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Digital television
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Digital television (DTV) is the sending and receiving of moving images and sound by discrete (digital) signals, in contrast to the analog signals used by analog TV.
The first country to make a wholesale switch to Digital Over-the-Air (terrestrial) broadcasting was the Netherlands, in 2006. This was
followed by Finland and Sweden in 2007.
In the United States, full-power television stations are scheduled to change over to digital on June 12, 2009.
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Encyclopedia
Digital television (DTV) is the sending and receiving of moving images and sound by discrete (digital) signals, in contrast to the analog signals used by analog TV.
The first country to make a wholesale switch to Digital Over-the-Air (terrestrial) broadcasting was the Netherlands, in 2006. This was
followed by Finland and Sweden in 2007.
In the United States, full-power television stations are scheduled to change over to digital on June 12, 2009. By special dispensation, some analog TV signals will cease, as previously scheduled, on February 17, 2009. This has to do with station resistance to a sudden change in schedule, that will cost broadcasters money and cause logistical nightmares.
In Japan, the switch to digital is scheduled to happen July 24, 2011. In Canada, it is scheduled to happen August 31, 2011. China is scheduled to switch in 2015. In the United Kingdom, the digital switchover has different times for each part of the country; however, the whole of the UK will be digital by 2012. Brazil switched to digital in December 2, 2007 in major cities and it is estimated 7 years for complete signal expansion over all of the Brazilian territory.
While the majority of the viewers of over-the-air broadcasting in the USA watch full-power stations (which number about 1800), there are three other categories of TV stations in the USA: low-power stations, Class A stations, and TV translator stations. There is presently no deadline for these stations, about 7100 in number, to convert to digital broadcasting.
Technical information
Formats and bandwidth
Digital television supports many different picture formats defined by the combination of size, aspect ratio (height to width ratio) and
interlacing.
With terrestrial broadcasting in the USA the range of formats can be coarsely divided into two categories: HDTV and SDTV. It should be noted that these
terms by themselves are not very precise, and many subtle intermediate cases exist.
High-definition television (HDTV), one of several different formats that can be transmitted over DTV, uses one of two formats: 1280 × 720 pixels in progressive scan mode (abbreviated 720p) or 1920 × 1080 pixels in interlace mode (1080i). Each of these utilizes a 16:9 aspect ratio. (Some televisions are capable of receiving an HD resolution of 1920 × 1080 at a 60 Hz progressive scan frame rate — known as 1080p60, but this standard is not currently used for transmission.) HDTV cannot be transmitted over current analog channels.
Standard definition TV (SDTV), by comparison, may use one of several different formats taking the form of various aspect ratios depending on the technology used in the country of broadcast. For 4:3 aspect-ratio broadcasts, the 640 × 480 format is used in NTSC countries, while 720 × 576 (rescaled to 768 × 576) is used in PAL countries. For 16:9 broadcasts, the 704 × 480 (rescaled to 848 × 480) format is used in NTSC countries, while 720 × 576 (rescaled to 1024 × 576) is used in PAL countries. However, broadcasters may choose to reduce these resolutions to save bandwidth (e.g., many DVB-T channels in the United Kingdom use a horizontal resolution of 544 or 704 pixels per line). The perceived quality of such programming is surprisingly acceptable because of interlacing—the effective vertical resolution is halved to 288 lines.
Each commercial terrestrial DTV channel in North America is permitted to be broadcast at a data rate up to 19 megabits per second, or 2.375 megabytes per second. However, the broadcaster does not need to use this entire bandwidth for just one broadcast channel. Instead the broadcast can be subdivided across several video subchannels (aka feeds) of varying quality and compression rates, including non-video datacasting services that allow one-way high-bandwidth streaming of data to computers.
A broadcaster may opt to use a standard-definition digital signal instead of an HDTV signal, because current convention allows the bandwidth of a DTV channel (or "multiplex") to be subdivided into multiple subchannels (similar to what most FM stations offer with HD Radio), providing multiple feeds of entirely different programming on the same channel. This ability to provide either a single HDTV feed or multiple lower-resolution feeds is often referred to as distributing one's "bit budget" or multicasting. This can sometimes be arranged automatically, using a statistical multiplexer (or "stat-mux"). With some implementations, image resolution may be less directly limited by bandwidth; for example in DVB-T, broadcasters can choose from several different modulation schemes, giving them the option to reduce the transmission bitrate and make reception easier for more distant or mobile viewers.
Reception
There are a number of different ways to receive digital television. One of the oldest means of receiving DTV (and TV in general) is using an antenna (known as an aerial in some countries). This way is known as Digital Terrestrial Television (DTT). With DTT, viewers are limited to whatever channels the antenna picks up. Signal quality will also vary.
Other ways have been devised to receive digital television. Among the most familiar to people are digital cable and digital satellite. In some countries where transmissions of TV signals are normally achieved by microwaves, digital MMDS is used. Other standards, such as DMB and DVB-H, have been devised to allow handheld devices such as mobile phones to receive TV signals. Another way is IPTV, that is receiving TV via Internet Protocol, relying on DSL or optical cable line. Finally, an alternative way is to receive digital TV signals via the open Internet. For example, there is a lot of P2P Internet Television software that can be used to watch TV on your computer.
Some signals carry encryption and specify use conditions (such as "may not be recorded" or "may not be viewed on displays larger than 1 m in diagonal measure") backed up with the force of law under the WIPO Copyright Treaty and national legislation implementing it, such as the U.S. Digital Millennium Copyright Act. Access to encrypted channels can be controlled by a removable smart card, for example via the Common Interface (DVB-CI) standard for Europe and via Point Of Deployment (POD) for IS or named differently CableCard.
Protection parameters for terrestrial DTV broadcasting
Digital television signals must not interfere with each other, and they
must also coexist with analog television until it is phased out.
The following table gives allowable signal-to-noise and
signal-to-interference ratios for various interference scenarios. This table is a crucial
regulatory tool for controlling the placement and power levels of stations. Digital TV is more tolerant
of interference than analog TV, and this is the reason fewer channels are needed to carry an all-digital set of television stations.
System Parameters
(protection ratios) | Canada [13] | USA [5] | EBU [9, 12]
ITU-mode M3 | Japan & Brazil [36, 37] |
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| C/N for AWGN Channel | +19.5 dB
(16.5 dB) | +15.19 dB | +19.3 dB | +19.2 dB |
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| Co-Channel DTV into Analog TV | +33.8 dB | +34.44 dB | +34 ~ 37 dB | +38 dB |
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| Co-Channel Analog TV into DTV | +7.2 dB | +1.81 dB | +4 dB | +4 dB |
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| Co-Channel DTV into DTV | +19.5 dB
(16.5 dB) | +15.27 dB | +19 dB | +19 dB |
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| Lower Adjacent Channel DTV into Analog TV | −16 dB | −17.43 dB | −5 ~ −11 dB | −6 dB |
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| Upper Adjacent Channel DTV into Analog TV | −12 dB | −11.95 dB | −1 ~ −10 | −5 dB |
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| Lower Adjacent Channel Analog TV into DTV | −48 dB | −47.33 dB | −34 ~ −37 dB | −35 dB |
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| Upper Adjacent Channel Analog TV into DTV | −49 dB | −48.71 dB | −38 ~ −36 dB | −37 dB |
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| Lower Adjacent Channel DTV into DTV | −27 dB | −28 dB | −30 dB | −28 dB |
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| Upper Adjacent Channel DTV into DTV | −27 dB | −26 dB | −30 dB | −29 dB |
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Interaction
Interaction happens between the TV watcher and the DTV system. It can be understood in different ways, depending on which part of the DTV system is concerned. It can also be an interaction with the STB only (to tune to another TV channel or to browse the EPG).
Modern DTV systems are able to provide interaction between the end-user and the broadcaster through the use of a return path. With the exceptions of coaxial and fiber optic cable, which can be bidirectional, a dialup modem, Internet connection, or other method is typically used for the return path with unidirectional networks such as satellite or antenna broadcast.
In addition to not needing a separate return path, cable also has the advantage of a communication channel localized to a neighborhood rather than a city (terrestrial) or an even larger area (satellite). This provides enough customizable bandwidth to allow true video on demand.
Advantages to conversion
DTV has several advantages over analog TV, the most significant being that digital channels take up less bandwidth (and the bandwidth needs are continuously variable, at a corresponding cost in image quality depending on the level of compression). This means that digital broadcasters can provide more digital channels in the same space, provide high-definition television service, or provide other non-television services such as multimedia or interactivity. DTV also permits special services such as multiplexing (more than one program on the same channel), electronic program guides and additional languages, spoken or subtitled. The sale of non-television services may provide an additional revenue source.
Digital signals react less fiercely to interference than analog signals. For example, common problems with analog television include ghosting of images, noise from weak signals, and many other potential problems. Digitized signals don't suffer from ghosting or noise because DTV Tuners and converter boxes receive numeric information by the antenna. The decoder only needs enough information to put the picture together. The only way it fails is when the decoder does not receive enough information from the antenna - there is too much interference in the signal for the decoder to read the number and produce the picture.
Effect on existing analog technology
The analog switch-off ruling, which so far has met with little opposition from consumers or manufacturers, would render all non-digital televisions obsolete on the switch-off date unless connected to an external off-the-air tuner, analog or digital cable, or a satellite system. An external converter box can be added to non-digital televisions to lengthen their useful lifespan. Several of these devices have already been shown and, while few were initially available, they are becoming more available by the day. In the United States, a government-sponsored coupon is available to offset the cost of an external converter box. Once connected to the converter unit, operation of non-digital units is achievable and, in most cases, rich in new features (in comparison to previous analog reception operation). At present, analog switchoff is scheduled for June 12, 2009 in the United States, August 31, 2011 in Canada, July 24, 2011 in Japan and 2012 in the United Kingdom, October 14, 2009 in some regions of North-Italy.
Some existing analog equipment will be less functional with the use of a converter box. For example, television remote controls will no longer be effective at changing channels, because that function will instead be handled by the converter box. Similarly, video recorders for analog signals (including tape-based VCRs, DVD recorders and hard-drive DVRs) will not be able to automatically select channels, limiting their ability to automatically record programs via a timer or based on downloaded program information. DTV-capable VCR tuners are also likely to be far less common than their analog-only counterparts, with most current digital offerings being VCR/DVD combo units.
Older handheld televisions, which rely primarily on over-the-air signals and battery operation, will be rendered impractical since most converter boxes are not portable nor powered with batteries and many portable televisions do not have the proper connectors to allow the use of a converter box. The additional power consumption of the converter limits portability for the few converter models (such as the Artec T3A or Winegard RCDT09A) which can operate from bulky external battery packs. Portable radios that are currently able to listen to frequency-modulated broadcast television audio would lose this ability.
A new TV containing only an ATSC tuner would be impractical, as this could prevent older devices such as VCRs and video game consoles with analog-only output from connecting to the TV. Connection would require an analog to digital converter box, which is the opposite of what is currently being sold. Such a box would be prohibitive in cost and also likely introduce additional delay into the video signal. Analog inputs suitable for connection to VCRs have therefore been retained on all current digital-capable TV's.
Environmental issues
The adoption of a broadcast standard incompatible with existing analog receivers has created the problem of large numbers of analog receivers being discarded during digital television transition. An estimated 99 million unused analog TV receivers are currently in storage in the US alone and, while some obsolete receivers are being retrofitted with converters, many more are simply dumped in landfills where they represent a source of toxic metals such as lead as well as lesser amounts of materials such as barium, cadmium and chromium.
While the glass in some cathode ray tubes may contain up to eight pounds (3.6 kg) of lead, which can have long-term negative effects on the environment if dumped as landfill, the glass envelope can be recycled at suitably-equipped facilities. Other portions of the receiver may be subject to disposal as hazardous material.
Local restrictions on disposal of these materials vary widely; in some cases second-hand stores have refused to accept working colour television receivers for resale due to the increasing costs of disposing of unsold TV's. Those thrift stores which are still accepting donated TV's have reported significant increases in good-condition working used television receivers abandoned by viewers who often expect them not to work after digital transition.
In Michigan, one recycler has estimated that as many as one household in four will dispose of or recycle a TV set in the next year. The digital television transition, migration to high-definition television receivers and the replacement of CRT's with flatscreens are all factors in the increasing number of discarded analog CRT-based television receivers.
Technical limitations
Compression artifacts and allocated bandwidth
DTV images have some picture defects that are not present on analog television or motion picture cinema, because of present-day limitations of bandwidth and compression algorithms such as MPEG-2.
When a compressed digital image is compared with the original program source, some hard-to-compress image sequences may have digital distortion or degradation. For example:
- quantization noise,
- incorrect color,
- blockiness,
- a blurred, shimmering haze.
Due to the lossy Discrete Cosine Transform compression used, the quantization noise is not uniformly distributed but tends to appear more near sharp edges (especially text and drawn lines as in cel animation), making it more noticeable than uniform Gaussian noise of comparable peak magnitude. Due to the motion-predictive temporal-differential encoding used, the quantization noise is increased in scenes with a lot of motion, especially motion that is fast, random, and/or complex (with many independent parts of the image moving differently.) (This is because the motion makes the encoding less efficient, so to compensate more data needs to be discarded by using coarser quantization.) In addition to pixellated noise near edges in the image, the quantization noise may also appear as banding in smooth shaded and gradient areas.
Because of the way the human visual system works, defects in an image that are localized to particular features of the image or that come and go are more perceptible than defects that are uniform and constant. However, the DTV system is designed to take advantage of other limitations of the human visual system to help mask these flaws, e.g. by allowing more artifacts during fast motion where the eye cannot track and resolve them as easily and, conversely, minimizing artifacts in still backgrounds that may be closely examined in a scene (since time allows).
Broadcasters attempt to balance their desires to show high quality pictures and to generate revenue by using a fixed bandwidth allocation for more services. The fact that the video entertainment industry is highly competitive and the observation that most viewers don't seem highly concerned about image quality tend to ensure that the quality of broadcast DTV pictures is substantially less than the optimal quality the system can technically support.
DVD Video, which also uses the MPEG-2 codec, has these same types of flaws. The same is true of the Dish Network (ECHOStar) DBS system, where the compression of standard-definition channels is heavy and artifacts are more noticeable.
Buffering and preload delay
Unlike analog televisions, digital televisions have a significant delay when changing channels, making "channel surfing" more difficult.
Different devices need different amounts of preload time to begin showing the broadcast stream, resulting in an audio echo effect when two televisions in adjacent rooms of a house are tuned to the same channel.
Effects of poor reception
Changes in signal reception from factors such as degrading antenna connections or changing weather conditions may gradually reduce the quality of analog TV. The nature of digital TV results in a perfect picture initially, until the receiving equipment starts picking up noise or losing signal. Some equipment will show a picture even with significant damage, while other devices may go directly from perfect to no picture at all (and thus not show even a slightly damaged picture). This latter effect is known as the digital cliff or cliff effect.
For remote locations, distant channels that as analog signals were previously usable in a snowy and degraded state may as digital signals be perfect or may become completely unavailable. In areas where transmitting antennas are located on mountains, viewers who are too close to the transmitter may find reception difficult or impossible because the strongest part of the broadcast signal passes above them. The use of higher frequencies will add to these problems, especially in cases where a clear line-of-sight from the receiving antenna to the transmitter is not available. Many intermittent signal fading conditions, such as the rapid-fade effect caused by reflections of UHF television signals from passing aircraft, will not produce intermittently-snowy video, but potential intermittent loss of the entire signal, which most receivers will display as a frozen ("paused") image or a black screen for the duration of the signal loss.
Multi-path interference is a much more significant problem for DTV than for analog TV and affects reception, particularly when using simple antennas such as rabbit ears. This is perceived as "ghosting" in the analog domain, but this same problem manifests itself in a much more insidious way with DTV. (What was "ghosting" in analog becomes intersymbol interference (ISI), which causes data corruption, in digital TV. Beyond a certain point, corrupt data is as good as no data.) IEEE engineers recommend using an attic or outdoor antenna for DTV, if possible, rather than an indoor antenna, because reflections and other interactions of the signal with objects (including bodies) in the room will increase multipath interference. Unlike the problems of the preceding paragraph, multi-path can be worse for DTV under high signal conditions. It is perceived by the viewer as a spotty loss of audio or picture freezing and pixelation as people move about in the vicinity of the antenna and is often worse in wet weather due to increased reflection or re-polarization of the DTV signal arriving from multiple paths. In extreme cases the signal is lost completely. The cure is to employ a directional antenna outdoors, aligned with the transmitting location.
Dynamic multipath interference, in which the delay and magnitude of reflections are rapidly changing, is particularly problematic for digital reception. While this just produces moving and changing ghost images for analog TV, it can render a digital signal impossible to decode. The 8VSB-based standards in use in North American ATSC broadcasts are particularly vulnerable to problems from dynamic multipath; this has the potential to severely limit mobile or portable use of digital television receivers. Solving the problem might require that different standards be adopted for mobile use.
Limitations
The greatest DTV detail level currently available is 1080i, which is a 1920 × 1080 interlaced widescreen format. Interlacing is done to reduce the image bandwidth to one-half of full-frame quality, which gives better frame update speed for quick-changing scenes such as sports, but at the same time reduces the overall image quality and introduces image flickering and "crawling scanlines" because of the alternating field refresh.
Full-frame progressive-scan 1920 × 1080 (1080p) is not part of the ATSC specification. High frame-rate 1080p may become an option in the near future, as a result of recent technology advances such as H.264/MPEG-4 AVC video coding, allowing more detail to be sent via the same channel bandwidth allocations that are used now.
The limitations of interlacing can be partially overcome through the use of advanced image processors in the consumer display device, such as the use of Faroudja DCDi and using internal frame buffers to eliminate scanline crawling.
Conversion
As of late 2007, six countries had completed the process of turning off analog terrestrial broadcasting. Many other countries had plans to do so or were in the process of a staged conversion. IEEE in Jamaica also decided to switch over at the same time as the US.
See also
External links
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