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 and decompress to an identical copy of the original audio data.
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 behind 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.
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
metaflac, and the plugins are available under the GNU General Public License.
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 volume of the music being compressed, and, with some music, file size can be reduced by as much as 80%.
- 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 (Vers. 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.
- 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[dubious ] (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.
- 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.
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.
|Compression option||Original||Compressed||Duration||Ratio||Encoding Time||Encoding Rate||Decoding Time||Decoding Rate|
|−0||2.030 GiB||1.435 GiB||03:18:21||70.67%||01:29||134x||01:24||141x|
|−5||2.030 GiB||1.334 GiB||03:18:21||65.72% (−4.95)%||03:44||53x (2.5x slower)||01:36||124x|
|−6||2.030 GiB||1.334 GiB||03:18:21||65.71% (−4.96, −0.01)%||03:51||52x (2.6x slower)||01:36||124x|
|−7||2.030 GiB||1.333 GiB||03:18:21||65.67% (−5, −0.04)%||07:47||25x (5.3x slower)||01:36||123x|
|−8||2.030 GiB||1.329 GiB||03:18:21||65.47% (−5.2, −0.2)%||10:17||19x (7x slower)||01:40||120x|
|−8 -A tukey(0.5) -A flattop||2.030 GiB||1.328 GiB||03:18:21||65.40% (−5.27, −0.07)%||16:39||12x (11x slower)||01:35||125x|
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, 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.
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, 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. The Windows operating system 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 insists on a lossless format for uploading, and has FLAC as a download option. The Wikimedia Foundation sponsored a free and open-source online ECMAScript FLAC tool for browsers supporting the required HTML5 features.
|Microsoft Windows||macOS||Linux||Android OS||iOS|
|Container support||FLAC (.flac)
Matroska (.mka, .mkv)
Core Audio Format (.caf)
Matroska (.mka, .mkv)
|FLAC (.flac)||FLAC (.flac)
Core Audio Format (.caf)
|Notes||Support introduced in Windows 10.||Support introduced in High Sierra.||FLAC has to be installed.||Support introduced in Android 3.1||Support introduced in iOS 11 (but depends on hardware used).|