1. Alex Chen Nader Shehad Aamir Virani Erik Welsh
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:
Encoded
Signal
Inverse
Quantization
Reconstruction
Filter Banks
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 won’t 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 Full-range Current input
Overhead cost
Full-range
Independent of current input
No overhead
Sampled Input
Quantized Version
Reconstructed Input

12. Quantization Narrow Band Full-Range More accurate
Lower compression ratio
Full-Range
Less accurate
Higher compression ratio
Using 3-bit Quantization
Input:
Levels:
Recon.:
Total Error: .16
Input:
Output:
Recon:
Total Error: .34

13. Demonstration Sine wave Chime Percussion Modern Full range
Narrow range
Chime
8-bit
Percussion
Full Range
Narrow Range
Modern
8-bit

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