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FLAC

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The Free Lossless Audio Codec (FLAC) logo.
The Free Lossless Audio Codec (FLAC) logo.

FLAC, an acronym for Free Lossless Audio Codec, is a popular format for audio compression. Unlike lossy codecs such as Vorbis, MP3 and AAC, it does not remove any information from the audio stream and is suitable both for everyday playback and audio archival. The FLAC format is currently well supported by many software projects and hardware support is growing.[link] FLAC also supports Replay Gain.

On January 29, 2003, Xiphophorus (now called the Xiph.Org Foundation) announced the incorporation of FLAC into their banner, adding it to Ogg, Vorbis, Theora, Speex, and others.

Contents

The project

The FLAC project consists of: "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 and 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. [link]

Comparisons

FLAC is for efficient packing of audio data, unlike general lossless algorithms such as ZIP and gzip. While ZIP may compress a CD-quality audio file by 10–20%, FLAC achieves compression rates of 30–50%.

Lossy codecs can achieve ratios of 80% or more by discarding data from the original stream. 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 codecs lie in its ability to be streamed and in a fast decode time, which is independent of compression level.

As with any lossless scheme, FLAC is also a popular archive format for owners of CDs and other media who wish to preserve their valuable 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 impossible from a lossy archive (e.g., MP3) of the same data. 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, silence between tracks, 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 Hydrogenaudio Wiki features a [comparison] of lossless codecs, including FLAC.

Technical details

FLAC supports only fixed-point samples, not floating-point. This is to eliminate any rounding errors to ensure bit-perfect reproduction. It can handle any PCM bit resolution from 4 to 32 bits per sample, any sampling rate from 1 Hz to 1,048,570 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 checksum 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 Rice parameter between 0-16, and upto 8 channels of audio and a wide range of sampling rates upto 192Khz, in various bits-per-sample width.

FLAC is implemented as libFLAC core encoder & decoder with the main distributable program flac being the reference program utilizing libFLAC API. This codec API is also available in the C++ as the libFLAC++.

The reference implementation of FLAC compiles on many platforms, including most Unix and Unix-like (including Linux, *BSD, Solaris, and Mac OS X), Windows, BeOS, and OS/2 operating systems. There are build systems for autoconf/automake, MSVC, Watcom C, and Project Builder.

API Organization

libFLAC API is organized into streams, seekable streams and files 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.

Encoder

FLAC encoder is created as follows
  1. Create an instance of the encoder using FLAC__file_encoder_new()
  2. Set various parameters of this encoder like SamplingRate, BitsPerSample, NumberofChannels, LPC order, Mid-Side stereo, Rice parameter search distance, min & max residual, BlockSize, output FileName (if no output file, use stream encoder). Function calls for each of these
  3. Initialize the FLAC encoder using FLAC__file_encoder_init()
  4. Encode the raw samples using FLAC__file_encoder_process() or FLAC__file_encoder_process_interleaver() functions for each samples read from input (either ADC or File).
  5. On finishing encoding process call FLAC__file_encoder_finish(), after which you can either destroy the encoder or redo the steps for another file following steps 1-5.

Software support

Hardware support

External links

 

From Wikipedia, the Free Encyclopedia. Original article here. Support Wikipedia by contributing or donating.
All text is available under the terms of the GNU Free Documentation License See Wikipedia Copyrights for details.

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