Compression without discarded samples
FLAC and ALAC look for repetition and structure in PCM data, then encode that information more compactly. Decoding reverses the process. If two valid lossless files were made from the same PCM source, their decoded sample data can be equivalent even when the file sizes and metadata differ.
WAV and AIFF often store uncompressed PCM. They can occupy more space without containing more musical information than a FLAC or ALAC made from the same source.
How lossy audio differs
MP3, AAC, Vorbis, and Opus use perceptual encoding to remove or simplify information. They can sound very good at suitable settings, but decoding cannot recreate sample data that the encode discarded. Converting an MP3 to FLAC makes a larger lossless container around the already altered signal; it does not restore the original.
What lossless does not tell you
A lossless file can come from an excellent master, a poor master, an upsample, or a previous lossy source. Its sample rate and bit depth are separate properties. A high-resolution label does not establish that the recording contains useful information above a lower bandwidth.
Nyquist's spectrum and bandwidth analysis can surface evidence such as a steep high-frequency cutoff or little content above a lower sampling limit. Those observations may suggest a file's history, but they are not an infallible provenance test. Some recordings, instruments, microphones, and production choices naturally have limited bandwidth.
Lossless playback in Nyquist
Nyquist supports FLAC, ALAC, and lossless PCM containers for local
playback. It displays the detected codec rather than assuming that
an .m4a or Ogg extension guarantees a particular
payload. For UPnP/DLNA playback it sends the original file without
transcoding, leaving decoding and any later processing to the
receiver.
Continue with FLAC vs. ALAC, the complete format reference, or sample rate and bit depth.