FLAC

Free Lossless Audio Codec

Developer(s)	Xiph.Org Foundation, Josh Coalson, Erik de Castro Lopo
Initial release	20 July 2001 (2001-07-20)
Stable release	1.3.1 / 26 November 2014 (2014-11-26)
Operating system	Cross-platform
Type	Codec
License	Command-line tools: GNU GPL Libraries: BSD
Website	xiph.org/flac

FLAC (/ˈflæk/; Free Lossless Audio Codec) is an audio coding format for lossless compression of digital audio, and is also the name of the reference codec implementation. Digital audio compressed by FLAC's algorithm can typically be reduced to 50–60% of its original size^[4] 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.

Since the introduction of FLAC in 2001, the number of products and services using the format has increased.^[5] It is generally supported by more hardware devices than competing lossless compressed formats that may have intellectual property constraints.

History

Development was started in 2000 by Josh Coalson.^[6] 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.^[6]

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 behind other free compression formats such as Vorbis, Theora, Speex and Opus.^[6][7][8]

Version 1.3.0 was released on 26 May 2013, at which point development was moved to the Xiph.org git repository.^[9]

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) of any kind.^[10]

Design

Audio sources encoded to FLAC are typically reduced to 50–60% of their original size, similar to other lossless formats.^[4]

Source encoder

• FLAC supports only fixed-point 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 655,350 Hz in 1 Hz increments,^[11] and any number of channels from 1 to 8.^[12]
• Channels can be grouped in some cases, for example stereo and 5.1 channel surround, to take advantage of interchannel correlations to increase compression.
• FLAC uses CRC checksums for identifying corrupted frames when used in a streaming protocol, and 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 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.^[11]
• 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 (smallest). 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 very dependent on the level of compression.^[13][14]

According to a WAV benchmark,^[15] 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 is 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. In a comparison of compressed audio formats, FFmpeg's FLAC implementation was noted to have the fastest and most efficient embedded decoder of any modern lossless audio format.^[16]

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 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. FLAC being lossless means it is highly suitable for transcoding e.g. to MP3, without the normally associated transcoding quality loss. 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.^[11]

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, 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.

FLAC playback support in portable audio devices and dedicated audio systems is limited compared to formats such as MP3^[17] or uncompressed PCM. FLAC support is included by default in Windows 10, Android, 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 (EBU) has adopted the FLAC format for the distribution of high quality audio over its Euroradio network.^[18] The Windows operating system has supported native FLAC integration since the introduction of Windows 10.^[19] The Android operating system has supported native FLAC playback since version 3.1.^[20][21]

Among others the Pono music player and streaming service uses the FLAC format.^[22][23] Bandcamp insists on a lossless format for uploading, and has FLAC as a download option.^[24] Since January 2014 gog.com offers video game soundtracks in FLAC format as bonus.^[25] The Wikimedia Foundation sponsored a free and open-source online ECMAScript FLAC tool for browsers supporting the required HTML5 features.^[26]

See also

• Comparison of audio coding formats

References

External links

Wikimedia Commons has media related to Free Lossless Audio Codec.

• Official website
• 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