Вопрос 9 speech coding standards

This book focuses mainly on the study of the foundation and historical evolution of many standardized coders. As a matter of principle, a technique is included only if it is part of some standard. Standards exist because there are strong needs to have common means for communication: it is to everyone’s best interest to be able to develop and utilize products and services based on the same reference.

By studying the supporting techniques of standardized coders, we are indeed concentrating our effort on understanding the most influential and successful ideas in this field of knowledge. Otherwise, we would have to spend an enormous amount of effort to deal with the endless papers, reports, and propositions in the literature; many of these might be immature, incomplete, or, in some instances, impractical. A standard, on the other hand, is developed by a team of experts over an extended period of time, with extensive testing and repeated evaluation to warrant that a set of requirements is met. Only organizations with vast resources can coordinate

such endeavors. According to Cox [1995], the time required to complete a standard from beginning to end under the best of circumstances is around

5. years.

This does not mean that a standard is error-free or has no room for improvement. As a matter of fact, new standards often appear as improvement on the existing ones. In many instances, a standard represents the state-of-the-art at the time; in other terms, a reference for future improvement. The relentless research effort will continuously push existent technology toward unknown boundaries.

Standard Bodies

The standard bodies are organizations responsible for overseeing the development of standards for a particular application. Brief descriptions of some well-known standard bodies are given here.

• International Telecommunications Union (ITU). The Telecommunications Standardization Sector of the ITU (ITU-T) is responsible for creating speech coding standards for network telephony. This includes both wired and wireless networks.

• Telecommunications Industry Association (TIA). The TIA is in charge of promulgating speech coding standards for specific applications. It is part of the American National Standards Institute (ANSI). The TIA has successfully developed standards for North American digital cellular telephony, including time division multiple access (TDMA) and code division multiple access (CDMA) systems.

• European Telecommunications Standards Institute (ETSI). The ETSI has memberships from European countries and companies and is mainly an organization of equipment manufacturers. ETSI is organized by application; the most influential group in speech coding is the Groupe Speciale Mobile (GSM), which has several prominent standards under its belt.

• United States Department of Defense (DoD). The DoD is involved with the creation of speech coding standards, known as U.S. Federal standards, mainly for military applications.

• Research and Development Center for Radio Systems of Japan (RCR). Japan’s digital cellular standards are created by the RCR.

The Standards Covered in this Book

As mentioned before, this book is dedicated to standardized coders. Table 1.2 con- tains the major standards developed up to 1999. The name of a standard begins with the acronym of the standard body responsible for development, followedby a label or number assigned to the coder (if available); at the end is the particular algorithm selected. The list in Table 1.2 is not meant to be exhaustive, and many other stan- dards are available either for special purpose or private use by corporations.

TABLE 1.2 Summary of Major Speech Coding Standards

Year Finalized	Standard Name	Bit-Rate (kbps)	Applications
1972a	ITU-T G.711 PCM	64	General purpose
1984b	FS 1015 LPC	2.4	Secure communication
1987b	ETSI GSM 6.10 RPE-LTP	13	Digital mobile radio
1990c	ITU-T G.726 ADPCM	16, 24, 32, 40	General purpose
1990b	TIA IS54 VSELP	7.95	North American TDMA
1990c	ETSI GSM 6.20 VSELP	5.6	digital cellular telephony GSM cellular system
1990c	RCR STD-27B VSELP	6.7	Japanese cellular system
1991b	FS1016 CELP	4.8	Secure communication
1992b	ITU-T G.728 LD-CELP	16	General purpose
1993b	TIA IS96 VBR-CELP	8.5, 4, 2, 0.8	North American CDMA
			digital cellular telephony
1995a	ITU-T G.723.1 MP-MLQ /	5.3, 6.3	Multimedia communications,
	ACELP		videophones
1995b	ITU-T G.729 CS-ACELP	8	General purpose
1996a	ETSI GSM EFR ACELP	12.2	General purpose
1996a	TIA IS641 ACELP	7.4	North American TDMA
1997b	FS MELP	2.4	digital cellular telephony Secure communication
1999a	ETSI AMR-ACELP	12.2, 10.2, 7.95,	General purpose
		7.40, 6.70, 5.90,	telecommunication
		5.15, 4.75

^a Coder is described only partially.

^b Coder is fully explained.

^c Coder is mentioned only briefly without detailed technical descriptions.

However, the major achievements in speech coding for the past thirty years are well represented by the coders on the list.

It is important to mention that the philosophy of this book is to explain the whys andhowsof a specific algorithm; mostimportantly, to justify the selection of a par- ticular technique for an application. Each standardized coder tends to have its own idiosyncrasies and minute operational tricks that might not be important for the understanding of the foundation of the algorithm and hence are often omitted. In order to develop a bit-stream compatible version of the coder, consultation with official documentation is mandatory. Even though many documents describing the standards are available to the general public, it doesn’t mean that it is free for anyone to use. The standards are created through the efforts of individuals, and licensing royalties are the way that these individuals get compensated. On incorporating many of the techniques discussed in this book to commercial

Figure 1.17 Performance comparison between some standardized coders.

products, readers must be aware of patent licenses and be ready to negotiate intellectual property rights agreements with the related organizations.

Figure 1.17 shows a quality/bit-rate/delay comparison plot for the various stan- dards, with quality informally referring to how good the synthetic speech sounds as a result of the encoding/decoding process associated with a speech coder. The plot is for illustration purposes and does not mean to be absolutely precise, since quality measurements (Chapter 19) must be done under various conditions and, in many instances, it is difficult to establish a fair comparison. The data are compiled from various sources and give a rough idea of relative performance among the dif- ferent coders. Delay is reflected by the height of a particular quality/bit-rate coor- dinate and refers to the encoder buffering delay.

Finally, the fact that certain proposed techniques have not become part of a standard does not mean that they are worthless. Sometimes there is a need for refinement; in other instances they are more suitable for special conditions. Since the standardization process is routinely linked to politics, power, and money, the adopted technology might not necessarily represent the best choice from a pure technical perspective. Serious researchers should be ready to learn from many sources and apply the acquired knowledge to optimize the solution for a particular problem.