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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

PAC/AAC audio coding standard

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

overview
Overview
  • Audio Recording
  • Coding-ultimate goal
  • AAC Encoder Block Diagram
  • Principles of Psychoacoustics
  • Perceptual Entropy
  • Quantization and Coding
  • Samples
introduction
Introduction

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

audio recording
Audio Recording
  • Edison, 1877
audio recording5
Audio Recording
  • Philips, 1978

A/D Converter

PCM

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

Bitrate = Fsa x Wordlength

aac encoder block diagram
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
Principles of Psychoacoustics
  • Source localization.

Two ears are necessary.

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

principles of psychoacoustics9

audible

inaudible

Principles of Psychoacoustics

Absolute Hearing Threshold

principles of psychoacoustics10
Principles of Psychoacoustics
  • Human Ear Loudness characteristic

Robinson and Dadson equi-loudness contours.

principles of psychoacoustics11
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.

principles of psychoacoustics12

audible

inaudible

Principles of Psychoacoustics
  • Simultaneous Masking: Frequency
principles of psychoacoustics13
Principles of Psychoacoustics

Simplified Paradigms:Noise Masking Tone

Tone Masking Noise

THN

1Bark

THT

1Bark

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

principles of psychoacoustics14

th

1Bark

Principles of Psychoacoustics

Spread of Masking

perceptual entropy
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 entropy17
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 entropy18
Perceptual Entropy

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

ApplyThresholdingrules

HannWindow

s(n)

MDCT

Spectral Flatness Measure

Coefficient of ‘Tonality’

Offset

JND Estimates

perceptual entropy19
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
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 diagram21
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
Quantization and Coding
  • Power-law quantizer
  • Huffman Coding (table can be chosen)
  • Global Gain -> Quantization step size
  • Scale Factors -> noise shaping factor
quantization and coding23
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();

else NOISE_CTL=0;

end

samples
Samples

128kbps AAC Stereo (48kHz)

Original 48kHz Stereo

Castanets

Piano

Timpani

references
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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