ACIDCAT . FILE FORMAT REFERENCE

FLAC Anatomy

Free Lossless Audionative stream
rev 2026.06

magic "fLaC"

endian big *

container metadata blocks

lossless yes

A FLAC is the fLaC marker, a run of metadata blocks, then audio frames. Each block opens with a 4-byte header: a last-block flag, a 7-bit type, and a 24-bit length. STREAMINFO is first and required. Hover a field to light its bits or bytes, click a + for its table. Color marks kind (see the key). * the one trap: FLAC is big-endian everywhere except the VORBIS_COMMENT block, whose lengths are little-endian.

block regions

Walked in order until the block whose last-block flag is set; the audio frames begin right after.

▸fLaC0x00 . 4 bytes

magic"fLaC" (0x66 4C 61 43)

▸STREAMINFOtype 0 . first . drawn below

Sample rate, channels, bit depth, total samples, and an MD5 of the unencoded audio. Must be the first block. See the byte map below.

▸VORBIS_COMMENTtype 4 . the metadata

lengthsLITTLE-endian here, unlike the rest of FLAC

vendore.g. "reference libFLAC 1.3.2"

commentsUTF-8 "FIELD=value": TITLE, ARTIST, BPM, KEY

the block, and the endianness flip (24 bytes)

Everything in FLAC is big-endian except the inside of this block.

one comment (16 bytes)

A length and a string. There is no key table and no type -- the key is text before an equals sign.

▸PICTUREtype 6 . cover art

fieldspicture type, MIME, description, w/h/depth, data

noteAPIC-like, but adds w/h/depth/colors and length-prefixes its strings

▸CUESHEETtype 5 . tracks + indices

prefix396 bytes: 128-byte catalog number, u64 lead-in, is-CD flag (top bit of byte 136), reserved, u8 num_tracks

tracks36 bytes each: u64 offset, u8 number, 12-byte ISRC, type + pre-emphasis bits, u8 index count

indices12 bytes each after their track: u64 offset, u8 number, 3 reserved

lead-outlast track, number 170 (CD-DA) or 255

▸SEEKTABLE / PADDING / APPLICATIONtypes 3 / 1 / 2

SEEKTABLE18-byte seek points (0xFFFF.. = placeholder)

PADDINGzeroed slack for later in-place edits

APPLICATION4-byte app id + opaque data

▸audio framesafter the last block

Opaque for metadata -- STREAMINFO already gives duration = total_samples / sample_rate -- but each frame opens with its own header:

frame sync14 bits, 0xFFF8 (byte0 == 0xFF and byte1 & 0xFE == 0xF8); the collision this would cause is why block type 127 is forbidden

blocking strategy1 bit after the sync: 0 = fixed block size (the coded number is a frame number), 1 = variable (it is a sample number)

coded numberthat frame-or-sample number in a UTF-8-like variable length, the 8-bit scheme stretched to carry up to 36 bits

rate / depth4-bit codes, unlike STREAMINFO's exact fields; escape values read the rate from STREAMINFO or inline as 8 or 16 bits

metadata block header

4 bytes before every block. Example: a final VORBIS_COMMENT block, 304 bytes long.

STREAMINFO body

34 bytes. Example: 44100 Hz, 2 channels, 16-bit, 88200 samples.