Advanced Audio Coding (AAC): The Codec Explained
Advanced Audio Coding (AAC): The Codec Explained
Advanced Audio Coding is the audio codec you use constantly without ever choosing it. Every MP4 you have downloaded, every track on Apple Music, the audio on nearly every YouTube video — that is AAC. It quietly replaced MP3 as the default lossy audio format and, unlike most format transitions, it happened without anybody having to care.
Which is fine until you have to make a decision about it. Then the profile names start mattering, the bitrate advice online contradicts itself, and it turns out "AAC" is not one thing.
What AAC Actually Is
AAC stands for Advanced Audio Coding. It was standardised twice: first in MPEG-2 Part 7 in 1997, then extended in MPEG-4 Part 3 in 1999, which is the version everything uses today. The design brief was explicit — build a successor to MP3 that sounds better at the same bitrate.
It is a lossy, perceptual codec. That means it does not try to reproduce the waveform faithfully. It models what human hearing can and cannot detect, then throws away everything in the second category. A loud sound masks quieter sounds near it in frequency and in time; AAC finds those masked components and does not spend bits on them.
The mechanism is a modified discrete cosine transform (MDCT) that converts short slices of audio into frequency coefficients, followed by quantisation guided by a psychoacoustic model. If lossy versus lossless compression is unfamiliar territory, that is the background this rests on.
The specification is generous with limits: sample rates from 8 kHz to 96 kHz, and up to 48 audio channels. Almost nobody uses the ceiling, but it is why AAC covers everything from a phone call to a cinema mix.
Why It Beats MP3
The advantage is real but frequently overstated. Three things drive it:
A longer transform window. MP3 works with 576 frequency lines per block; AAC uses 1024. Finer frequency resolution means the encoder can target its bit-spending more precisely instead of blurring across a wider band.
Better stereo handling. AAC's joint stereo — mid/side and intensity coding — is more flexible than MP3's, so correlated information between channels gets stored once rather than twice.
No legacy baggage. MP3 inherited a hybrid filterbank from MPEG-1 Layer 2 that constrains what it can do. AAC was designed clean.
The practical result: a 128 kbps AAC file lands roughly where a 192 kbps MP3 does. The gap widens as bitrate falls and narrows as it rises. At 256 kbps both are close enough to the source that arguing about it is a hobby, not engineering.
The Profiles Are the Confusing Part
This is where most explanations go wrong, including some that confidently state AAC-LD and AAC-LC are the same thing. They are not. AAC is a family of profiles sharing a core, each tuned for a different job.
AAC-LC (Low Complexity) — the one you almost always mean. Plain AAC, no extra tools, cheap to decode. This is Apple Music, iTunes purchases, the audio in your MP4s. Best from roughly 96 kbps upward.
HE-AAC (High Efficiency), also called aacPlus, adds Spectral Band Replication. Rather than encoding high frequencies directly — expensive, and where the ear is least precise — the encoder stores a compact description of them and the decoder reconstructs them from the low band. This transforms low-bitrate performance. HE-AAC at 64 kbps is genuinely listenable; AAC-LC at 64 kbps is not.
HE-AAC v2 stacks Parametric Stereo on top, encoding one channel plus a small set of parameters describing the stereo image. It targets 32 kbps and below — digital radio, low-bandwidth streaming.
AAC-LD and AAC-ELD (Low Delay / Enhanced Low Delay) trade compression efficiency for latency. Standard AAC's encode-decode round trip runs over 100 ms, which is unusable for conversation. ELD cuts that to around 15 ms. This is the codec behind FaceTime audio and a lot of professional intercom hardware.
xHE-AAC (Extended HE-AAC) is the newest, built on USAC. It scales smoothly from about 12 kbps up to transparent quality in a single codec, and carries loudness metadata. Streaming services have been adopting it gradually.
The short version: music gets AAC-LC at 128 kbps or higher. Speech and low-bandwidth streams get HE-AAC. Real-time conversation gets ELD. Everything else is a special case.
AAC Versus the Alternatives
| Codec | Best use | Royalty-free | Notes |
|---|---|---|---|
| AAC-LC | Music, MP4 audio | No | Universal hardware support |
| MP3 | Legacy compatibility | Yes (expired) | Worse per bit, plays anywhere |
| Opus | Web, voice, low bitrate | Yes | Better below 96 kbps, thinner hardware support |
| FLAC / ALAC | Archival, audiophile | Yes | Lossless, roughly 5-8x the size |
| AC-3 / E-AC-3 | Broadcast, surround | No | Entrenched in TV and Blu-ray |
The honest comparison is against Opus, not MP3. Opus is royalty-free, generally outperforms AAC below 96 kbps, and has lower latency. AAC's advantage is that it is decoded in hardware by every phone, TV, car stereo and Bluetooth chipset shipped in the last fifteen years. Compatibility is winning that argument and will for a while.
AAC's patent position matters more than people assume. It is licensed through Via LA, and that non-free status is precisely why the Alliance for Open Media built Opus for audio and AV1 for video.
Containers, Extensions, and the .m4a Confusion
A recurring source of trouble: AAC is a codec, not a file format. The same AAC bitstream shows up as:
.m4a— an MP4 file with only an audio track. The most common wrapper for standalone AAC..mp4/.m4v— AAC as the audio track alongside video. This is the default pairing in MP4 H.264 files..aac— a raw ADTS stream with no container. Works, but carries no metadata and no seek index..3gp— mobile container, usually HE-AAC.
Renaming .m4a to .mp3 does not convert anything; it produces a file that lies about its contents and confuses whatever opens it. If you need an actual MP3, convert M4A to MP3 properly.
One more wrinkle: .m4a can also hold ALAC, Apple's lossless codec. Same extension, completely different compression, roughly five times the size. The extension tells you the box, never the contents.
Choosing a Bitrate
The advice that survives contact with reality:
256 kbps AAC-LC — the practical ceiling for stereo music. Apple Music's standard. Beyond this you are paying storage for differences you will not hear.
128 kbps AAC-LC — the sensible default. Fine for casual listening, podcasts, background music, and anything played through phone speakers or ordinary earbuds.
96 kbps AAC-LC — noticeably compromised on dense music, acceptable on speech.
64 kbps HE-AAC — the low-bandwidth workhorse. Transparent enough for talk content, and dramatically better than AAC-LC would be at the same rate.
Above 320 kbps — stop. You are asking a lossy codec to behave like a lossless one. If you need that fidelity, use FLAC or ALAC and accept the file size.
Two things that matter more than the number:
Never re-encode lossy to lossy. Converting a 128 kbps MP3 to 128 kbps AAC applies a second round of destruction to audio already missing information. The result is worse than either original. Encode from the highest-quality source you still have. The bitrate fundamentals apply here in full.
Prefer VBR where the encoder offers it. Constant bitrate spends the same budget on silence as on a dense chorus. Variable bitrate targets a quality level and lets file size follow, which is almost always the better trade.
Where AAC Shows Up
- Apple Music, iTunes, Apple devices — AAC-LC at 256 kbps
- YouTube — AAC in MP4 streams, Opus in WebM streams, chosen by client
- Bluetooth — the AAC codec is one of the standard A2DP options, and the default between iPhones and AirPods
- Broadcast — DVB, ATSC 3.0, and most digital radio outside the US
- Video files — the default audio track in essentially every MP4 produced by a camera, phone, or editor
That last one is why AAC matters even if you never think about audio. When you compress a video, the audio track is AAC, and leaving it at a needlessly high bitrate wastes space in a file you are explicitly trying to shrink.
Compressing AAC Audio on a Mac
Most oversized audio files are not oversized because AAC is inefficient. They are oversized because something encoded them at a bitrate nobody chose deliberately — a 320 kbps export default, or a lossless master that never got converted.
Compresto handles audio compression and format conversion in batch on macOS. Drop in a folder of files, pick a target bitrate or size, and it re-encodes everything at once, including the audio tracks inside video files.
Related: how to compress audio files and what is audio bitrate.
Frequently Asked Questions
What is Advanced Audio Coding?
AAC is a lossy audio codec standardised in MPEG-2 Part 7 (1997) and extended in MPEG-4 Part 3 (1999), designed as the successor to MP3. It delivers better quality at the same bitrate, especially below 128 kbps, and is the default audio track in MP4 files, Apple Music, and most digital broadcasting.
Is AAC better than MP3?
At equal bitrate, yes. A larger transform window, better joint stereo, and a cleaner design mean 128 kbps AAC sounds roughly like 192 kbps MP3. The advantage grows as bitrate falls and shrinks above 256 kbps, where both are close to transparent.
What is the difference between AAC-LC and HE-AAC?
AAC-LC is plain AAC, used for music at 128 kbps and up. HE-AAC adds Spectral Band Replication, reconstructing high frequencies from a compact description instead of storing them, which makes it far stronger at 64 kbps and below. HE-AAC v2 adds Parametric Stereo for even lower rates. AAC-LD and AAC-ELD are separate low-latency profiles, not synonyms for AAC-LC.
Is AAC lossy or lossless?
Lossy. It permanently discards information its psychoacoustic model judges inaudible. For lossless audio, use FLAC or ALAC — expect roughly five to eight times the file size.
What bitrate should I use for AAC?
256 kbps AAC-LC for music you care about, 128 kbps for everything casual, 64 kbps HE-AAC for speech and low-bandwidth streaming. Above 320 kbps you should be using a lossless format instead.
Is AAC royalty-free?
No. It is patent-licensed through Via LA. Licensing is normally handled by the OS or hardware vendor, so end users never encounter it directly, but it is the reason royalty-free Opus exists.
Why is my .m4a file actually AAC?
.m4a is a container — an MP4 holding only audio — and the codec inside is almost always AAC. The same stream can live in .mp4, .3gp, or a raw .aac file. Confusingly, .m4a can also contain lossless ALAC, so the extension never tells you the codec on its own.