1. III Digital Audio
III.6 (Fr Oct 20) The MP3 algorithm with PAC

2. 6. Frame Outputstream Formatting
The MP3 encoder chain
5. Huffman Compression
6. Frame Outputstream Formatting
Audio Data
Filter Bank 32 Subbands
Subbands
Psychoacoustical Model
Quantization and Encoding (Check of Quantization loop)
External Check
Encoding
Encoding of Additional Information
Datastream Formatting
to Frames etc.
Additional Data
Data Stream 2*16 to
Line
1. Digital Datastream
4. Quantization
2. FFT with Filter Bank
3. Psychoacoustical Model
(Perceptual-Audio-Coding Model PAC)

3. The MP3 encoder chain 1. Digital Datastream 2 ~ stereo
Audio Data
Filter Bank 32 Subbands
Subbands
Psychoacoustical Model
Quantization and Encoding (Check of Quantization loop)
External Check
Encoding
Encoding of Additional Information
Datastream Formatting
to Frames etc.
Additional Data
Data Stream 2*16 to
Line
1. Digital Datastream
2 ~ stereo
768 kbit/s ~ × 16 b/s

4. Since # sample rate = # Fourier coefficients,
The MP3 encoder chain
Audio Data
Filter Bank 32 Subbands
Subbands
Psychoacoustical Model
Quantization and Encoding (Check of Quantization loop)
External Check
Encoding
Encoding of Additional Information
Datastream Formatting
to Frames etc.
Additional Data
Data Stream 2*16 to
Line
2. FFT with Filter Bank
Important:
Since # sample rate = # Fourier coefficients,
speak of “Fourier samples per second”
2.1 Cut spectrum 0 – 20 kHz into 32 subbands of 625 Hz each (32 × 625 = ) for 1/40 sec windows.
2.2 Use MDCT (Modified Discrete Cosine Transformation ~ variant of FFT) to split each 625 Hz band into 18 subbands with variable widths, according to psychoacoustical criteria. Get 576 = 18 × 32 “lines”.

5. 5. Huffman lossless Compression
The MP3 encoder chain
5. Huffman lossless Compression
Audio Data
Filter Bank 32 Subbands
Subbands
Psychoacoustical Model
Quantization and Encoding (Check of Quantization loop)
External Check
Encoding
Encoding of Additional Information
Datastream Formatting
to Frames etc.
Additional Data
Data Stream 2*16 to
Line
4. lossy Quantization
Already discussed, Ok!!!!
40% of compression

6. 3. Psychoacoustical Model (Perceptual-Audio-Coding Model PAC)
The MP3 encoder chain
Audio Data
Filter Bank 32 Subbands
Subbands
Psychoacoustical Model
Quantization and Encoding (Check of Quantization loop)
External Check
Encoding
Encoding of Additional Information
Datastream Formatting
to Frames etc.
Additional Data
Data Stream 2*16 to
Line
3. Psychoacoustical Model
(Perceptual-Audio-Coding Model PAC)

7. PAC 1: hearing thresholds PAC 2: auditory masking
The MP3 encoder chain
Psychoacoustical Model
(Perceptual-Audio-Coding Model PAC) =
core features of MP3,
it covers 60 % of MP3 compression
The PAC Model is based upon three limitations of human audio-perception:
PAC 1: hearing thresholds
PAC 2: auditory masking
PAC 3: temporary masking
All three PAC components generate lossy compression

8. PAC 1: hearing thresholds
The MP3 encoder chain
you don’t hear sinusoidal sounds below this threshold of loudness
PAC 1: hearing thresholds
loudness
frequency (kHz)

9. The MP3 encoder chain PAC 2: auditory masking
frequency
loudness
For every sinusoidal frequency component of frequency f and loudness l, there is a surrounding masking surface, where other frequency/loudness components cannot be heard together with the given one. Example: the 4 kHz/40 dB component (red) masks the blue one.

10. PAC 3: temporary masking
The MP3 encoder chain
PAC 3: temporary masking
loudness
time
For every sinusoidal frequency component of frequency f and loudness l (red) another subsequent component (blue) cannot be heard below the given curve of loudness in time, because the ear needs some time to “recover” from that first component’s perception. This is even true for sounds before the given one (red curve), because the perception needs to be built up!

11. 6. Frame Outputstream Formatting
The MP3 encoder chain
6. Frame Outputstream Formatting
Audio Data
Filter Bank 32 Subbands
Subbands
Psychoacoustical Model
Quantization and Encoding (Check of Quantization loop)
External Check
Encoding
Encoding of Additional Information
Datastream Formatting
to Frames etc.
Additional Data
Data Stream 2*16 to
Line