Romualdo Picco

Statistical analysis of the 2D-DCT coefficients of the differential signal for images

Abstract Block-matching motion compensation techniques are widely used in image coding algorithms. A differential signal with different characteristics from the original signal is then generated. It is important to know the statistical properties of the signal source in order to correctly characterize some parameters of the digital image coding scheme. In this paper a statistical study of the 2D-DCT coefficients of the motion-compensated blocks is performed. The results of this study indicate that the statistics of the coefficients are best approximated by a Laplacian pdf (probability density function). The influence of some types of normalization is investigated and the corresponding pdfs are estimated. An analysis indicates that the Laplacian pdf may be used as a good approximation for the statistical properties of the normalized coefficients.

Motion compensation and multiresolution coding

Abstract Multiresolution techniques have become more and more appealing in current image coding. Image multispectral representation produces many important features such as spectral shaping of coding noise according to human eye perception, good image energy compaction, coder tuning with respect to any band characteristics, and allows for multilevel layered transmission that is one of the main targets pursued by the broadcaster. Despite these appealing capabilities, multiresolution techniques have failed to give the expected results. One of the reasons for this failure is the difficulty of exploiting the temporal redudancy present in image sequences. This paper addresses the problem of motion compensation in a multiresolution environment, considering both QMF-SBC and wavelet transform approaches. Different motion compensation schemes are derived and their efficiency is considered with regard to scalability and to the lengths of subband analysis and synthesis filters. Simulation results are used to support relevant conclusions where needed.

Video signal coding with DCT and vector quantization

An image coding scheme using the discrete cosine transform is analyzed when the transform coefficients are vector quantized. The coding method is based on the known scheme proposed by Chen and Smith (1977) which sorts the picture blocks into classes according to the level of image activity. The coding scheme is modified to allow for vector quantization of the ac coefficients, in particular a pyramid vector quantizer (PVQ) is used. This is based on the statistical and geometric properties of a Laplacian source which, in fact, is the best model for the ac coefficients of the two-dimensional discrete cosine transform (2D-DCT) of an image. A method for forming almost statistically independent vectors is also suggested and improves quantization performance. Images are encoded with both the PVQ and standard scalar quantizer transform coders, demonstrating that the PVQ coder reduces the mean square encoding error and improves image quality. In particular, emphasis is given to how the use of fractional bit rates affects the objective and subjective gains obtained. The results presented (i.e. mean square error values and printed images) have been obtained experimentally, working with a statistical criterion in a group of images whose size was in accordance with the 50 Hz CCIR Recommendation 601 Standard. >

Combining DCT and subband coding into an intraframe coder

Abstract The advent of broadband digital networks will make it possible to transmit high quality digital television. Subband coding is a very interesting coding scheme as it accommodates a transmission quality that can flexibly adapt to the available channel capacity. Subband coding (SBC) and block cosine-transform techniques have already been used together, but only to code the low-pass image. Residual spatial correlation present in higher frequencies bands has only partially been taken care of. This paper presents the results obtained with a simulated coder in which DCT is used in all bands where expected spatial correlation is high enough. The use of a pyramid vector quantizer is proposed. Results are good and compare favourably with other results published in the literature.

MPEG-2 Coded Video Traces Transmitted Over aSatellite Link: Scalable and Non-Scalable Solutionsin Rain Fading Conditions

The literature is poor in the analyses of the effects produced by corrupted bits in compressed video bitstreams. This paper presents the results of a transmission experiment of MPEG-2 coded video data over a satellite link affected by noise, in order to investigate under which conditions this type of transmission is economically feasible. The signal-to-noise ratio scalability feature of the MPEG-2 encoder was used to produce different bitstreams of the same movie sequence. The scope of the study was to verify which are the best combinations of video and channel codings in the presence of attenuation on the satellite link, in order to optimize the bandwidth utilisation for a requested image quality. The results obtained give indications about the data channel codings to be used to counter the rain fade on the transmission link, which is a non negligible problem especially when satellite transmissions are in the Ka band. Moreover, the results highlight the flexibility of the scalable video coding in the examined scenario.

Evolution and trends of HDTV

The evolution of scientific activity related to HDTV is summarized from its first steps to the present and future situation. The problems connected to the definition of HDTV standards are examined, and an overview of the programs of the main international standards bodies (CCIR, CCITT) is given. The HDTV systems already realized and the most important projects regarding HDTV, which include MUSE, HD-MAC, SMPTE, and various Eureka projects, are discussed. A brief overview of Italian activity concerning HDTV is reported, focusing on the CNR-Telecommunicazioni project. The HDTV coding scheme realized by CSTV-CNR is presented, and its performance on two test sequences is evaluated.<<ETX>>

MPEG-2 sources: exploiting source scalability for an efficient bandwidth allocation

Abstract. The paper investigates efficient bandwidth allocation schemes for the transmission of MPEG-2 video traffic on high-speed networks. To this end we performed an extensive analysis of the traffic generated by an MPEG-2 encoder. Specifically, we encoded “The Sheltering Sky” movie according to the MPEG-2 standard. By the analysis of the generated traffic it results that a constant-quality transmission can be performed with a poor bandwidth utilization. In the paper we identified that the low bandwidth utilization is caused by rare high-rate periods in the codec bitstream. Hence, we identified the source scalability as a promising approach to achieve a “quasi-constant” quality transmission and an efficient bandwidth utilization. The effectiveness of this approach is evaluated in the paper via simulation. Specifically, by defining a Markovian model for an MPEG-2 scalable source we performed a set of simulation experiments which indicate that the source scalability approach significantly increases the utilization, while maintaining the quality of the video signal at the highest value for most of the time, e.g., a 50% of the network utilization with the highest quality for the 99.7% of the time.

A pyramid vector quantizer chip for HDTV applications

The quantization process in bit-rate reduction for video coding has been taken into account. Because, at least in theory, vector quantization obtains better performance than scalar quantization, a pyramid vector quantizer has been analysed. Some figures of performance have been estimated by Monte Carlo simulation and, after analysing the problem of vector formation, directly on a typical video coding scheme. In the past, a single-chip dedicated processor for implementation of the PVQ was presented with the conclusion that the allowed quantization sample rate was no greater than 16 kHz. In this work ASIC-oriented modifications of the base algorithm are described and an ASIC design for the PVQ is presented. The architecture of the integrated circuit was obtained by means of the systolic macrocell approach. A system throughput greater than 70 MSa/s was allowed that extends the range of application of the PVQ up to HDTV digital video coding.

Matrix multiplier transform analysis and design of a general purpose transformer

Abstract Transform techniques are becoming more and more important in digital signal processing, with particular reference to coding problems, owing to their capability of redundancy reduction by signal decorrelation. The MMT (Matrix Multiplier Transform) is a high speed device able to perform any type of two-dimensional separable unitary transform. It is realized with 1.5 μm technology and can work at operating frequencies up to 16.75 MHz, and so is able to work on TV signal in real time. An analysis of the truncation errors due to the use of a finite arithmetic and their propagation has been performed. That analysis, together with the decision to make use of a silicon compiler in the design stage, made it possible to maintain a parametrical architecture so that MMT can be easily adapted to many applications.

Analysis of a 2-D DCT image coding scheme with motion compensation and vector quantization

A new effective adaptive coding scheme for data compression of images is presented. It is based on a reduction of the spatial, temporal and statistical redundancy. Inter-frame motion compensation technique was used to take advantage of the image temporal correlation. A unitary 2D-DCT was performed on each (8x 8) pixel block in order to exploit spatial correlation and in order to decorrelate the block elements and to make the relative quantization error subjectively less disturbing. The Rate Distortion Function of a Laplacian source was used as a classification strategy by relating a block to a coding bit rate in accordance with its energy. The a.c. transform coefficients were vector quantized by using the pyramidal vector quantizer based on the cubic lattice. Compression ratios in the 8 — 12 range give a distribution quality for colour image sequences according to the CCIR Rec. 601 (12-20 Mbit/sec) or HDTV sequences (50-70 Mbit/sec). Experimental results obtained by computer simulations are presented and the influence of the motion compensation and the vector quantization on the objective error are analysed.