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Introduction

Motion Picture Experts Group (MPEG)

I

nternational Standards Organization (I

SO) First High Fidelity Audio standard

Part of a multiple standard for

Video compression Audio compression

Audio, Video and Data synchronization at an

aggregate rate of1.5 Mbit/sec

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MPEG Audio Physically Lossy compression algorithm

Perceptually lossless, transparent algorithm Exploits perceptual properties of human ear

Psychoacoustic modeling

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Medium quality audio

compression Code Excited Linear Prediction

for speech coding

-law

Adaptive Differential Pulse Code

Modulation

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The MPEG Audio standard

Ensures inter-operability

Defines coded bitstream syntax

Defines decoding process

Guarantees decoders accuracy

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MPEG audio acceptance Wide acceptance

Large number of MPEG audio codecs

produced

Stand-alone, Mobile phone add-ons etc

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MPEG audio features No assumptions about the nature of the

audio source

Exploitation of human auditory system

perceptual limitations

Removal of perceptually irrelevant parts of

audio signal

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MPEG audio sampling rates 32 kHz

44.1 kHz

48 kHz

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MPEG audio supports One or two audio channels in

a monophonic mode for a single audio channel

a dual monophonic mode for two independent

audio channels

a stereo mode with sharing of bits

a joint stereo mode based on the correlation orthe phase difference between channels

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MPEG audio supports Several predefined fixed bit rates ranging

from 32 to 224 kbits/sec per channel

Free bit rate other than the predefined rates

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MPEG audio offers Three independent layers of compression

A wide range of tradeoffs between codec

complexity and compressed audio quality

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MPEG Audio LayerII

Intermediate complexity

Bit rates around 128 kbits/sec per channel

Digital Audio Broadcasting (DAB)

Synchronized Video and Audio on CD-

ROM

Full motion CD-I

Video-CD

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MPEG Audio LayerIII

Most complex coding

Best audio quality

Bit rates around 64 kbits/sec per channel

Suitable for audio overISDN

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MPEG Audio extras All three layers allow single chip real-time

decoder implementation

Optional Cyclic Redundancy Check (CRC)

error detection

Ancillary data may be included in the bit

stream

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Overview Quantization, the key to MPEG audio

compression

Transparent, perceptually lossless

compression

No distinction between original and 6-to-1

compressed audio clips

stereo, 16 bit/sample, sampled at 48 kHz,

compressed at 256 kbits/sec

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The Polyphase Filter Bank

Key component common to all layers

Divides the audio signal into 32 equal-width

frequency subbands

The filters provide good time and

reasonable frequency resolution

Critical bands associated with

psychoacoustic models

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Psychoacoustics The aim is to remove acoustically irrelevant

parts of the audio signal

The human auditory system is unable to

hear quantization noise under conditions of

auditory masking

Masking occurs whenever a strong signalmakes a neighborhood of weaker audio

signals imperceptible

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Critical bands The human auditory system has a limited,

frequency dependent resolution

This frequency dependence is expressed in

the form of critical band widths, less then

100 Hz for low and more then 4kHz for

high frequencies The human ear blurs the various signal

components inside a critical band

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Noise masking threshold Human ear resolving power is frequency

dependent

Noise masking threshold, at any frequency,

depends only on the signal energy within a

limited bandwidth neighborhood that

frequency

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The Psychoacoustic Model

Analyzes the audio signal and computes the

amount of noise masking as a function of

frequency

The encoder decides how best to represent

the input signal with a minimum number of

bits

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Basic Steps Time align audio data Convert audio to frequency domain

representation Process spectral values into tonal and non-

tonal components

Apply a spreading function Set a lower bound for threshold values

Find the threshold values for each subband

Calculate the signal to mask ratio

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MPEG LayerIII

coding Based on LayerI&II filter banks

Compensation of filter deficiencies by

processing outputs with a Modified Discrete

Cosine Transform

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LayerIII

enhancements Alias reduction

Non uniform quantization

Scalefactor bands

Entropy coding of data values

Use of a bit reservoir

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