What is the AAC audio format? What is the difference between AAC-LC and AAC-HE

Advanced Audio Coding (Advanced Audio Coding) is a lossy audio compression format defined by the MPEG-4 standard, developed by Fraunhofer, with Dolby, Sony and AT&T as the main contributors.

In the new multimedia MPEG-4 standard that uses MP4 as a container format for various contents, it is the next successor to MPEG Layer III/MP3. AAC can include 48 full bandwidth (up to 96khz) audio channels in a single audio track, plus 15 low frequency enhancement (LFE, limited to 120Hz) channels, up to 15 data streams and more.

The two are different configurations that comply with the MPEG4 AAC standard. LC means "low complexity" (low complexity) HE means "high efficiency" (high efficiency) HE-AAC is also called AAC SBR/AAC+/aacplus, etc. Note that HE-AAC focuses on low bitstream encoding and is very suitable for multi-channel files (smaller file sizes).

Academically speaking, HE-AAC mixes AAC and SBR technology. SBR stands for Spectral Band Replication. The key to SBR is to provide full-bandwidth encoding at low bit rates without generating redundant signals.

Traditionally, it is believed that audio coding at low bit rates means reducing bandwidth and sampling rate (see MP3 FAQ #7) or generating unpleasant noise signals. SBR solves the problem by letting the core code to encode the low-frequency signal, while the SBR decoder analyzes the low-frequency signal to generate a high-frequency signal and some guide signals retained in the bitstream (usually the bitstream is extremely low, ~2kbps). This is the reason for using a non-SBR decoder, so your bandwidth (the reason for frequency Replication, it only increases the audio bandwidth, not reconstruction. Similar to the MPEG-4 video format, AAC also has different configurations, among which Low Complexity (LC AAC) configuration (MAIN Level 2) is most widely used in the commercial market (such as Apple’s very famous iTunes music library). Others include Long Term Prediction configuration (LTP) (long-term prediction), Scalable Sampling Rate (SSR) (variable sampling) Rate) and Low Delay (LD) (low delay).

Generally speaking, AAC-HE format can be used for low bitrate source conversion; but if you want to make something compatible with more devices for playback, it is strongly recommended that you use AAC-LC format.

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