AAC Plus Format: What Exactly is it Used For?

Does the plus version of AAC make it better under all circumstances?

AAC Plus Format
AAC Plus (V1 & V2) Heirachy. Source: Wiki Commons
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What is AAC Plus?

You might think that Apple is responsible for developing the AAC Plus format (sometimes called AAC+). But, it is in fact a trade name used by Coding Technologies for their HE-AAC V1 compression format. If you are wondering what the HE part of the name stands for, then it is short for High Efficiency. In fact, AAC Plus is often referred to as HE-AAC rather than using the plus name or + symbol.

The audio format file extensions associated with AAC Plus are:

  • .m4a
  • .MP4
  • .,M4b
  • .m4V

But, what is the difference between this and the standard AAC format?

The main purpose of HE-AAC (High Efficiency Advanced Audio Encoding) is when audio needs to be encoded efficiently at low bit rates. One of the best examples of this is when songs need to be streamed over the Internet using the least amount of bandwidth possible. Compared to standard AAC it is much better at preserving the perceived quality at bit rates less than 128 Kbps -- more typically around 48 Kbps or less.

You might assume that it is also better at encoding audio at high bit rates too. After all, doesn't the Plus after AAC (or HE before it) give you the sense that it is better all-round?

Sadly this isn't the case. No format can be good at everything and this is where AAc Plus has a disadvantage when compared to standard AAC (or even MP3).

When you want to preserve the quality of an audio recording using a lossy codec, then it is still better to use standard AAC when bitrate and file sizes aren't your main issue.

Is it Compatible With iOS And Android Devices?

Yes, most (if not all) portable devices that are based on iOS and Android will be able to decode audio in the AAC Plus format.

For iOS devices higher than version 4, AAC Plus files are decoded with maximum quality. If you have an Apple device that is older than this then you'll still be able to playback these files, but there will be a reduction in fidelity. This is because the SBR part, which contains high frequency detail (treble), isn't used when decoding. Files will be treated as if they were encoded with AAC-LC (Low Complexity AAC).

How About Software Media Players?

Software media programs like iTunes (version 9 and higher) and Winamp (pro version) support the encoding and decoding of AAC Plus. While other software like VLC Media Player and Foobar2000 can only playback HE-AAC encoded audio files.

How Does the Format Efficiently Encode Audio?

The AAC Plus algorithm (used by streaming music services like Pandora Radio), employs a technology called Spectral Band Replication (SBR) to enhance audio reproduction while maximizing compression efficiency. This system actually replicates missing higher frequencies by transposing lower frequencies -- these are stored at 1.5 Kbps. Incidentally SBR is also used in other formats like MP3Pro.

Streaming Audio

As well as software media players supporting AAC Plus, online music services like Pandora Radio mentioned earlier (and other Internet radio services) can use this format for streaming content.

It is an ideal audio compression scheme to use due to its low bandwidth requirements -- for speech broadcasts in particular where even going as low as 32 Kbps is normally acceptable quality.