Dither Modulation Research Articles

Improving the performance of RDM watermarking by means of trellis coded quantisation

We propose a method to improve the performance of the recently introduced rational dither modulation (RDM) watermarking scheme. The improvement is obtained by modifying the essentially scalar nature of RDM with the introduction of a vector quantiser. The vector quantiser is based on a properly modified version of classical trellis coded quantisation, thus leading to the new trellis-coded RDM scheme (TC-RDM). Due to the infinite memory of the trellis TC-RDM relies on, the standard Viterbi algorithm is no longer optimum, hence we introduce a new suboptimal encoding algorithm that keeps the computational complexity reasonably low while ensuring a minimal loss with respect to the optimal scheme. The simulations we carried out on synthetic sequences show that TC-RDM permits improvement of the performance of RDM by 2–4 dB in terms of robustness against Gaussian noise addition.

Rational dither modulation: a high-rate data-hiding method invariant to gain attacks

A novel quantization-based data-hiding method, called Rational Dither Modulation (RDM), is presented. This method retains most of the simplicity of the conventional dither modulation (DM) scheme, which is largely vulnerable to amplitude scalings and modifies it in such a way that the result becomes invariant to gain attacks. RDM is based on using a gain-invariant adaptive quantization step-size at both embedder and decoder. This causes the watermarked signal to be asymptotically stationary. Mathematical tools are used to determine the stationary probability density function, which is later used to assess the performance of RDM in Gaussian channels. It is also shown that by increasing the memory of the system, it is possible to asymptotically approach the performance of DM, still keeping invariance against gain attacks. RDM is compared with improved spread-spectrum methods, showing that the former can achieve much higher rates for the same bit error probability. Experimental results confirm the validity of the theoretical analyses given in the paper. Finally, a broader class of methods, that extends gain-invariance to quantization index modulation (QIM) methods, is also presented.

Robustness improvement of known-host-state data-hiding using host statistics

In this paper we considered the problem of performance improvement of known-host-state (quantization-based) watermarking methods undergo additive white Gaussian noise (AWGN) and uniform noise attacks. We question the optimality of uniform high rate quantizer based design of dither modulation and distortion compensate dither modulation methods from their robustness to these attacks point of view in terms of bit error rate probability. Motivated by superior performance of uniform deadzone quantizer over the uniform one in lossy source coding, we propose to replace the latter one by a former one designed according to the statistics of the host data. Based on suggested modifications we obtained analytical expressions for bit error rate probability analysis of quantization-based watermarking methods in AWGN and uniform noise channels. Experimental results of computer simulations demonstrated performance enhancement of known-host-state watermarking techniques in comparison to the classically elaborated schemes at negative watermark-to-noise ratios.

Joint watermarking and compression using scalar quantization for maximizing robustness in the presence of additive Gaussian attacks

In joint watermarking and compression (JWC), a key process is quantization which embeds watermarks into a host signal while digitizing the host signal subject to requirements on the embedding rate, compression rate, quantization distortion, and robustness. Using fixed-rate scalar quantization for watermarking and compression, in this paper, we mainly consider how to design binary JWC systems to maximize the robustness of the systems in the presence of additive Gaussian attacks under constraints on the compression rate and quantization distortion. We first investigate optimum decoding of a binary JWC system, and demonstrate by experiments that in the distortion-to-noise ratio (DNR) region of practical interest, the minimum distance (MD) decoder achieves performance comparable to that of the maximum likelihood decoder in addition to having advantages of low computation complexity and being independent of the statistics of the host signal. We then present optimum binary JWC encoding schemes using fixed-rate scalar quantization and the MD decoder. Simulation results show that optimum binary JWC systems using nonuniform quantization are better than optimum binary JWC systems using uniform quantization. Furthermore, in comparison with separate watermarking and compression systems, optimum binary JWC systems using nonuniform quantization achieve significant DNR gains in the DNR region of practical interest. Finally, spread transform dither modulation is applied to improving the robustness of the JWC systems at low DNRs.

Performance Analysis of ST-DM Watermarking in Presence of Nonadditive Attacks

The performance of spread-transform dither modulation (ST-DM) watermarking in the presence of two important classes of non additive attacks, such as the gain attack plus noise addition, and the quantization attack are evaluated. The analysis is developed under the assumption that the host features are independent and identically distributed Gaussian random variables, and that a minimum distance criterion is used to decode the hidden information. The theoretical bit-error probabilities are derived in closed form, thus permitting to evaluate the impact of the considered attacks on the watermark at a theoretical level. The analysis is validated by means of extensive Monte Carlo simulations. In addition to the validation of the theoretical analysis, Monte Carlo simulations permitted to abandon the hypothesis of normally distributed host features, in favor of more realistic models adopting a Laplacian or a generalized Gaussian probability density function. The general result of our analysis is that the excellent performance of ST-DM are confirmed in all cases with the only noticeable exception of the gain attack.

Image Watermarking using Dither Modulation in Dual-Transform Domain

A new method for inserting invisible watermark into a digital image is described, which uses two layers of orthogonal transforms, pseudorandom data shuffling and a dither modulation technique. Neither the original host image nor the inserted data string is needed in extraction of the watermark. Experiments show that watermark embedded in this way is sufficiently transparent and robust. A desired tradeoff between in- visibility, robustness, and embedding capacity can be achieved by appropriately choosing the system pa- rameters and the candidate coefficients for embedding.

Quantization index modulation: a class of provably good methods for digital watermarking and information embedding

We consider the problem of embedding one signal (e.g., a digital watermark), within another signal to form a third, signal. The embedding is designed to achieve efficient tradeoffs among the three conflicting goals of maximizing the information-embedding rate, minimizing the distortion between the host signal and composite signal, and maximizing the robustness of the embedding. We introduce new classes of embedding methods, termed quantization index modulation (QIM) and distortion-compensated QIM (DC-QIM), and develop convenient realizations in the form of what we refer to as dither modulation. Using deterministic models to evaluate digital watermarking methods, we show that QIM is against arbitrary bounded and fully informed attacks, which arise in several copyright applications, and in particular it achieves provably better rate distortion-robustness tradeoffs than currently popular spread-spectrum and low-bit(s) modulation methods. Furthermore, we show that for some important classes of probabilistic models, DC-QIM is optimal (capacity-achieving) and regular QIM is near-optimal. These include both additive white Gaussian noise (AWGN) channels, which may be good models for hybrid transmission applications such as digital audio broadcasting, and mean-square-error-constrained attack channels that model private-key watermarking applications.

Quantization Index Modulation Methods for Digital Watermarking and Information Embedding of Multimedia

Copyright notification and enforcement, authentication, covert communication, and hybrid transmission applications such as digital audio broadcasting are examples of emerging multimedia applications for digital watermarking and information embedding methods, methods for embedding one signal (e.g., the digital watermark) within another "host" signal to form a third, "composite" signal. The embedding is designed to achieve efficient trade-offs among the three conflicting goals of maximizing information-embedding rate, minimizing distortion between the host signal and composite signal, and maximizing the robustness of the embedding. We present a class of embedding methods called quantization index modulation (QIM) that achieve provably good rate-distortion-robustness performance. These methods, and low-complexity realizations of them called dither modulation, are provably better than both previously proposed linear methods of spread spectrum and nonlinear methods of low-bit(s) modulation against square-error distortion-constrained intentional attacks. We also derive information-embedding capacities for the case of a colored Gaussian host signal and additive colored Gaussian noise attacks. These results imply an information embedding capacity of about 1/3 b/s of embedded digital rate for every Hertz of host signal bandwidth and every dB drop in received host signal quality. We show that QIM methods achieve performance within 1.6 dB of capacity, and we introduce a form of postprocessing we refer to as distortion compensation that, when combined with QIM, allows capacity to be achieved. In addition, we show that distortion-compensated QIM is an optimal embedding strategy against some important classes of intentional attacks as well. Finally, we report simulation results that demonstrate the performance of dither modulation realizations that can be implemented with only a few adders and scalar quantizers.

Cancellation of laser dither modulation from optical frequency standards

We demonstrate the removal of the dither modulation from an iodine-stabilized He-Ne laser by using a frequency-modulated acousto-optic modulator and feed-forward techniques. This procedure reduces the linewidth of the beat between this laser and a flywheel He-Ne laser from 6 MHz to 8 kHz, the undithered beat linewidth being ~7 kHz. Dither suppression greatly reduces counter errors during beat measurements from stroboscopic effects between the counter's gate and the frequency of the dither modulation and increases the utility of the already formidable array of dithered laser frequency standards by making locking to them an easier task.

Optimization of CO/sub 2/ laser frequency stabilization via external Doppler dither modulation

An external Doppler dither modulation scheme for locking the CO/sub 2/ laser frequency to the center of the 4.2- mu m fluorescence Lamb dip of CO/sub 2/ is discussed. The optimum values for the frequency dither, fluorescence cell temperature and CO/sub 2/ absorber gas pressure depending on the experimental conditions are determined. The design of two different dither mirror arrangements is described. With optimum parameters and with tau =40 ms integration time, it is possible to lock the CO/sub 2/ laser to within +or-10 kHz to line center over several minutes. Measurements of ultrahigh-resolution Lamb dip spectra of the upsilon /sub 2/ as R