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Download - MP3 Tutorial

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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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