DIGITAL WATERMARKING OF AUDIO SIGNALS USING A PSYCHOACOUSTIC AUDITORY MODEL AND SPREAD SPECTRUM THEORY By: Ricardo A. Garcia University of Miami School of Music 1999

Objectives: Design an algorithm and implement a system capable of embedding digital watermarks into audio signals Use spread spectrum techniques to generate the watermark. Use a psychoacoustic auditory model to shape the watermark

Characteristics: Not perceptible (transparent) Resistant to degradation –Removal attempts –Transmission by analog/digital channel –Sub-band coders Original audio is not required in recovery

Design Approach:

SPREAD SPECTRUM Communication system –Uses all the available spectrum –Each channel uses an orthogonal code –All other channels appear as “noise”

TDMAFDMA CDMA spread spectrum

Direct Sequence Spreading Uncoded Direct Sequence Binary Phase Shift Keying Uncoded DS/BPSK

De-Spreading and Data Recovery

Coded DS/BPSK Transmitter: –Repeat Code –Interleaving Receiver: –De-interleaving –Decoder (decision rule)

PSYCHOACOUSTIC AUDITORY MODEL Simultaneous frequency masking Calculate an approximated masking threshold T(z) LINEAR LOGARITHMIC

Frequency  Bark Scale Mapping Critical bands Basilar membrane spreading function B(z)

Psychoacoustic Auditory Model

Noise Shaping Replace components below masking threshold with components from watermark Level of the watermark below threshold Each band has its own scaling factor

Noise Shaping

PROPOSED SYSTEM Transmission: watermark generation and embedding

Reception: watermark recovery

SYSTEM PERFORMANCE Survival over different channels –MPEG –Mini Disc –Two consecutive D/A - A/D –Analog Tape –FM Stereo Radio –FM Mono Radio –FM Mono Radio (weak signal) –AM Radio

MPEG LAYER 3 Level: -2 dB

Listening Test Transparency was achieved for all the watermarking levels. Total listening trials: 40 level = -2 dB 24 correct identifications level = -4 dB 19 correct identifications level = -6 dB 19 correct identifications

CONCLUSIONS The perceptual quality of the audio signal was retained The watermark signal survives to different removal attacks (redundancy) Few parameters are needed at the receiver to recover the watermark

FURTHER RESEARCH Performance with different types of music Changes in the playback speed of the signal Bit error detection and recovery Optimal spread spectrum parameters Multiple watermark embedding Crosstalk interference