How FLAC compression actually works
FLAC stands for Free Lossless Audio Codec. The trick that makes it useful is that it compresses audio without throwing anything away. After decoding, the output is the same as the original uncompressed audio, sample for sample. Compare that to MP3 or AAC, which discard frequency content they predict you will not notice.
FLAC achieves smaller files in two stages. First, linear prediction estimates each sample from the recent past samples and stores only the prediction error (usually a small number). Then entropy coding (Rice coding, similar to Huffman in spirit) packs those small numbers into fewer bits. Music with predictable waveforms compresses well, very noisy material compresses less.
FLAC vs ALAC vs Apple Lossless
ALAC (Apple Lossless Audio Codec) is the same idea as FLAC, done independently by Apple. Both achieve similar compression ratios (usually within a few percent of each other on the same source). FLAC has wider tooling support outside the Apple ecosystem; ALAC dominates inside iTunes, Apple Music, Voice Memos Lossless mode. We accept both, the transcript is the same.
Why FLAC did not win consumer adoption
FLAC was published in 2001, free and open-source. Yet most consumer streaming runs on AAC, MP3, or Opus, not FLAC. The short reason: for ordinary listening on phones and laptops, lossy formats sound identical to most people while being a fraction of the size. FLAC wins where the listener can actually tell the difference (high-end home audio) or where preservation matters (libraries, archives), neither of which is the consumer mass market.
FLAC for speech recognition: helpful or unnecessary?
For most speech recordings, FLAC vs MP3 makes no measurable difference to Whisper accuracy. Where lossless starts to help is at the edges: very quiet voices, heavy background noise, or recordings that are already marginal. In those cases the high-frequency tail FLAC preserves can carry the consonant information Whisper uses to disambiguate similar words. Most podcast and interview audio sits well inside the comfort zone where MP3 and FLAC produce identical transcripts.