Transcript Audio Compression

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.
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