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MPEG Audio Compression. by V. Loumos. Introduction. Motion Picture Experts Group (MPEG) International Standards Organization (ISO) First High Fidelity Audio standard Part of a multiple standard for Video compression Audio compression

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Introduction
Introduction

  • Motion Picture Experts Group (MPEG)

  • International Standards Organization (ISO)

  • 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


Mpeg audio
MPEG Audio

  • Physically Lossy compression algorithm

  • Perceptually lossless, transparent algorithm

  • Exploits perceptual properties of human ear

  • Psychoacoustic modeling


Medium quality audio compression
Medium quality audio compression

  • Code Excited Linear Prediction

    • for speech coding

  • μ-law

  • Adaptive Differential Pulse Code Modulation


The mpeg audio standard
The MPEG Audio standard

  • Ensures inter-operability

  • Defines coded bitstream syntax

  • Defines decoding process

  • Guarantees decoder’s accuracy


Mpeg audio acceptance
MPEG audio acceptance

  • Wide acceptance

  • Large number of MPEG audio codecs produced

  • Stand-alone, Mobile phone add-ons etc


Mpeg audio features
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


Mpeg audio sampling rates
MPEG audio sampling rates

  • 32 kHz

  • 44.1 kHz

  • 48 kHz


Mpeg audio supports
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 or the phase difference between channels


Mpeg audio supports1
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


Mpeg audio offers
MPEG audio offers

  • Three independent layers of compression

  • A wide range of tradeoffs between codec complexity and compressed audio quality


Mpeg audio layer i
MPEG Audio Layer I

  • Simplest coding

  • Suitable for bit rates above 128 kbits/sec per channel

  • Philips Digital Compact Cassette


Mpeg audio layer ii
MPEG Audio Layer II

  • 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


Mpeg audio layer iii
MPEG Audio Layer III

  • Most complex coding

  • Best audio quality

  • Bit rates around 64 kbits/sec per channel

  • Suitable for audio over ISDN


Mpeg audio extras
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


Overview
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


The polyphase filter bank
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


Psychoacoustics
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 signal makes a neighborhood of weaker audio signals imperceptible


Critical bands
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


Noise masking threshold
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


The psychoacoustic model
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


Basic steps
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


Mpeg layer iii coding
MPEG Layer III coding

  • Based on Layer I&II filter banks

  • Compensation of filter deficiencies by processing outputs with a Modified Discrete Cosine Transform


Layer iii enhancements
Layer III enhancements

  • Alias reduction

  • Non uniform quantization

  • Scalefactor bands

  • Entropy coding of data values

  • Use of a “bit reservoir”


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