Advanced Audio Coding

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Advanced Audio Coding (AAC) is one of the Moving Picture Experts Group ( MPEG) developed, lossy audio data compression method, which was specified as the further development of MPEG -2 multi-channel MPEG- 2 standard.

  • 2.2.1 profiles

History

AAC provides a completely new development dar. It is based not like MPEG Layer 3 ( MP3) on the filter bank design of MPEG Layer 2, but is using it as Dolby Digital and Ogg Vorbis alone the MDCT as a basis transformation. With lower complexity than MP3 better qualities are achieved with the same data rate. Multi-channel encoding is supported directly. Moreover, various tools are supported. These are methods to encode critical signals better. They are allowed in profiles with higher complexity.

Standardization

MPEG- 2 (1997)

AAC has been standardized in Part 7 of the MPEG- 2 standards first in 1997. For the new format three profiles have been specified. A profile is a standard within the format, which specifies the used encoding and decoding process exactly.

Profiles

  • Low Complexity ( LC): For medium to high bit rates. It is ( and hardware ) as well as the online music stores of Apple and RealNetworks used in most AAC implementations and is still the most widely used and supported profile. If in the current language of AAC is mentioned, so that format is usually meant. As a container format MPEG -2, however, was largely due to the newer MPEG -4 ( file extension. M4a or. Mp4) replaced.
  • Main
  • Scalable Sample Rate (SSR ): For streaming. It enables the uninterrupted playback of streams by reducing the bit rate, if the available bandwidth decreases suddenly, but hitherto publicly available in any implementation.

MPEG-4 ( 1999)

In 1999, AAC was ( also known as MPEG -4 audio ) included in the new standard MPEG -4 Part 3 and there again in Subpart 4 (General Audio Coding). The new standard categorizes its various audio formats in so-called object types ( object types ). The original version of AAC, was combined with the new technology Perceptual Noise Substitution, in profile Main object type 1 and in profile Low Complexity ( LC) was added as an object type 2. As more types of objects some optional extensions to the original format were defined:

  • Low Delay (LD ): For small delay times ( 20 ms) at medium to high bit rates. Application in the communication field, for example in video conferencing systems.

Profiles

The MPEG -4 Part 3 standard defines so called profiles, which represent a combination of several object types. The following is a selection of the most important profiles in which AAC is included.

  • HE- AAC (High- Efficiency AAC ), also known as AAC or aacPlus: Combines the object types AAC LC and SBR ( Spectral Band Replication ). HE- AAC is optimized for use with low bit rates (in the range 32-80 kbit / s) and cuts in this area in listening tests better than normal from AAC.
  • HE- AAC v2, also known as eAAC , AAC v2 or aacPlus v2: How HE -AAC, but also with the object type Parametric Stereo (PS). This compression method is again lower bit rates ( 16-40 kbit / s) and is optimized in this area still achieve an acceptable quality.

HE- AAC and HE- AAC v2 represent extensions of the AAC, but not replace it. They are exclusively optimized for low bitrates and come about during a live broadcast of digital radio and television used. At higher bit rates ( from 96 kbit / s) cut in comparison to AAC from poor and should not be used.

Importance and dissemination

The format for lossy audio compression, which was taken by the ISO as standard, in competition with other formats such as MP3, WMA and Vorbis occurs. AAC reached as early as 64 kbit / s acceptable, albeit limited stereo quality. 96 kbit / s correspond to good FM quality and 128 kbit / s have a high transparency. Bit rates from 192 kbit / s and 224 kbit / s are comparable with the lossless CD format. AAC finds its use everywhere, where normally also MP3 is used. Therefore, more and more MP3 players, music websites and online internet radio stations rely on these audio compression. Also support the video container formats 3GP, MP4, MPEG -2 and other formats such as AAC.

Licenses and Patents

Manufacturer of an implementation of AAC must purchase a license. For this reason, it is free software that implements the codec, often only in the source code distributed to avoid patent infringements.

Data compression#Audio Technical standard Filename extension Transparency (data compression)
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