Signal Compression

The following sections are included:

• CONTENTS

This article is an introduction to a special issue on signal coding and compression. We begin by defining the concepts of digital coding and audiovisual signal compression. We then describe the four dimensions of coding performance: bit rate, signal quality, processing delay and complexity. We illustrate the two basic principles of audiovisual coding, removal of signal redundancy and the matching of the quantizing system to the properties of the human perceptual system, with specific recent examples of coding algorithms. We then summarize standards for, and applications of audiovisual signal compression. A fast-emerging application is the internetworking of audiovisual information, a field that is too recent to be covered in the articles in this collection. We conclude our article by presenting our views about future research directions in the field.

Digital speech coding is used in a wide variety of every day applications that the ordinary person takes for granted, such as network wireline and cellular telephony and telephone answering machines. This article reviews some of the most currently used methods of speech coding. This includes reviewing the basic attributes of these coders, the methods used for coding and some of the most important speech coding standards. It also reviews the methods used to realize digital speech coders, especially those for implementing these coders in a cost effective manner.

The Moving Pictures Expert Group within the International Organization of Standardization (ISO/MPEG) has developed, and is presently developing, a series of audiovisual standards. Its audio coding standard MPEG Phase 1 is the first international standard in the field of high quality digital audio compression and has been applied in many areas, both for consumer and professional audio. Typical application areas for digital audio are in the fields of audio production, program distribution and exchange, digital sound broadcasting, digital storage, and various multimedia applications. This paper will describe in some detail the main features of MPEG Phase 1 coders.

As a logical further step in digital audio a multichannel audio standard MPEG Phase 2 is being standardized to provide an improved stereophonic image for audio-only applications including teleconferencing and for improved television systems. The status of this standardization process will be covered briefly.

The area of image and video compression has made tremendous progress over the last several decades. The successes in image compression are due to advances and better understanding of waveform coding methods which take advantage of the signal statistics, perceptual methods which take advantage of psychovisual properties of the human visual system (HVS) and object-based models especially for very low bit rate work. Recent years have produced several image coding standards—JPEG for still image compression and H.261, MPEG-I and MPEG-II for video compression. While we have devoted a special section in this paper to cover international coding standards because of their practical value, we have also covered a large class of nonstandard coding technology in the interest of completeness and potential future value. Very low bit rate video coding remains a challenging problem as does our understanding of the human visual system for perceptually optimum compression.

The wide range of applications and bit rates, from video telephony at rates as low as 9.6 kbps to HDTV at 20 Mbps and higher, has acted as a catalyst for generating new ideas in tackling the different challenges characterized by the particular application. The area of image compression will remain an interesting and fruitful area of research as we focus on combining source coding with channel coding and multimedia networking.

Most data that is inherently discrete needs to be compressed in such a way that it can be recovered exactly, without any loss. Examples include text of all kinds, experimental results, and statistical databases. Other forms of data may need to be stored exactly, such as images—particularly bilevel ones, or ones arising in medical and remote-sensing applications, or ones that may be required to be certified true for legal reasons. Moreover, during the process of lossy compression, many occasions for lossless compression of coefficients or other information arise.

This paper surveys techniques for lossless compression. The process of compression can be broken down into modeling and coding. We provide an extensive discussion of coding techniques, and then introduce methods of modeling that are appropriate for text and images. Standard methods used in popular utilities (in the case of text) and international standards (in the case of images) are described.