The progression toward digital video standards

Given history, the development of accepted standards for digital video could be a challenge. The author takes a look at standards bodies and the industry’s past standards

A standard can best be described as an agreement between several parties and, consequently, as a compromise that restrains the freedom of the participants. Agreement to a standard indicates that the parties have certain perceived advantages that offset the limitations of freedom of choice.

Many telecommunications and broadcasting industries operate and prosper thanks to the existence of worldwide (as well as regional) accepted standards. They have a tradition of participating in standards bodies and usually assign their best personnel to help achieve uniform standards. In doing so, they are driven by their assumed role of public service providers.

Invariably, governments play a major role in the development of industry standards for telegraphy, telephone, radio and television. History shows that their involvement has both helped and hampered the creation of international standards.

The history of standards

1865: The first International Telegraph Convention agreement was signed. It harmonized the different systems used.

1885: The Telegraph Union started drawing up international rules for telephony.

1906: The first Radiotelegraph Convention agreement was signed.

1924: The International Telephone Consultative Committee (CCIF) was created.

1925: The International Telegraph Consultative Committee (CCIT) was created.

1927: The International Radio Consultative Committee (CCIR) was created.

(The CCIF, CCIT and CCIR were made responsible for drawing up international standards.)

1927: The Telegraph Union allocated frequency bands to the various radio services existing at the time, such as fixed, maritime and aeronautical, broadcasting, amateur and experimental.

1934: The International Telegraph Convention of 1865 and the International Radiotelegraph Convention of 1906 merged to become the International Telecommunication Union (ITU).

1941: The United States National Television System Committee (NTSC) developed the 525/60 NTSC television system, embodying such revolutionary concepts as negative video modulation, vestigial sideband transmission, FM sound and a multichannel frequency allocation. The original VHF channel allocation, with the exception of the removal of channel 1, has withstood the test of time, and in the 1950s, UHF channels were added.

1953: The second NTSC committee developed the NTSC-compatible color television system, a revolutionary application of frequency division multiplexing of chrominance and luminance information.

1956: The CCIT and the CCIF were amalgamated to the International Telephone and Telegraph Consultative Committee (CCITT).

Today: The CCITT is called ITU-T and the CCIR is called ITU-R. All these bodies are operating under the umbrella of the United Nations. Other internationally recognized regional standards bodies are the European Broadcasting Union (EBU) and the Society of Motion Picture and Television Engineers (SMPTE). In the United States, transmission standards are regulated and enforced by the FCC. In Canada, the Department of Communications (DOC) regulates it. Similar organizations exist in other countries.

The international standards

After World War II, most of Europe adopted the 625/50 scanning standard. France adopted a high-definition 819/50 scanning standard, as did Belgium, Monaco, Algeria and Morocco. England retained the prewar 405/50 scanning standard. Both France and England later adopted the 625/50 scanning standard while retaining the alternate standards for the benefit of older TV set owners.

While the United States and other countries in the Americas recognized the need to have identical transmission standards and VHF/UHF channel allocations, Europe did not feel the need to have either a common scanning standard nor a common transmission standard.

In addition to several line standards, there were quite a few transmission standards in Europe — creating a nightmare of channel allocations. By the early 1960s, there were no less than three scanning standards and seven incompatible transmission standards in Europe.

With the advent of color television, the choices multiplied dramatically, as NTSC was chosen by 525/60 countries with the exception of Brazil; PAL was adopted by the majority of 625/50 countries; and SECAM was chosen by France (as well as former and remaining French territories), most of the East European (communist) countries (with the exception of Romania, the former Yugoslavia and Albania) and some Middle Eastern countries (such as Egypt, Lebanon and Iran).

CCIR Report 624-4 uses 33 pages to describe all these television standards. All these standards had in common was interlaced scanning and film aspect ratio (4:3). By the early 1980s, the French transmissions in 819/50 and the British transmissions in 405/50 were phased out. And currently, Europe shares a single SDTV scanning standard (625/50), two color standards (PAL and SECAM) and four incompatible transmission standards — a marked improvement.

Other industries, particularly the consumer electronics and the computer industries, have in the past taken a different approach. Individual companies — or a group of companies — develop standards and offer them to the marketplace for use.

Typical examples include the competing Betamax and VHS consumer VCR formats and the non-compatible and non-standard component video signal levels of Betacam and MII as marketed in North America. Occasionally, these non-standards are offered to a standards body for ratification. Among the standards developed by the manufacturers and submitted to standards bodies for ratification was the composite digital (4fSC) standard. The short-lived D2 and D3 digital VTR formats are based on this standard.

Standard success

In the early 1980s, an interest evolved in the standardization of studio digital signals. The first successful internationally accepted digital television approach was Recommendation 601 of the then CCIR. This concept consists of using time-division multiplexing of three digital component video signals — Y, CR, CB — for parallel or serial distribution. What resulted was a component digital sampling family known as 4:4:4, 4:2:2 and 4:1:1, with common sampling frequencies in the two standard definition scanning standards (625/50 and 525/60).

The dominant format is 4:2:2. The SMPTE 259 standard defines the characteristics of the bit-serial interface for 4:2:2 digital signals with a bit rate of 270Mb/s. A host of SMPTE standards relating to the new component digital technologies were subsequently developed.

The late 1980s witnessed the development of a new concept: the compression of 270Mb/s signals by a factor of 25 to 35 while preserving the original picture quality. What resulted was the MPEG compression.

The majority of the MPEG-2 levels and profiles start with a 4:2:2, 10-bit component digital signal. The bit rate is reduced by using such methods as downsampling to 4:2:0, discrete cosine transform, requantizing, variable length coding, run length coding and buffering to achieve the constant bit rate required in transmission in a given RF bandwidth or variable bit rate, such as used in DVD recordings. What resulted was the appearance of various digital video and audio compression schemes with application to recording and transmission of digital video.

In tandem with the MPEG developments, there evolved a number of advanced digital modulation techniques resulting in superior bandwidth-saving transmission methods of digital audio and video. The result was the ATSC standard, which uses a 6MHz television channel to transmit a single HDTV signal compressed from 1.5Gb/s to a bit rate of 19.38Mb/s or several SDTV signals compressed and multiplexed into a bit rate of 19.38Mb/s.

So what's next?

The trend towards DTV 16:9 formats produced three line-scanning standards: 525, 750 and 1125 total lines per picture. The 750 line standard specifies progressive scanning. The 525 and 1125 line standards may use progressive or interlaced scanning. The refresh rates are based on film rates (24Hz) or power line frequencies (50Hz or 60Hz) at nominal or NTSC-friendly rates.

Until recently, the ATSC-compatible 1125 line standard was restricted to interlaced scanning due to the ATSC transmitted bit rate of 19.38Mb/s, which could not accommodate progressive scanning with an uncompressed bit rate of 3Gb/s when using MPEG-2 compression. The appearance on the market of more efficient MPEG-4 technologies allows the compression of a 3Gb/s 1125 line progressive scanning signal into a 19.38Mb/s bit rate with an excellent picture quality. The technology is available at competitive costs as demonstrated by DirecTV, which is in the process of migrating from MPEG-2 to MPEG-4 to carry all the present and future HDTV programs.

The ATSC would do well to take this new technology into consideration. Interestingly, Europe, which was late in migrating to HDTV, now has the advantage of new and highly efficient MPEG-4 technologies. Stay tuned!

Michael Robin, a fellow of the SMPTE and former engineer with the Canadian Broadcasting's engineering headquarters, is an independent broadcast consultant located in Montreal. He is co-author of “Digital Television Fundamentals,” published by McGraw-Hill and translated into Chinese and Japanese.