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8/17/2019 Adsp 03 Ac Intro Ec623 Adsp

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Audio Coding

Introduction

S. R. M. Prasanna

Dept of ECE,

IIT Guwahati,

[email protected]

Audio Codin – . 1/

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http://prosper.sourceforge.net/http://prosper.sourceforge.net/


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Goal of Audio Coding

Terms Coding and Compression are usedinterchangeably.

Goal of audio coding is to develop methods for compactdigital representation of audio signals.

Efficient transmission or storage.

Minimum number of bits with transparent perceptualquality.




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First Generation Audio Coders

Digital representation of audio signals.

Compact Disc (CD) is the digital storage medium.

Sampling frequency is 44.1 kHz and Bit rate 16bits/sample

20 kHz audio spectrum + 2.05 guard band = 22.05kHz

Sampling freq = 22.05× 2 = 44.1kHz .

Data rate:44100× 16 = 705.6 kb/s for mono705.6× 2 = 1.41 Mb/s for stereo



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Second Generation Audio Coders

For network and wireless multimedia digital audio.

Bandwidth is the severe constraint.

At the same time, end-users need CD quality.Conflicting requirements.

Goal is to reduce data rate without compromising on

the perceptual quality.Led to several audio compression algorithms.

Exploit both perceptual irrelevancies and statistical

redundancies.



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Third Generation Audio Coders

Lossless audio

Spatial audio

Real-time source localizationHead related transfer function (HRTF)

Immersive audio



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Audio Coding Methods

PCM (1.41 Mb/s).

DPCM (0.75 x PCM data rate).

ADPCM (0.5 x PCM data rate).Not much data rate reduction.

Need for high compression methods driven by potential

applications.New approaches for audio coding based on theprinciples of psychoacoustics.



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Psychoacoustics

Characterizing human auditory perception.

Time-frequency analysis capabilities of the inner ear.

Perceptually irrelevant audio signal information.Contributions from psychoacoustics:

Perceptual entropy

Auditory filter bankPerceptual entropy deals with estimate of thefundamental limit of transparent audio signal

compression.Auditory filter bank based on the time-frequencyanalysis capabilities of the inner ear.


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Some Audio Coding Standards

MPEG-1 Audio (1992).

MPEG-2 Audio (1996).

MPEG-4 Audio v1 (1999).MPEG-4 Audio v2 (2000)



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Block Diagram of Generic Audio Coder



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Principle of Generic Audio Coder

Segment input signals into quasi-stationary frames of2-50 ms.

Time-frequency analysis estimates the temporal andspectral components of each frame.

TFA approach employed is based on human auditorysystem.

Objective is to extract a set of time-frequencyparameters that are robust to quantization according toa perceptual distortion metric.

Perceptual distortion control is achieved by apsychoacoustic signal analysis section that estimatessignal masking power based on psychoacoustic

principles.


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Principle of AC (contd.)

Psychoacoustic model delivers masking thresholds thatquantify the maximum amount of distortion at eachpoint in the time-frequency plane such that quantization

of the time-frequency parameters does not introduceaudible artifacts.

Psychoacoustic model allows the quantization section

to exploit perceptual irrelevancies.Final redundancy removal based on the perceptualentropy coding scheme.


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Audio Coder Attributes

Audio reproduction quality.

Operating bit rates.

Computational complexity.Codec delay.

Channel error robustness.

High quality audio at low bit rates (


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Types of Audio Coders

Based on the signal model or analysis-synthesistechnique.

LP

Transform

Subband

Sinusoidal


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AC-Expt.1

Effect of Sampling Frequency and Bit Resolution

Objective is to analyze the effect of sampling frequency

and bit resolution on the perceptual quality of audio.Take a CD quality music signal of 1 sec, sampled at44.1 kHz with 16 bits/sample and perform the following.

Change its sampling frequency to 16, 8 and 4 kHz.Keep bit resolution constant at 16 bits/sample.Consider about 50 ms segment in a high energyregion.

Plot the time domain and DFT spectra for all thefour cases.Comment on the effect of different sampling

frequency.Comment also on the perceptual quality of theaudio.



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AC-Expt.1

Effect of Sampling Frequency and Bit Resolution

Change its bit resolution to 8, 4 and 1 bits/sample.

Keep sampling frequency constant at 44.1 kHz.Consider the same 50 ms segment in a high energyregion.

Plot the time domain and DFT spectra for all the

four cases.Comment on the effect of different bit resolutions.Comment also on the perceptual quality of the

audio.



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