Advanced Audio Coding (AAC) is an audio coding standard. Like MP3, it is a "lossy" standard, meaning that some audio data (that is perceived by the algorithm to be irrelevant) is removed in the compression process.
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Benefits of AAC Encoding
AAC utilises more advanced compression algorithms than MP3. As such, the quality of AAC-compressed audio for low-bit-rate streams is significantly better than the MP3 counterpart. We recommend using AAC streams for mobile applications as it is much easier to stream over patchy connections and the audio quality fares well under these circumstances. The relative quality of the audio taper off as you increase the bit rate therefore we still recommend using MP3 for streams 94Kbps and above as at this stage the audio quality difference is essentially negligible.
AAC comes in a number of different codecs the primary categories are AAC-LC (Low Complexity) and AAC-HE (High Efficiency, commonly referred to as AAC+). LC, as the name suggests, is the more rudimentary version of AAC compression. As such, the perceived audio quality at a low bit rate is noticeably poorer than its HE counterpart.
Should you wish to stream a higher bit rate AAC, we would always recommend using the LC codec as the audio superiority of HE is less noticeable at high bit rates and not worth the bandwidth trade-off.
HE/AAC+ Usage Profiles
HE/AAC+ formats predominantly encapsulate two popular usage profiles: AAC-HE v1 and AAC-HE v2.
HEv1 utilised the baseline AAC LC compression algorithms alongside an enhancement process called SBR (Spectral Band Replication), commonly used to enhance speech-based audio. HEv2 takes this one step further by also adding some PS (Parametric Stereo) compression to the mix which virtually recreates the stereo field of an audio signal to recreate a stereo file at a fraction of the size. These technologies in conjunction make AAC-HE codecs a high-quality option for low-bitrate streams.
With that said, for those wishing to integrate programmatic monetisation into their stream using AdsWizz inaudible tone markers, AAC-HE may not be a good option. Part of the compression algorithms remove frequencies below 20hz, further more, the PS processing on the v2 usage profile will render the stereo inversion AdsWizz apply to the tones (to make them appear silent on mono sound sources) as silence and AIS will not be able to detect the tones to insert the file. As standard, we suggest using AAC-LC for low-bitrate monetised streams.
In terms of behaviour, we have found that the different usage profiles play differently in Chrome browser when streamed directly. Some will progressively download, others play in an html5 player in-browser. As such we recommend using a dedicated network player like VLC to monitor streams. All AAC stream types will be compatible with major web players and referrers (Tune-In, RadioPlayer, JPlayer, etc.).