Dolby Digital (AC-3) file signature
Magic number at the start of the file · .w
A Dolby Digital (AC-3) audio frame begins with the two-byte sync word 0B 77, repeated at the start of every frame in the stream.
What this magic number means
A magic number is a short, fixed run of bytes at a known position that tells a program what a file really is, no matter what the filename says. If a file starts with the bytes 0B 77 (the text .w), it is a Dolby Digital (AC-3) file.
What each byte of 0B 77 means
Here is the signature byte by byte, the way a hex editor shows it: the position in the file, the value in hex and in decimal, and the character that value stands for in ASCII. Bytes with no printable character show a dot.
| Byte offset | Hex | Decimal | ASCII |
|---|---|---|---|
| 0 | 0B | 11 | . |
| 1 | 77 | 119 | w |
Formats that use the 0B 77 signature
These file types in our database carry this signature:
How to check a file's signature
You can read the bytes of any file yourself. They are shown in hex, the same way this page lists them.
Linux & macOS
- xxd -l 16 example.ac3
- hexdump -C -n 16 example.ac3
- file example.ac3
Windows (PowerShell)
- Format-Hex -Path example.ac3 -Count 16
Python
- open("example.ac3","rb").read(2).hex()
In your browser
- Drop the file into the WhatFileType identifier, which reads the signature without uploading it.
Frequently asked questions
What is the Dolby Digital (AC-3) file signature?
Dolby Digital (AC-3) files start with the hex bytes 0B 77 (.w in ASCII). This magic number identifies the format regardless of the file's name or extension.
How do I check a file's magic number?
Open the file in a hex editor, or run a command such as xxd -l 16 example.ac3 on Linux or macOS, or Format-Hex -Path example.ac3 -Count 16 in Windows PowerShell, and read the first bytes.
Can a file fake the Dolby Digital (AC-3) signature?
Renaming a file does not change its bytes, so the extension can lie but the signature usually cannot. A genuine Dolby Digital (AC-3) file has these exact bytes; a file with the wrong bytes is not really Dolby Digital (AC-3), whatever its name says.
Related signatures
Sources
Reuse this signature
This entry is part of the WhatFileType file signature reference, published under a CC BY 4.0 licence with the full provenance for every entry, a copy-paste embed and a JSON export. Credit WhatFileType and the data is yours to republish.