Free Lossless Audio Codec (FLAC) is an audio compression codec primarily authored by Josh Coalson. FLAC employs a lossless data compression algorithm. A digital audio recording compressed by FLAC can be decompressed into an identical copy of the original audio data. Audio sources encoded to FLAC are typically reduced to 50–60% of their original size.
FLAC is an open and royalty-free format with a free software implementation made available. FLAC has support for tagging, cover art, and fast seeking. Though FLAC playback support in portable audio devices and dedicated audio systems is limited compared to formats like MP3, FLAC is supported by more hardware devices than competing lossless formats like WavPack.
History
Development 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 others.
On 17 September 2007, the version 1.2.1 was released.
The Project
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 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 Replay Gain
- 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)
"Free" means that 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 that neither the FLAC format nor any of the implemented encoding/decoding methods are covered by any patent. It also means that the reference implementation is free software. The sources for libFLAC and libFLAC++ are available under Xiph.org's BSD license, and the sources for flac, metaflac, and the plugins are available under the GPL.
In its stated goals, the FLAC project encourages its developers not to implement copy prevention features of any kind.
Comparisons
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 compress a CD-quality audio file by 10–20%, FLAC achieves compression rates of 30–50% for most music, with significantly greater compression for voice recordings.
FLAC uses linear prediction to convert the audio samples to a series of small, uncorrelated numbers (known as the residual), which are stored efficiently using Golomb-Rice coding. It also uses run-length encoding for blocks of identical samples, such as silent passages. The technical strengths of FLAC compared to other lossless formats lie in its ability to be streamed and decoded quickly, which is independent of compression level.
As a lossless scheme, FLAC is also a popular 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 transcode 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, pregaps, and CD-Text. However, additional data present on some audio CDs such as lyrics and CD+G graphics are beyond the scope of a CUE file and most ripping software, so that data will not be archived.
The European Broadcasting Union (EBU) has adopted the FLAC format for the distribution of high quality audio over its Euroradio network.
Technical details
FLAC supports only fixed-point samples, not floating-point. This is to remove the imprecision of floating point arithmetic so as to ensure the encoder is fully lossless. 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, and any number of channels from 1 to 8. Channels can be grouped in cases like 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 has a complete MD5 hash of the raw PCM audio stored in its STREAMINFO metadata header. FLAC allows for a Rice parameter between 0–16. FLAC supports Replay Gain.
FLAC is implemented as the libFLAC core encoder & decoder library with the main distributable program flac being the reference program utilizing 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 and Mac OS X) and Unix-like (including Linux, BSD), 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.
For tagging, FLAC uses the same system as Vorbis comments.
API organization
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.
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