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
• Physically Lossy compression algorithm
• Perceptually lossless, transparent algorithm
• Exploits perceptual properties of human ear
• Psychoacoustic modeling
Medium quality audio compression
• Code Excited Linear Prediction– for speech coding
• μ-law
• Adaptive Differential Pulse Code Modulation
The MPEG Audio standard
• Ensures inter-operability
• Defines coded bitstream syntax
• Defines decoding process
• Guarantees decoder’s accuracy
MPEG audio acceptance
• Wide acceptance
• Large number of MPEG audio codecs produced
• Stand-alone, Mobile phone add-ons etc
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 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 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
• Three independent layers of compression
• A wide range of tradeoffs between codec complexity and compressed audio quality
MPEG Audio Layer I
• Simplest coding
• Suitable for bit rates above 128 kbits/sec per channel
• Philips Digital Compact Cassette
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
• Most complex coding
• Best audio quality
• Bit rates around 64 kbits/sec per channel
• Suitable for audio over ISDN
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
• 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
• 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
• 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
• 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
• 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
• 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• 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
• Based on Layer I&II filter banks
• Compensation of filter deficiencies by processing outputs with a Modified Discrete Cosine Transform
Layer III enhancements
• Alias reduction
• Non uniform quantization
• Scalefactor bands
• Entropy coding of data values
• Use of a “bit reservoir”