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Audio Compression

• Audio compression has become well entrenched in consumer and professional digital audio products such as the compact disc (CD), digital versatile disc (DVD), digital audio broadcasting (DAB) and motion picture experts group (MPEG) audio layer 3 (MP3) distribution on the Internet.

• Audio and speech compression schemes can be conveniently partitioned into applications reflecting some measure of acceptable quality, ranging from telephone speech to wideband audio.

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MPEG Layers I, II and III

• The International Standards Organisation (ISO) and Motion Picture Experts Group (MPEG) audio coding standard describes audio compression for synchronized audio to accompany the compressed video known as MPEG.

• It combines features of MUSICAM (Masking pattern adapted Universal Subband Integrated Coding and Multiplexing) and ASPEC (Adaptive Spectral Perceptual Entropy Coding).

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• It consists of three layers (codes) of increasing complexity and improving subjective performance, and it operates with input sampling rates of for example 32, 44.1 and 48 kHz, and it outputs bit rates per monophonic channel between 32 and 192 kbit/sec, or per stereophonic channel between 64 and 384 kbit/sec.

• The standard supports single channel mode, stereo mode, dual channel mode (for bilingual audio programs) and an optional joint stereo mode.

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• The encoder operates in conjunction with a real-time model of the human spectral perception threshold.

• This threshold is a frequency-dependent boundary or threshold that marks sound pressure levels (SPL) below which the human ear cannot detect sounds.

• Signal spectral components below the threshold level that cannot be heard are declared irrelevant, and they are not encoded in the compression process.

• The encoder operation is quite complex!

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• The Advanced Audio Coding in MPEG-4 Part 3 was enhanced relative to the previous standard MPEG-2 Part 7, in order to provide better sound quality for a given encoding bitrate.

• AAC's best known use is as the default audio format of Apple's iPhone, iPod, iTunes.

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• AAC's multiple codecs are:

- Low Complexity Advanced Audio Coding (LC-AAC)

- High-Efficiency Advanced Audio Coding (HE-AAC)

- Scalable Sample Rate Advanced Audio Coding (AAC-SSR)

- Bit Sliced Arithmetic Coding (BSAC)

- Long Term Predictor (LTP)

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• AAC has been standardized by ISO and IEC, as part of the MPEG-2 and MPEG-4 specifications. The MPEG-2 standard contains several audio coding methods, including the MP3 coding scheme. AAC is able to include 48 full-bandwidth (up to 96 kHz) audio channels in one stream plus 16 low frequency effects (LFE, limited to 120 Hz) channels, up to 16 "coupling" or dialog channels, and up to 16 data streams.

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• The quality for stereo is satisfactory to modest requirements at 96 kbit/s in joint stereo mode, however hi-fi transparency demands data rates of at least 128kbit/s. The MPEG-2 audio tests showed that AAC meets the requirements referred to as "transparent" for the ITU at 128 kbit/s for stereo, and 320kbit/s for 5.1 audio.