compared to formats such as MP3<ref>{{cite web |url=https://xiph.org/flac/links.html#hardware |title=Links |website=FLAC |access-date=24 March 2009}}</ref> or uncompressed [[PCM]]. FLAC support is included by default in [[Windows 10]],<ref>{{Cite web |title=Audio snobs rejoice: Windows 10 will have system-wide FLAC support |url=http://www.pcworld.com/article/2852595/audio-snobs-rejoice-windows-10-will-have-system-wide-flac-support.html |website=PC World |access-date=10 July 2015}}</ref> [[Android (operating system)|Android]],<ref>{{cite web |url=https://developer.android.com/media/platform/supported-formats |title=Android Supported Media Formats |date=27 September 2024 |website=Android.com |access-date=20 December 2024}}</ref> [[macOS]] and [[iOS]].<ref>{{cite web |url=http://www.idownloadblog.com/2017/06/09/ios-11-brings-lossless-flac-audio-to-iphone-and-ipad/ |title=iOS 11 brings lossless FLAC audio playback to iPhone and iPad |date=9 June 2017 |website=idownloadblog.com |access-date=26 September 2017}}</ref>

{| class="wikitable"
|+ FLAC support by different operating systems
|-
! scope="row" | 
! [[Microsoft Windows]]
! [[macOS]]
! [[Android (operating system)|Android]]
! [[iOS]]
|-
! scope="row" | Codec support
| {{yes}} || {{yes}} || {{yes}} || {{yes}}
|-
! scope="row" | Container support
| FLAC (.flac) <br/>[[Matroska]] (.mka, .mkv) <br /> [[Ogg]] (.oga)<ref>{{cite web |url=https://wiki.xiph.org/MIME_Types_and_File_Extensions |title=MIME Types and File Extensions - XiphWiki }}</ref>
| FLAC (.flac) <br/>[[Core Audio Format]] (.caf)
| FLAC (.flac)
| FLAC (.flac) <br/>[[Core Audio Format]] (.caf)
|-
! scope="row" | Notes
| <p>Support introduced in [[Windows 10]].</p>
<p>[[Windows Media Player (2022)]] also supports FLAC in an Ogg container for live streams (e.g. [[Icecast]] [[internet radio]]).<ref>{{cite web |url=https://hydrogenaud.io/index.php/topic,117036.0.html |title=FLAC vs Ogg FLAC }}</ref></p>
| Support introduced in macOS 10.13 [[macOS High Sierra|High Sierra]].
| <p>Support introduced in Android 3.1.</p>
<p> Android natively supports regular FLAC (.flac), but not Ogg FLAC (.oga).<ref>{{cite web |url=https://issuetracker.google.com/issues/36906426 |title=The Android Platform Doesn't Fully Support OGG Container Formats (OGA) [36906426] - Visible to Public - Issue Tracker }}</ref> However, support for both regular FLAC and Ogg FLAC were later added to the [[Files (Google)]] file manager.</p>
| Support introduced in iOS 11 (but depends on hardware).
|}
Various other containers are supported, independently from used operating system, depending on used playback software.

=== Use in archives ===
The standardization process of the FLAC format into {{IETF RFC|9639}} was driven by the specific use case of archival and preservation in mind.<ref>{{cite web|url=https://datatracker.ietf.org/doc/charter-ietf-cellar/|title=Codec Encoding for LossLess Archiving and Realtime transmission}}</ref> The [[National Archives and Records Administration]] has FLAC listed as a preferred format for digital audio.<ref>{{cite web|url=https://www.archives.gov/records-mgmt/policy/transfer-guidance-tables.html|title=Appendix A: Tables of File Formats {{pipe}} National Archives}}</ref>

== See also ==
{{Portal|Free and open-source software}}
* [[Comparison of audio coding formats]]
* [[Comparison of audio player software#Audio format ability|List of audio player software that supports FLAC]]

== References ==
{{Reflist}}

== External links ==
{{Commons category|Free Lossless Audio Codec}}
* {{Official website|https://xiph.org/flac/}}
* [https://www.bobulous.org.uk/misc/lossless_audio_2006.html Lossless audio formats comparison]: measuring FLAC against five other lossless audio formats
* [https://wiki.hydrogenaud.io/index.php?title=Lossless_comparison Lossless comparison]: FLAC against seven other lossless audio formats on [[Hydrogenaudio]]
* [https://www.gsmarena.com/results.php3?sName=&sFreeText=FLAC GSMArena Phone Finder]: all phones & tablets with FLAC support

{{Compression formats}}
{{Compression software implementations}}
{{Xiph.org}}

[[Category:2001 software]]
[[Category:Cross-platform software]]
[[Category:Free audio codecs]]
[[Category:Lossless audio codecs]]
[[Category:Xiph.Org projects]]
[[Category:Open formats]]