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FLAC

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For other uses, see FLAC (disambiguation).
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Free Lossless Audio Codec
Developer(s)Xiph.Org Foundation, Josh Coalson, Erik de Castro Lopo
Initial release20 July 2001; 23 years ago (2001-07-20)
Stable release
1.4.3[1] / 23 June 2023; 17 months ago (23 June 2023)
Repository
Written inC, C++
Operating systemCross-platform
TypeCodec
LicenseCommand-line tools: GNU GPL
Libraries: BSD
Websitexiph.org/flac
Free Lossless Audio Codec
Filename extension
.flac
Internet media type
audio/flac
Uniform Type Identifier (UTI)org.xiph.flac
Magic numberfLaC[2]
Type of formatLossless audio
StandardRFC 9639
Open format?Yes[3]
Free format?Yes

FLAC (/flæk/; 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[4] 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

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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.[5]

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.[5][6][7]

Version 1.3.0 was released on 26 May 2013, at which point development was moved to the Xiph.org git repository.[8]

In 2019, FLAC was proposed as an IETF standard,[9] and was published in December 2024 as RFC 9639.

Design

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FLAC is a lossless encoding of linear pulse-code modulation data.[10]

File structure

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A FLAC file consists of the magic number fLaC, metadata, and encoded audio.[10]

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 bit depth. The data block contains the audio information.[10]

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.[10] FLAC uses Vorbis comments for textual metadata like track title and artist name.

Encoding and decoding

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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, 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.[10]

The decoding process is the reverse of encoding. The compressed residual is first decoded.[11][12] 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.[13][14] 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.[15]

Even though the reference encoder uses a single block size for the whole stream,[10] FLAC allows the block size in samples to vary per block.

Compression

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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 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.[16][17]

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.[18]

Implementation

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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 programs flac and metaflac, are also part of the reference implementation.

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

Comparison to other formats

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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.[citation needed]

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

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 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 additional data such as lyrics and CD+G graphics.[19] 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

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See also: List of hardware and software that supports FLAC and Comparison of video container formats § Audio coding formats support

Reference implementation

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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), 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

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FLAC playback support in portable audio devices and dedicated audio systems is limited compared to formats such as MP3[20] or uncompressed PCM. FLAC support is included by default in Windows 10[21], Android[22], macOS and iOS[23].

FLAC support by different operating systems
Microsoft Windows macOS Android BlackBerry OS iOS
Codec support Yes Yes Yes Yes Yes
Container support FLAC (.flac)
Matroska (.mka, .mkv)
Ogg (.oga)[24] 		FLAC (.flac)
Core Audio Format (.caf) 		FLAC (.flac) FLAC (.flac) FLAC (.flac)
Core Audio Format (.caf)
Notes

Support introduced in Windows 10.

Windows Media Player (2022) also supports FLAC in an Ogg container for live streams (e.g. Icecast internet radio).[25]

 Support introduced in macOS 10.13 High Sierra.

Support introduced in Android 3.1.

Android natively supports regular FLAC (.flac), but not Ogg FLAC (.oga).[26] However, support for both regular FLAC and Ogg FLAC were later added to the Files (Google) file manager.

 Support introduced in BlackBerry OS 5.0 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

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The standardization process of the FLAC format into RFC 9639 was driven by the specific use case of archival and preservation in mind[27]. The National Archives and Records Administration has FLAC listed as a preferred format for digital audio[28].

See also

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References

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  1. ^ "FLAC 1.4.3 Released With More Optimizations, Drops PowerPC-Specific Code". 23 June 2023. Retrieved 19 February 2024.
  2. ^ Coalson, Josh. "FLAC – format". Xiph.Org Foundation. Retrieved 4 April 2013. "fLaC", the FLAC stream marker in ASCII, meaning byte 0 of the stream is 0x66, followed by 0x4C 0x61 0x43
  3. ^ "PlayOgg!". Free Software Foundation. 17 March 2010. Retrieved 1 October 2013.
  4. ^ "Looking at Flac Compression Ratios". Steven Pigeon. 7 February 2012. Retrieved 10 May 2018.
  5. ^ a b "Features". Xiph.Org Foundation. Retrieved 23 November 2023.
  6. ^ "FLAC Joins Xiph.org". Xiph.org Foundation. 29 January 2003. Retrieved 31 August 2009.
  7. ^ Plant, Emmett. "FLAC Joins Xiph!". Xiph.org Foundation. Archived from the original on 29 May 2008. Retrieved 31 August 2009.
  8. ^ "FLAC – changelog". Xiph.org Foundation. Retrieved 15 October 2013.
  9. ^ van Beurden, Martijn; Weaver, Andrew. Free Lossless Audio Codec. I-D draft-ietf-cellar-flac. Retrieved 14 May 2023.
  10. ^ a b c d e f Coalson, Josh. "About the FLAC Format". Retrieved 13 February 2022.
  11. ^ "stream_decoder.c". libFLAC. Retrieved 19 February 2022.
  12. ^ libFLAC/stream_decoder.c, line 2734.
  13. ^ "lpc.c". libFLAC. Retrieved 19 February 2022.
  14. ^ libFLAC/lpc.c, lines 813–820.
  15. ^ "FLAC Format Specification". FLAC. Retrieved 19 February 2022.
  16. ^ "CUETools FLAC encoders comparison". CUETools Wiki. Retrieved 27 May 2013.
  17. ^ "Encoding Settings". JRiver Media Centre. Retrieved 27 May 2013.
  18. ^ "Lossless Codec Comparison". Synthetic-soul.co.uk. Archived from the original on 2 February 2009. Retrieved 26 November 2016.
  19. ^ "FAQ". FLAC. Retrieved 23 September 2014.
  20. ^ "Links". FLAC. Retrieved 24 March 2009.
  21. ^ "Audio snobs rejoice: Windows 10 will have system-wide FLAC support". PC World. Retrieved 10 July 2015.
  22. ^ "Android Supported Media Formats". Android.com. 27 September 2024. Retrieved 20 December 2024.
  23. ^ "iOS 11 brings lossless FLAC audio playback to iPhone and iPad". idownloadblog.com. 9 June 2017. Retrieved 26 September 2017.
  24. ^ "MIME Types and File Extensions - XiphWiki".
  25. ^ "FLAC vs Ogg FLAC".
  26. ^ "The Android Platform Doesn't Fully Support OGG Container Formats (OGA) [36906426] - Visible to Public - Issue Tracker".
  27. ^ "Codec Encoding for LossLess Archiving and Realtime transmission".
  28. ^ "Appendix A: Tables of File Formats | National Archives".
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