Fons Bruekers

Audio watermarking for monitoring and copy protection

Based on existing technology used in image and video watermarking, we have developed a robust audio watermarking technique. The embedding algorithm operates in frequency domain, where the magnitudes of the Fourier coefficients are slightly modified. In the temporal domain, an additional scale parameter and gain function are necessary to refine the watermark and achieve perceptual transparency. Watermark detection relies on the Symmetrical Phase Only Matched Filtering (SPOMF) cross-correlation approach. Not only the presence of a watermark, but also its cyclic shift is detected. This shift supports a multi-bit payload for one particular watermark sequence. The watermarking technology proved to be very robust to a large number of signal processing “attacks” such as MP3 (64 kb/s), all-pass filtering, echo addition, time-scale modification, resampling, noise addition, etc. It is expected that this approach may contribute in a wide variety of existing (e.g. monitoring and copy protection) and future applications.

High capacity reversible watermarking for audio

A digital watermark can be seen as an information channel, which is hidden in a cover signal. It is usually designed to be imperceptible to human observers. Although imperceptibility is often achieved, the inherent modification of the cover signal may be viewed as a potential disadvantage. In this paper, we present a reversible watermarking technique for digital audio signals. In our context reversibility refers to the ability to restore the original input signal in the watermark detector. In summary, the approach works as follows. In the encoder, the dynamic range of the input signal is limited (i.e. it is compressed), and part of the unused bits is deployed for encoding the watermark bits. Another part of these bits is used to convey information for the bit-exact reconstruction of the cover signal. It is the purpose of the watermark detector to extract the watermark and reconstruct the input signal by restoring the original dynamic range. In this study we extensively tested this new algorithm with a variety of settings using audio items with different characteristics. These experiments showed that for 16bit PCM audio, capacities close to 1-bit per sample can be achieved, while perceptual degradation of the watermarked signal remained acceptable.

Reversible watermarking for images

Reversible watermarking is a technique for embedding data in a digital host signal in such a manner that the original host signal can be restored in a bit-exact manner in the restoration process. In this paper, we present a general framework for reversible watermarking in multi-media signals. A mapping function, which is in general neither injective nor surjective, is used to map the input signal to a perceptually equivalent output signal. The resulting unused sample values of the output signal are used to encode additional (watermark) information and restoration data. At the 2003 SPIE conference, examples of this technique applied to digital audio were presented. In this paper we concentrate on color and gray-scale images. A particular challenge in this context is not only the optimization of rate-distortion, but also the measure of perceptual quality (i.e. the distortion). In literature distortion is often expressed in terms of PSNR, making comparison among different techniques relatively straightforward. We show that our general framework for reversible watermarking applies to digital images and that results can be presented in terms of PSNR rate-distortions. However, the framework allows for more subtle signal manipulations that are not easily expressed in terms of PSNR distortion. These changes involve manipulations of contrast and/or saturation.

Baby-Posture Classification from Pressure-Sensor Data

The activity of babies and more specifically the posture of babies is an important aspect in their safety and development. In this paper, we studied the automatic classification of baby posture using a pressure-sensitive mat. The posture classification problem is formulated as the design of features that describe the pressure patterns induced by the child in combination with generic classifiers. Novel rotation invariant features constructed from high order statistics obtained from the concentric rings around the center of gravity. Non-constant ring radii are used in order to ensure uniform cell areas and therefore equal importance of features. A vote fusion of various generic classifiers is used for classification. Temporal information was shown to improve the classification performance. The obtained results are promising and open new opportunities for applications and further research in the area of baby safety and development.

Child-activity recognition from multi-sensor data

The automatic recognition of child activity using multi-sensor data enables various applications such as child-development monitoring, energy-expenditure estimation, child-obesity prevention, child safety in and around the home, etc. We formulate the activity recognition task as a classification problem based on multiple sensors embedded in a wearable device. The approach we propose in this paper isto apply spectral analysis techniques of multiple sensor data for activity recognition. Quadratic Discriminant Analysis (QDA) classifieris then trained using manually annotated data and applied for activity recognition. The obtained experimental results for the recognition of 7 activities based on a limited data set are promising and show the potential of the proposed method.

Reduction of Symmetric Complex Filters

Due to their linear-phase property, symmetric filters are an interesting class of finite-impulse-response (FIR) filters. Moreover, symmetric FIR filters allow an efficient implementation. In this paper we extend the classical definition of Hermitian symmetry to a more general symmetry that is also applicable to complex filters. This symmetry is called generalized-Hermitian symmetry. We show the usefulness of this definition as it allows for a unified treatment of even and odd-length filters. Central in this paper is a theorem on the reduction of generalized-Hermitian-symmetric filters to Hermitian-symmetric filters, both with finite precision coefficients. A constructive proof of this theorem is presented and an associated procedure for reducing generalized-Hermitian-symmetric filters is derived. Two of the examples show the application of the reduction procedure and the achieved savings on arithmetic costs. Finally, all three examples show that a special instance of the generalized-Hermitian-symmetric filters with finite precision coefficients, may have lower arithmetic costs than the Hermitian-symmetric filter from which it is derived.