Digital Television Defined | A Brief History of Television | Transmission and Reception
How DTV Began : The Grand Alliance and The ATSC | The DVB Project
The Standards: MPEG-2 | Dolby Digital | The Transmission Schemes : 8-VSB | COFDM Conditional Access
The Display Formats: HDTV | SDTV
The Transition to DTV: Simulcast | What You Need
Digital television (DTV) is the transmission of television signals using digital rather than conventional analog methods. Analog transmission is in the form of a constantly variable wave; digital transmission consists of an electrical pulse which has two possibilities: on and off (or positive and negative), which are represented by a one and a zero (this is binary data, the same type of information that a computer understands). Because a digital signal does not fluctuate, but is either perfectly intact or totally absent, a digital transmission is more precise than an analog transmission. Although both signals are transmitted in the same basic way and have the same range, they behave differently at the limits of their ranges. An analog signal degrades over distance and may be barely detectable at the farther reaches of the broadcast area - this is why the signal from a distant radio station fades in and out. As the signal reaches the farther limits of its range, the signal-to-noise ratio (SNR) decreases and the quality of the broadcast suffers, although the range remains the same. In comparison, when the SNR decreases in a digital signal, the quality of the broadcast does not degrade, but the range shrinks (this is called the "cliff effect").
Advantages of DTV over analog TV include:
- Superior image resolution (detail) for a given bandwidth
- Smaller bandwidth for a given image resolution
- Compatibility with computers and the Internet
- Superior audio quality
- Consistency of reception over varying distances
- Capacity for multicasting
Conventional standards - NTSC , PAL , and SECAM - specify analog transmission. However, both the audio and video components of a television signal can be transmitted in digital form. Because much more data can be sent digitally, a digital transmission will allow - depending on the display format chosen - either a dramatically improved resolution and sound quality, in the case of high definition television (HDTV), or simultaneous broadcasts (multicasting) of up to five programs, in the case of standard definition television (SDTV). Another SDTV option, known as datacasting is the transmission of data such as games, still pictures, and other material while the viewer is watching a program. Datacasting might allow, for example, someone watching a baseball game to choose a different camera angle, or to select a display of player or team statistics.
"DTV" is often spoken of as synonymous with "HDTV", but it is not really the same thing. HDTV defines a new television display format but does not specify how the format will be transmitted; HDTV can be transmitted in either analog or digital signals. DTV technology, on the other hand, is focused more on the method of transmission. HDTV features include a greater pixel density and a new, wider aspect ratio . The aspect ratio refers to the screen shape: aspect ratio is defined as the ratio s/h, where s is the length of the longest side of the element and h is the distance to the opposite side - in terms of a rectangular screen, this translates as a width to height ratio. Although a wider aspect ratio is a feature of digital televisions in general, DTV can be displayed in either HDTV or standard-definition television (SDTV) format and digital television sets are available with a range of resolutions below that of HDTV.
The possibility of television (literally seeing from a distance , combining the Greek tele , meaning from a distance - and especially transmission over a distance - with vision ) excited many inventors after the photo-sensitive qualities of selenium were discovered in 1873: it was discovered that selenium's electrical conductivity varies in response to exposure to light. This discovery meant both that it was possible to convert image data to electrical signals and that the data could be transmitted.
In 1884, Paul Nipkow patented a simple mechanical television that he called the Elektrisches Telescop (electric telescope), the first system that scanned, transmitted, and received images. Nipkow's invention featured a scanning disk and a selenium cell. The scanning disk is a round, flat disk with holes spiralling outward from the center. Light from a 1000-watt bulb reflected from an image and passed through the revolving disk to stimulate the photo-sensitive selenium cell. The effect of the light on the cell was to create an electrical current, which fluctuated in response to variations in light and charged the light source in a receiver. In front of the receiver, a second disk revolved in synchronization with the first. Light passed through the holes in the second disk to create a flickering image which the viewer saw through an eyepiece. Nipkow's electric telescope afforded a blurry picture on a two or three inch screen.
The first cathode ray tube, which serves as the prototype for modern television picture tubes, was developed by William Crookes, who proved the existence of cathode rays by building a tube in which they could be displayed. Ambrose Flemming subsequently discovered that cathode rays could be deflected and focused by wrapping the tube with wire to create a magnetic field and passing an electrical current through it. In 1897, Karl Braun demonstrated the capacity of cathode rays to control a magnetic field by placing fluorescent matter at the end of the cathode ray tube. Braun developed the cathode ray oscilliscope, which would lead to the development of the modern scanning system.
In 1932, the scanning disk was replaced by Vladimir Zworykin's iconoscope, an electronic camera tube used to record images. A cathode-ray tube was used in the television receiver to display the recorded images on the television screen. These two inventions replaced the components of the earlier mechanical television system and eventually brought television broadcasting to the masses. Later color television was introduced, for which several different systems have been developed.
The video and audio signals of a television program are transmitted simultaneously on separate high frequency radio waves. These high frequency waves create a band, or channel. High frequency waves are used instead of low frequency waves, which are used for radio broadcasting, because they are fairly free from static. High frequency waves are called UHF (ultrahigh frequency) and VHF (very high frequency) waves.
Each high frequency wave is transmitted point to point in a straight line to antennas in one of three ways: through terrestrial transmission (this is the conventional ground-based method), through satellite transmission, or through cable. The antenna connected to the back of the television receives the audio and video signals and sends them into special electrical circuits that separate the signals located inside the television. The audio signal is converted into sound, and the video signal passes into the cathode-ray tube, which in turn produces the images on the television screen. Black and white receivers process only the brightness signal. Color receivers separate the signal into components: brightness, saturation, and hue are recombined to produce primary colour signals for the picture tube.
A major breakthrough in television broadcasting happened when local cable television networks began offering cable services. These networks distribute television signals from a central receiver to television sets using coaxial cables. Improved reception and more channels caused many television viewers to switch to cable television.
In 1987, the United States Federal Communications Commission (FCC) formed the Advisory Committee on Advanced Television Service (ACATS) whose purpose it was to advise the FCC on the development of advanced television (ATV). At first it was thought to simply improve the existing NTSC analog television system, but General Instrument proposed an all-digital television system. In the following months, three additional digital systems were proposed by several companies.
From July 1991 to October 1992, the four all-digital systems were tested along with two improved analog systems. The digital systems tested fair with some deficiencies requiring improvement. The first decision the ACATS special panel made was to not consider further improvements to the NTSC analog television system and to concentrate on solely on DTV. They also decided to use basic elements from all four digital systems instead of elements from just one system. This decision caused the proponents of the four digital systems to join forces and form the Grand Alliance. The Grand Alliance then built a prototype of the system in a modular fashion at various locations. The complete system was then integrated and tested in April 1995.
The Advanced Television Systems Committee ( ATSC ) is a standards organization that was created in 1982 as part of ATV to promote the establishment of technical standards for all aspects of advanced television systems. Based in Washington, D.C., ATSC has an international membership of over 200 organizations (up from an original 25), including broadcasters, motion picture companies, telecommunications carriers, cable TV programmers, consumer electronics manufacturers, and computer hardware and software companies.
The ATSC developed standards for digital television (DTV) that specify technologies for the transport, format, compression, and transmission of DTV in the U.S. ATSC DTV Standards include digital high definition television (HDTV), standard definition television (SDTV), data broadcasting, multichannel surround-sound audio, and satellite direct-to-home broadcasting. For SDTV and HDTV, ATSC chose MPEG-2 for video and Dolby Digital for audio. The ATSC is currently finalizing DTV standards for satellite services, conditional access (methods, such as encryption or electronic locking systems, used to restrict service access to authorized users), datacasting, and interactive services.
ATSC standards for HDTV are currently being adopted internationally.
In the early 1990s, European broadcasters, consumer equipment manufacturers, and regulatory bodies formed the European Launching Group (ELG) to discuss introducing DTV throughout Europe. The ELG realized the importance of establishing a common frame of reference among members and drafted a document called the Memorandum of Understanding (MoU) to establish a basis of understanding. The MoU was signed by all ELG members in 1993. The DVB (Digital Video Broadcasting) Project was created from the ELG membership in September of 1993.
DVB is an open system as opposed to a closed system. Closed systems are content-provider specific, not expandable, and optimized only for the system they were developed for. An open system, such as DVB, allows the subscriber to choose different content providers and allows integration of PCs and televisions. DVB systems are optimized for television, but home shopping and banking, private network broadcasting, and interactive viewing are supported.
DVB opens the possibilities of providing crystal-clear television programming to television sets in buses, cars, trains, and even hand-held televisions. DVB is beneficial to content providers because they can offer their services anywhere DVB is supported regardless of geographic location. They can also expand their services easily and inexpensively and ensure restricted access to subscribers reducing lost revenues due to unauthorized viewing.
Today, the DVB Project consists of over 220 organizations in more than 29 countries worldwide and DVB broadcast services are available in Europe, Africa, Asia, Australia, and parts of North and South America.
MPEG-2 (Motion Picture Experts Group standard 2) reduces a digital signal from 166 Mbits to 5 Mbits, allowing broadcasters to transmit digital signals using existing cable, satellite, and terrestrial systems. MPEG-2 uses the lossy compression method, which means that the digital signal sent to the television is compressed and some data is lost, but this lost data does not affect how the human eye views the picture. Both the ATSC and DVB standards selected MPEG-2 for video compression and transport.
Dolby Digital is a digital audio coding technique that reduces the amount of data needed to produce high quality sound. Dolby Digital takes advantage of how the human ear processes sound. When coding noise is close to the frequency of an audio signal, that audio signal masks the noise so that the human ear hears only the intended audio signal. Sometimes the coding noise is not in the same frequency of an audio signal and must be reduced or eliminated. Reducing or eliminating the noise reduces the amount of data to one tenth of the data on a compact disk (CD). Dolby Digital is used with digital versatile disks (DVD), DTV, and digital cable and satellite transmissions. The DVB standard does not use Dolby Digital, but instead uses MPEG technology for both audio and video signals.
Dolby Digital provides five full bandwidth channels: front left, front right, center, surround left, and surround right, for true surround sound quality. A low frequency effect (LFE) channel is included that provides the sound needed for special effects and action sequences in movies. The LFE channel is one tenth of the bandwidth of the other channels and is sometimes erroneously called the subwoofer channel. This multichannel scheme is known as 5.1 channel.
Because not everyone has the equipment needed to take advantage of Dolby Digital's 5.1 channel, developers included a downmixing feature that ensures compatibility with any playback device. The decoder in the playback device delivers the audio signal specific to that particular device's ability. For example, a 5.1 channel audio signal is delivered to a mono television. The playback device's decoder downmixes the 5.1 channel signal to a mono signal allowing the television to use the received audio signal. Because the playback device does the downmixing, producers do not have to create multiple audio signals for each playback device.
The ATSC decided to use Dolby Digital as a standard for DTV because of its popularity with film producers and consumers, its ability to use a single audio stream because of the downmixing feature, and its high-quality sound. The U.S. cable television industry also adopted Dolby Digital for their DTV applications. Most television facilities are not equipped to produce 5.1 channel sound. For this reason, many DTV programs use two-channel sound. Five-to-one channel is used primarily for pay-per-view movies and at theaters.
A great deal of controversy has surrounded the ATSC's decision to use 8-VSB (8-level vestigial sideband) as a transmission system for DTV. The ATSC adopted the VSB transmission system because of its large bandwidth, which is needed to transmit HDTV programming. Critics, however, state that this larger bandwidth is irrelevant if customers cannot view the transmitted program because of multipath effects.
8-VSB (8-level vestigial sideband) is the FCC standard radio frequency (RF) modulation (a process of adding data to a signal carrier) format chosen by the ATSC for the broadcast of digital television (DTV) to consumers.
The 8-VSB mode includes eight amplitude levels that support up to 19.28 Mbps of data in a single 6 Mhz channel (there is also a 16-VSB is mode that has 16 amplitude levels and supporst up to 38.57 Mbps of data on a 6 Mhz channel). 8-VSB is perfect for the simultaneous transmission of more than one DTV program (multicasting) and the transmission of data along with a television program (datacasting) because it supports large data payloads.
The ATSC adopted the VSB transmission system because of its large bandwidth, which is needed to transmit HDTV (high definition television) programming. Critics, however, state that this larger bandwidth is irrelevant if customers cannot view the transmitted program because of multipath effects. When a signal is transmitted, it is met with obstructions such as canyons, buildings, and even people, which scatter the signal causing it to take two or more paths to reach its final destination, the television. The late arrival of the scattered portions of the signal cause ghost images. Multipath effects can occur simply by an individual walking into the room. For this reason, some consumers in metropolitan areas or areas with rugged terrain opt for cable television instead of fighting their antennas for better reception.
Because a VSB signal is transmitted on one carrier, it scatters like water blasted on a wall when met with obstacles. This interference can cause a digital television screen to darken or even lock up, which is not a problem with the DVB standard, COFDM because it transmits a signal on multiple carriers. VSB advocates state that simply buying an outdoor antenna that rotates solves this problem, but critics worry that customers do not want to buy an expensive rotating outdoor antenna to view free television programs. They also worry that the poor reception and the added expense of an outdoor antenna are slowing the transition to DTV in ATSC-compliant countries. The VSB scheme also does not support mobile television viewing. VSB equipment manufacturers are developing ways to solve these two problems.
Coded Orthogonal Frequency Division Multiplexing (COFDM) is a modulation scheme that divides a single digital signal across 1,000 or more carriers simultaneously transmitting the signal. The signals are sent at right angles to each other (orthogonal) so they do not interfere with each other.
The key element in Europe's decision to use COFDM is its ability to overcome multipath effects 100 percent. When a signal is transmitted, it is met with obstructions such as canyons, buildings, and even people, which scatter the signal causing it to take two or more paths to reach its final destination, the television. The late arrival of the scattered portions of the signal cause ghost images. Multipath effects can occur simply by an individual walking into the room. For this very reason, some consumers in metropolitan areas or areas with rugged terrain opt for cable or satellite television instead of fighting their antennas for better reception. COFDM is resistant to multipath effects because it uses multiple carriers to transmit the same signal. Instead of the signal scattering when met with an obstacle, it flows around the obstacle like a river flows around a rock making it perfect for free DTV programming and for mobile television viewing. Problems with multipath effects were often cited in early evaluations of 8-VSB, although it is expected that devices such as internal antennas will overcome them.
COFDM is ideal for Europe because stations transmit the same signal 100 percent of the time across many borders using single frequency networks. A single frequency network is a network of several stations that broadcast the same signal simultaneously using multiple transmitters. This allows television viewers to watch the same broadcast anywhere in Europe without interference. In NTSC-compliant countries, however, different programs along with local advertising are broadcast at different times throughout the day depending upon geographic location eliminating COFDM as a transmission scheme for the ATSC set of standards.
Conditional access (CA) is a technology used to control access to DTV services to authorized users by encrypting the transmitted programs. CA has been used for years for pay-TV services. There are numerous ATSC and DVB compliant CA systems available for a broadcaster to choose from. The CA system provider provides the equipment and software to the broadcaster who then implements the CA system into his equipment. CA is not designed solely for DTV. It can be used for digital radio broadcasts, digital data broadcasts, and non-broadcast information and interactive services. A CA system consists of several basic components:
- Subscriber Management System (SMS): The SMS is a subsystem of the CA system that manages the subscriber's information and requests entitlement management messages (EMM) from the Subscriber Authorization System (SAS). An EMM provides general information about the subscriber and the status of the subscription. The EMM is sent with the ECM, a data unit that contains the key for decrypting the transmitted programs.
- Subscriber Authorization System (SAS): The SAS is a subsystem of the CA system that translates the information about the subscriber into an EMM at the request of the SMS. The SAS also ensures that the subscriber's security module receives the authorization needed to view the programs, and the SAS also acts as a backup system in case of failure.
- Security module: The security module, usually in the form of a smart card, extracts the EMM and ECM necessary for decrypting the transmitted programs. The security module is either embedded within a set-top box or in a PC card that plugs into a set-top box.
- Set-top box: The set-top box houses the security module that gives authorization for decrypting the transmitted programs. The set-top box also converts the digital signal to an analogue signal so an older television can display the program.
The following describes the conditional access process:
- The receiver receives the digital data stream.
- The data flows into the conditional access module, which contains the content provider's unscrambling algorithms.
- The conditional access module verifies the existence of a smart card that contains the subscriber's authorization code.
- If the authorization code is accepted, the conditional access module unscrambles the data and returns the data to the receiver. If the code is not accepted, the data remains scrambled restricting access.
- The receiver then decodes the data and outputs it for viewing.
For years, smart cards (see smart card) have been used for pay TV programming. Smart cards are inexpensive, allowing the content provider to issue updated smart cards periodically to prevent piracy. Detachable PC cards allow subscribers to use DVB services anywhere DVB technology is supported.
There are two DVB protocols used by CA systems: SimulCrypt and MultiCrypt . SimulCrypt uses multiple set-top boxes, each using a different CA system, to authorize the programs for display. The different ECMs and EMMs required by each CA system are transmitted simultaneously. Each set-top box recognizes and uses the appropriate ECM and EMM needed for authorization. The ATSC standard uses SimulCrypt. MultiCrypt allows multiple CA systems to be used with one set-top box by using a PC card with an embedded smart card for each CA system used. Each card is then plugged into a slot in the set-top box. Each card recognizes the ECM and EMM needed for authorization.
There are 18 officially sanctioned standards for DTV display, all of which fall into one of these two categories: digital television sets are either HDTV-capable, or SDTV-capable. The sets can receive both types of broadcasts, but will convert signals received to their own resolution capabilities.
One important feature of the ATSC standards is the support of two scanning methods for the television screen: interlacing and progressive scanning. Interlacing takes two screen scans to complete a frame: one scan for the odd-numbered horizontal lines, and another scan for the even-numbered lines. Interlacing is important to some broadcasters. Progressive scanning, also called non-interlaced scanning, scans the lines sequentially. Progressive scanning is computer friendly.
In the United States, the FCC has assigned broadcast channels for DTV transmissions. In SDTV formats, DTV makes it possible to use the designated channels for multiple signals at current quality levels instead of single signals at HDTV levels, which would allow more programming with the same bandwidth usage. Commercial and public broadcast stations are currently deciding exactly how they will implement their use of ATV and HDTV.
HDTV (high definition television) is the high-end television display technology that provides picture quality similar to 35 mm. movies with sound quality similar to that of today's compact disc. Some television stations have begun transmitting HDTV broadcasts to users on a limited number of channels. HDTV generally uses digital rather than analog signal transmission, however, in Japan, the first analog HDTV program was broadcast on June 3, 1989. The first image to appear was the Statue of Liberty and the New York Harbor. It required a 20 Mhz channel, which is why analog HDTV broadcasting is not feasible in most countries.
HDTV offers a vertical resolution display from 720p to 1080i and higher. The p stands for progressive scanning , which means that each scan includes every line for a complete picture, and the i stands for interlaced scanning which means that each scan includes alternate lines for half a picture. These rates translate into a frame rate of up to 60 frames per second, twice that of conventional television. One of HDTV's most prominent features is its wider aspect ratio (the width to height ratio of the screen) of 16:9, a development based on research showing that the viewer's experience is enhanced by screens that are wider. HDTV pixel numbers range from one to two million, compared to SDTV's range of 300,000 to one million.
SDTV is a DTV format that provides a picture quality similar to DVD. SDTV has a range resolutions below those of HDTV and no defined aspect ratio. Because a compressed SDTV digital signal is smaller than a compressed HDTV signal, broadcasters can multicast or transmit up to five SDTV programs simultaneously instead of just one HDTV program. With today's analog television system, only one program at a time can be transmitted.
When the United States decided to make the transition from to DTV, the FCC decided to let broadcasters decide whether to broadcast SDTV or HDTV programs. Most have decided to broadcast SDTV programs in the daytime and to broadcast HDTV programs during prime time broadcasting. Both SDTV and HDTV are supported by both the DVB and ATSC set of standards.
The attraction of DTV is not only the clearer picture and higher quality sound, but the ability to interact with the television program that is broadcast. Interaction could include a viewer playing a game while watching an educational program, downloading a recipe from a favorite cooking show, or choosing a program to watch from an electronic program guide (EPG). An EPG is an onscreen television guide that displays channel and program information at no extra cost to the viewer. In DTV, the EPG is sent as a separate data packet in the MPEG-2 transport stream.
There are two types of EPGs: text-based and multimedia. A text-based EPG is similar to a printed television guide. A multimedia EPG allows the viewer to choose and record a program once, daily, or weekly at the touch of a button on the remote, sort through movies by theme, review top national and world news stories, and access comprehensive coverage of news, weather, sports, and entertainment stories. A typical multimedia EPG includes channel grids that contain present and future television programs along with a video window that displays a current broadcast from a selected channel and a brief description of the program.
In the United States, the FCC's DTV rollout schedule set deadlines for stations to complete the DTV transition process. Commercial television stations were supposed to have completed construction of DTV facilities by May 1, 2002. Public television stations were supposed to have completed their DTV facilities by May 1, 2003. Of course, extensions were granted based on individual circumstances. Once a station is DTV ready, that station may broadcast DTV programming. The FCC's schedule for transition to DTV originally proposed that everyone in the U.S. should have access to DTV by 2002, although analog transmissions continued after that date. After the switch to digital has been completed, regular television sets will need converters to receive broadcasts, although their sets will remain functional indefinitely. Actions are being taken to make the transition as smooth as possible. For example, in the Telecommunications Act of 1996, the FCC allotted to each existing broadcaster an additional 6MHz channel for digital transmissions, so that they could send out both analog and digital transmissions simultaneously. This procedure is known as simulcasting. The FCC mandated that all analog transmission would cease in 2009.
Simulcast is the simultaneous transmission of the same television program in both an analog and a digital version using two different channels or frequencies. At the end of the DTV transition period, analog transmission will cease and current analog channels will be used solely for DTV. The extra channels that were used for digital broadcasting will then be auctioned and used for more television channels or other services such as datacasting. Simulcast is also used for the transmission of simultaneous television and Internet services, the transmission of analog and digital radio broadcasts, and the transmission of television programs in different screen formats such as the traditional format and the wide screen format. Simulcast broadcasting is used worldwide.
The equipment needed for DTV depends on whether you use terrestrial, cable, or satellite services. Consumers will, at a minimum, have to purchase a converter to view DTV transmissions on their old television sets. In addition, consumers that use terrestrial services or antennas to receive television signals need an antenna equipped for digital signals. A consumer located in mountainous terrain in an ATSC-compliant country may not be able to receive terrestrial-based digital signals because of multipath effects. This is common even with today's analog television system. In DVB compliant countries, terrain does not affect the reception of digital signals. Satellite users are already enjoying DTV broadcasting, but a larger satellite dish might be needed to view HDTV programming.
A set-top box is a device that enables a television set to become a user interface to the Internet and also enables an analog television set to receive and decode DTV broadcasts. DTV set-top boxes are sometimes called receivers. It is estimated that 35 million homes will use digital set-top boxes by the end of 2006, the estimated year ending the transition to DTV.
A typical digital set-top box contains one or more microprocessors for running the operating system, usually Linux or Windows CE , and for parsing the MPEG transport stream. A set-top box also includes RAM, an MPEG decoder chip, and more chips for audio decoding and processing. The contents of a set-top box depend on the DTV standard used. DVB-compliant set-top boxes contain parts to decode COFDM transmissions while ATSC-compliant set-top boxes contain parts to decode VSB transmissions. More sophisticated set-top boxes contain a hard drive for storing recorded television broadcasts, for storing downloaded software, and for other applications provided by the DTV service provider. Digital set-top boxes can be used for satellite and terrestrial DTV but are used mostly for cable television. Set-top boxes are commonly leased from a service provider.
In the Internet realm, a set-top box is really a specialized computer that can "talk to" the Internet - that is, it contains a Web browser (which is really a Hypertext Transfer Protocol client) and the Internet's main program, TCP/IP. The service to which the set-top box is attached may be through a telephone line as, for example, with Web TV or through a cable TV company like TCI.
To take advantage of Dolby Digital 5.1 channel for satellite broadcasts, a satellite receiver that provides a Dolby Digital output is necessary. For cable users, all digital set-top boxes are equipped with a Dolby Digital two-channel decoder. To use 5.1 channel sound, a 5.1 channel-compliant set-top box is needed or an external 5.1 channel decoder unit.
The most dramatic demonstration of digital television's benefits is through a high-end HDTV, because of the larger screen, wider aspect ratio and better resolution. Like most new technologies, however, HDTV is expensive. Nevertheless, less expensive digital TVs provide a markedly improved viewing experience over regular TV, and for those who choose to retain their old sets, even the addition of a set-top converter will deliver a discernably improved picture and sound.
The FCC's schedule for transition to DTV proposed that everyone in the U.S. should have access to DTV by 2002 and that the switch to digital transmission must be completed either by 2006 or when 85% of the households in a specific area have purchased digital television sets or set-top converters. The transition is scheduled to complete in 2009.