AAC (ADTS) file signature
Magic number at the start of the file
A raw AAC stream in an ADTS wrapper starts with a 12-bit sync word, so the first bytes are FF followed by F1 (MPEG-4) or F9 (MPEG-2). AAC inside an MP4 container instead shows the ftyp signature.
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 FF F1, it is a AAC (ADTS) file.
Signature variants
AAC (ADTS) appears with more than one byte pattern. These all identify the same family:
| Hex signature | Offset | Note |
|---|---|---|
| FF F1 | 0 | ADTS, MPEG-4 |
| FF F9 | 0 | ADTS, MPEG-2 |
Formats that use the FF F1 signature
These file types in our database carry this signature:
How to check a file's signature
You can read the first bytes of any file yourself. The magic bytes are shown in hex, the same way this page lists them.
Linux & macOS
- xxd -l 16 example.aac
- hexdump -C -n 16 example.aac
- file example.aac
Windows (PowerShell)
- Format-Hex -Path example.aac -Count 16
Python
- open("example.aac","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 AAC (ADTS) file signature?
AAC (ADTS) files start with the hex bytes FF F1. 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.aac on Linux or macOS, or Format-Hex -Path example.aac -Count 16 in Windows PowerShell, and read the first bytes.
Can a file fake the AAC (ADTS) signature?
Renaming a file does not change its bytes, so the extension can lie but the signature usually cannot. A genuine AAC (ADTS) file has these exact bytes; a file with the wrong bytes is not really AAC (ADTS), whatever its name says.