.AAC File Format

Introduction

AAC is the lossy audio codec of the MPEG-2 and MPEG-4 standards, designed as the successor to MP3. At the same bit rate it usually sounds better. It is the standard audio of the iTunes Store and Apple Music, YouTube, digital television and most phones, and comes in several profiles, such as AAC-LC and the lower-rate HE-AAC.

AAC audio reaches users in two kinds of file. A .aac file is usually a raw stream of ADTS frames. An .m4a file holds AAC inside an MPEG-4 container with tags and cover art; the same extension is used for Apple Lossless.

Structure

An ADTS stream is a run of frames, each with its own header so a player can start anywhere. The header is 7 bytes, or 9 with a CRC. It starts with a 12-bit sync word of ones (FFF), then gives the MPEG version, whether a CRC is present, the profile, a sampling-rate index, the channel configuration and the 13-bit frame length including the header. The compressed audio follows. Each AAC frame normally decodes to 1024 samples per channel.

In an .m4a file the frames carry no headers. They sit in an mdat box, and the moov box holds the codec settings (in an mp4a sample entry), the sample tables and the tags.

Also used for

• Apple Lossless Audio Codec – Apple’s lossless codec, stored in an MPEG-4 .m4a file like AAC.

Links