Mainstream use of AAC encoders

AAC English is all called Advanced Audio Coding, Chinese meaning is the advance audio encoding. As compared with the current popular MP3 audio format, with more audio track channel support, higher sampling rate and sampling accuracy, while the compatibility of multiple languages, higher decoding efficiency, has gradually become the successor of the MP3 format bar. In general, AAC can provide better sound quality when comparing MP3 files by a 30% reduction.

There are currently three types of AAC encoders used in the mainstream, namely:

1. Nero AAC encoder: Released free of charge by Nero, while supporting AAC LC low complexity specifications and AAC HE high efficiency specifications.

2, FAAC encoder: A command line encoder, free free software, support AAC main specifications, AAC LC Low complexity specifications and AAC LTP long-term forecast specifications.

3. Quicktime/itunes built-in AAC encoder: Apple's two software offers AAC encoding, and its coding technology comes from Dolby Laboratories.

In summary, the current use of the Nero AAC encoder, followed by the Quicktime/itunes built-in AAC encoder and FAAC encoder. In contrast, the FAAC encoder in the high-frequency portion of the sound than Nero aac, and in the middle and low-frequency FAAC sound quality is not as good as Nero aac, as for the Quicktime/itunes AAC go the middle route, between the two.

The above mentioned AAC specifications, here to add. AAC is divided into 9 kinds of specifications to meet the needs of different occasions. AAC is based on the MPEG-2 audio coding technology, in 2000, after the advent of MPEG-4 standards, AAC re-integrated its features, added SBR technology and PS technology, in order to distinguish between the traditional MPEG-2 AAC is also known as MPEG-4 AAC.

1, MPEG-2 AAC LC Low Complexity specification (complexity).

2, MPEG-2 AAC main specifications.

3, MPEG-2 AAC SSR Variable sample rate specification (scaleable sample rates).

4, MPEG-4 AAC LC Low Complexity specification (complexity).

5, MPEG-4 AAC main specifications.

6, MPEG-4 AAC SSR Variable sample rate specification (scaleable sample rates).

7, MPEG-4 AAC LTP Long Term Forecast specifications (long predicition).

8, MPEG-4 AAC LD Low Latency specification (slow delay).

9, MPEG-4 AAC HE High Efficiency specification (higher efficiency).

In the above specification, the main specification (main) contains all functions except gain control, with the best sound quality, while the low Complexity specification (LC) is relatively simple, without gain control, but improves the coding efficiency. As for the "SSR" on the "LC" specification is generally the same, but more gain control function. In addition, MPEG-4 aac/ltp/ld/he, are used in low-level meta-rate coding, especially "HE" is a Nero ACC encoder support, is a recently used encoder, but generally speaking, the Main specifications and LC specifications of the sound quality is not very small, so the current use of the most AAC Specifications are mostly LC and he specifications, because the current memory capacity of the mobile phone is not up to a reasonable level.

What is HE-AAC and LC-AAC.

The two are different profiles (classes) that conform to the MPEG4 AAC standard. LC means "low complexity" (lower complexity) and he means "high efficiency" (efficient). HE-AAC also known as AAC Sbr/aac+/aacplus. Note that the HE-AAC focuses on low code stream encoding and is well suited for multichannel files (smaller file sizes). For the He-aac file, you also need a usable he decoder, see Q6. If you want to compare AAC with other popular audio codes in 64kbps

Academically speaking, HE-AAC mixes AAC with SBR technology. SBR represents the spectral Band Replication (frequency band replication). The key to SBR is to provide full-bandwidth coding at low code streams without generating unwanted signals. Traditionally, audio coding in low code flow means reduced bandwidth and reduced sampling rates (see MP3 FAQ #7) or produce unpleasant noise signals. SBR solves the problem by allowing the core encoding to encode low-frequency signals, while the SBR decoder generates high-frequency signals by analyzing low-frequency signals and some guidance signals that remain in the bitstream (usually with very low bitstream, up to Kbps). This is why no SBR decoder is used, so your bandwidth (frequency response/frequency response) can be severely wasted. This is why it is called spectral Band replication, it just increases the bandwidth of the audio, not the rebuild. For more information about SBR technology, see here and there.

Like MPEG-4 video, AAC has different profiles, where low complexity profiles (also known as Main @ Level 2) are most widely used in commercial markets (such as Apple's very famous itunes music library), Other there are like long term prediction profiles (ltp/long predictions), scalable sampling rate profiles (ssr/variable sample rates) and low delay profiles (ld/lower latency).