Editing FLAC (section)

=== Encoding and decoding ===
[[File:FLAC_1.5.0_screenshot.webp|thumb|250x250px|Screenshot of <code>flac</code> command-line software]]
The FLAC encoding algorithm consists of multiple stages. In the first stage, the input audio is split into blocks. If the audio [[stereo|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]].<ref name="format-about"/>

The decoding process is the reverse of encoding. The compressed residual is first decoded.<ref>{{cite web |title=stream_decoder.c |url=https://gitlab.xiph.org/xiph/flac/-/blob/a2fe43f64e7ce8057fb274c64996568d69b301b8/src/libFLAC/stream_decoder.c |website=libFLAC |access-date=19 February 2022 |ref=libflac_stream_decoder}}</ref><ref>[[#libflac_stream_decoder|libFLAC/stream_decoder.c]], line 2734.</ref> 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.<ref>{{cite web |title=lpc.c |url=https://gitlab.xiph.org/xiph/flac/-/blob/a2fe43f64e7ce8057fb274c64996568d69b301b8/src/libFLAC/lpc.c |website=libFLAC |access-date=19 February 2022 |ref=libflac_lpc}}</ref><ref>[[#libflac_lpc|libFLAC/lpc.c]], lines 813–820.</ref> As FLAC [[lossless compression|compresses losslessly]], the decoded waveform is identical to the waveform before encoding.

For two-channel stereo, the encoder may choose to [[joint stereo|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.<ref>{{Cite web |title=FLAC Format Specification |url=https://xiph.org/flac/format.html |access-date=19 February 2022 |website=FLAC}}</ref>

Even though the [[reference implementation|reference encoder]] uses a single block size for the whole stream,<ref name="format-about"/> FLAC allows the block size in samples to vary per block.