RF64

Chunk sizes in a WAV file are 32-bit, so a file stops at 4 GB: a little over 4 hours of stereo audio at 48 kHz and 24 bits. RF64, defined by the EBU in Tech 3306, keeps the WAV layout and adds a chunk with 64-bit sizes.

How it differs

The file starts RF64 instead of RIFF, and the RIFF size is set to 0xFFFFFFFF.
A ds64 chunk comes first, straight after WAVE, holding the real sizes.
The data chunk’s size, and any other size too large for 32 bits, is set to 0xFFFFFFFF, meaning “look in ds64”.

The ds64 chunk

Offset	Size	Type	Name	Description
0	4	char[4]	`ckID`	`ds64`
4	4	u32	`ckSize`	28 + 12 × `tableLength`
8	8	u64	`riffSize`	the real RIFF size: the file size minus 8
16	8	u64	`dataSize`	the real size of the data chunk
24	8	u64	`sampleCount`	the real value for the `fact` chunk
32	4	u32	`tableLength`	the number of entries that follow
36	12 each	record	`table`	for any other large chunk: its ID (4 bytes) and its 64-bit size

00000000  52 46 36 34 FF FF FF FF 57 41 56 45 64 73 36 34  RF64....WAVEds64
00000010  1C 00 00 00 48 F2 05 2A 01 00 00 00 00 F2 05 2A  ....H..*.......*
00000020  01 00 00 00 80 7C 81 4A 00 00 00 00 00 00 00 00  .....|.J........

The start of an RF64 file holding 5,000,000,000 bytes of 16-bit stereo audio.

Here riffSize is 5,000,000,072, dataSize is 5,000,000,000, sampleCount is 1,250,000,000 frames and the table is empty. The format chunk follows at offset 48, then a data chunk whose size reads FF FF FF FF.

The JUNK chunk trick

A recorder does not know in advance whether a file will pass 4 GB. So it writes an ordinary WAV file with a 28-byte JUNK chunk straight after the RIFF header, the same size as an empty ds64. If the recording stays small, the file is a normal WAV. If it grows past 4 GB, the recorder changes RIFF to RF64 and JUNK to ds64, and fills in the 64-bit sizes, without moving any audio.

BW64 and Wave64

BW64, from ITU-R BS.2088, has the same layout with the ID BW64, and adds chunks for describing object-based and immersive audio. Sony’s Wave64 (.w64) is a separate format that solves the same problem with 64-bit sizes and GUIDs as chunk IDs.