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PAC/AAC audio coding standard. A. Moreno [email protected] Georgia Institute of Technology ECE8873-Spring/2004. Overview. Audio Recording Coding-ultimate goal AAC Encoder Block Diagram Principles of Psychoacoustics Perceptual Entropy Quantization and Coding Samples. Introduction.

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PAC/AAC audio coding standard

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PAC/AAC audio coding standard

A. [email protected] Institute of TechnologyECE8873-Spring/2004


Overview

  • Audio Recording

  • Coding-ultimate goal

  • AAC Encoder Block Diagram

  • Principles of Psychoacoustics

  • Perceptual Entropy

  • Quantization and Coding

  • Samples


Introduction

"If a tree falls in the forest with no one around to hear it, does it make a sound?"


Audio Recording

  • Edison, 1877


Audio Recording

  • Philips, 1978

A/D Converter

PCM


Coding

  • Ultimate Goal: reduce the number of bits needed to represent the data.

    Bitrate = Fsa x Wordlength


AAC Encoder Block Diagram

Perceptual Model

Iterative Rate Control Loop

ScaleFactorExtract

EntropyCoding

s(n)

Quant

Gain Control

Multi-ChannelM/S, Intensity

MDCT

TNS

Prediction

z^-1

Side information coding, Bitstream

channel


Principles of Psychoacoustics

  • Source localization.

    Two ears are necessary.

    Brain uses intensity differences, and time delays between the two perceived signals.


audible

inaudible

Principles of Psychoacoustics

Absolute Hearing Threshold


Principles of Psychoacoustics

  • Human Ear Loudness characteristic

    Robinson and Dadson equi-loudness contours.


Principles of Psychoacoustics

  • Critical BandsConcept introduced by Harvey Fletcher 1940.

Frequency to Place Transform.Function of frequency that quantifies the cochlear filter passbands.

Example: The critical band for a 1kHz is about 160Hz in width.

A narrow band noise centered at 1kHz is perceived with the same loudness as long as the width < 160Hz.


audible

inaudible

Principles of Psychoacoustics

  • Simultaneous Masking: Frequency


Principles of Psychoacoustics

Simplified Paradigms:Noise Masking Tone

Tone Masking Noise

THN

1Bark

THT

1Bark

K=3dB...5dB (constant)


th

1Bark

Principles of Psychoacoustics

Spread of Masking


Principles of Psychoacoustics

  • Masking: Temporal


Perceptual Entropy

  • Perceptual Entropy, objective metric of perceptually relevant introduced by J. Johnston

    The perceived information from an audio signal is only a fraction of the total information emanated by the source.


Perceptual Entropy

  • Procedure:

  • Window and transform to frequency.

  • Masking Threshold is computed using perceptual rules

  • A determination is made of the number of bits required to quantize the spectrum, without injecting perceptible noise.


Perceptual Entropy

Determine nature(Noise-like)(Tone-like)

ApplyThresholdingrules

HannWindow

s(n)

MDCT

Spectral Flatness Measure

Coefficient of ‘Tonality’

Offset

JND Estimates


Perceptual Entropy

i:index of critical band;

bli, blh: lower and upper bounds of band i;

ki:number of transform component in band i;

Ti:masking threshold in band i;

nint:rounding to the nearest integer.


Returning

  • "If a tree falls in the forest with no one around to hear it, does it make a sound?"

    From a Perceptual Coding standpoint, if no one can hear it, THERE IS NO TREE.


AAC Encoder Block Diagram

Perceptual Model

Iterative Rate Control Loop

ScaleFactorExtract

EntropyCoding

s(n)

Quant

Gain Control

Multi-ChannelM/S, Intensity

MDCT

TNS

Prediction

z^-1

Side information coding, Bitstream

channel


Quantization and Coding

  • Power-law quantizer

  • Huffman Coding (table can be chosen)

  • Global Gain -> Quantization step size

  • Scale Factors-> noise shaping factor


Quantization and Coding

while NOISE_CTL

while FINDING_RATE

Nr_bits= get_bits_needed();

if (Nr_bits > max_bits)

adjust_global_gain();

else

FINDING_RATE=0;

end

q_noise=get_quant_noise_level();

if (q_noise> Th(band))

adjust_band_scale_factor();

elseNOISE_CTL=0;

end


Samples

128kbps AAC Stereo (48kHz)

Original 48kHz Stereo

Castanets

Piano

Timpani


References

[1] Ted Painter and Andreas Spanias. Perceptual coding of digital audio. Proceedings of the IEEE, 88(4):449-513. Abril 2000.

[2] Karlheinz Brandenburg, MP3 and AAC explained, AES 17th International Conference on High Quality Audio Coding, 1999.

[3] J.D. Johnston, A.J. Ferreira, Sum-Difference Stereo Transform Coding, Proc. ICASSP 1992.

[4] Deepen Sinha, James D. Johnston. Audio Compression at low bit rates using a Signal Adaptive switched Filterbank. Proc. of the ICASSP 1996, pp. 1053-1056 .


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