W'A'V'S' Compression

1
W.A.V.S. Compression

• Alex Chen
• Nader Shehad
• Aamir Virani
• Erik Welsh

2
Overview

• Approach
• Psychoacoustic Modeling
• Filter Banks
• Quantization
• Demonstration
• Results
• Further Research

3
Approach
Encoding
Filter Banks
Quantization
Input
Encoded Signal
Psychoacoustic Model
Decoding
Inverse Quantization
Reconstruction Filter Banks
Encoded Signal
Output
4
Psychoacoustic Model

• Based on studies that show hearing capabilities
  affected by
• Environment
• Limitations of human auditory system
• Used to eliminate portions of signal average
  human wont hear
• Two key properties
• Absolute threshold of hearing
• Auditory masking

5
Absolute Threshold of Hearing

• Experiment
• Plot audible threshold of tone
• Observations
• Auditory system sensitive to some frequencies
• Frequencies within critical bandwidth treated
  similarly
• Basis for Bark scale

6
Auditory Masking

• Tones and noise drown out less powerful sounds
• Affect neighboring frequencies
• Affect critical bandwidth
• Effects add to produce overall masking threshold
• Mask quantization

7
Filter Banks Theory

• Array of bandpass filters
• Break up signal into frequency subbands
• Allows for variable coding scheme

8
Analysis and Synthesis Banks
1) Analysis filters divide up the signal 2)
Down-sample 3) Quantize

• 4) Up-sample
• 5) Synthesis filters remove distortions
• 6) Reconstruct the signal

9
Filter Bank Design

• Phase
• Tradeoff between fine and coarse frequency
  resolution
• Piccolo vs. Castanets
• Non-stationary signals
• We used non-adaptive approach

10
Filter Bank Implementation

• We used Cosine Modulated PR
• (perfect reconstruction) filter banks with 32
  filters each
• Output is a delayed version of the input (linear
  phase)
• Distortion arises from quantization only

11
Quantization

• Two types
• Narrow-band
• Current input
• Overhead cost
• Full-range
• Independent of current input
• No overhead

Sampled Input
Quantized Version
Reconstructed Input
12
Quantization

• Narrow Band
• More accurate
• Lower compression ratio
• Full-Range
• Less accurate
• Higher compression ratio

• Using 3-bit Quantization
• Input -.4 -.22 .14 .4
• Levels 1 3 6 8
• Recon. -.4 -.2 .1 .3
• Total Error .16

• Input -.4 -.22 .14 .4
• Output 3 4 6 7
• Recon -.5 -.25 .25 .50
• Total Error .34

13
Demonstration

• Sine wave
• Full range
• Narrow range
• Chime
• 8-bit
• Full range
• Narrow range

• Percussion
• Full Range
• Narrow Range
• Modern
• 8-bit
• Full Range
• Narrow Range

14
Sine Wave (time)
Full-Range Quantization
Narrow Quantization
15
Sine Wave (freq)
Full-Range Quantization
Narrow Quantization
16
Sine Wave (freq error)
Full-Range Quantization
Narrow Quantization
17
Modern (time)
Full-Range Quantization
Narrow Quantization
18
Modern (freq)
Full-Range Quantization
Narrow Quantization
19
Modern (freq error)
Full-Range Quantization
Narrow Quantization
20
Results

• Full Range Smallest File, Worst Sound Quality
• Narrow Range Better Sound Quality, Larger File
• MP3 Industry Standard

21
Further Research

• Filter Banks
• Wavelets
• Dynamic Frequency Ranges
• Better Psychoacoustic Model
• Tone Designation
• Pre- and Post- Echo
• Bit Allocation
• Writing a File