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MPEG Audio Compression - PowerPoint PPT Presentation


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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”