Analog to Digital TV: How to “Get” HDTV
What are you really looking at when you gaze at a color TV screen or a newspaper photo? On a TV screen or in a newspaper or a magazine picture, or even the computer monitor screen on which you read this text, the images are formed by groups of red, green and blue dots or “pixels” (short for picture elements). In the case of a conventional television display, the groups of dots glow in varying degrees of brightness and hues, triggered by electrical pulses from the TV set's electronics. These pulses perfectly track the signals broadcast by the television station or cable system that routes the TV signals into your home.
If we sit far enough away from the TV screen, the individual clusters of colored dots blend together to form a smooth and coherent color picture that changes 30 times every second. Our eyes and brain blend the rapidly changing still images into continuous and smooth action, through a process called “persistence of vision.”
Lines and Lines The 60-year-old analog TV system all of us grew up watching is based on 525 horizontal scanning lines, although only 480 lines are actually used to scan the picture from top to bottom across the TV screen every 1/30 of a second. A single frame of TV is composed of two “fields” of 240 lines each, presented every 1/60 of a second, which are “ interlaced ” or interwoven to form a full picture every 1/30 second. This happens so quickly that our eyes blend the 60 still fields together into 30 frames (each frame is a still picture) and we see only continuous motion.
Definition: Standard and High Because the image is composed of 480 interlaced lines, the system is called 480i, and is known as standard definition in the world of digital TV. A DVD image represents standard definition. The number of scanning lines determines the amount of detail in the image (vertical detail, because the horizontal lines are stacked from the top to the bottom of the screen). Understanding this process will help you understand the differences in clarity or resolution (sharpness) between regular analog TV and digital TV, including High Definition TV (HDTV). Incidentally, just because a new TV is “digital” does NOT mean it is HDTV. Although all HDTV pictures are digital, not all digital TV is high-definition, because the North American Digital TV standard, established in 1995 by the Advanced Television Systems Committee (ATSC), enables 18 different formats and various levels of clarity— Standard Definition , Enhanced Definition (EDTV), and High Definition (HDTV).
If you get a digital TV, a DVD's 480-line image can also be progressively scanned, one line after the other in sequence, rather than interlaced. This is called 480p, and represents Enhanced Definition (EDTV). A 480p TV image has a smoother, more film-like look, especially when it's viewed on a larger screen, but a digital TV set is usually required to display progressive-scan (480p) pictures (most analog sets will not, except for a few equipped with component-video inputs that are designed to handle progressive-scan signals.) Almost all newer DVD players on the market, even inexpensive ones, will output both interlaced and progressive-scan signals, so if you are getting a new DVD player, make sure it has progressive-scan outputs as well as interlaced (you must switch your DVD player to the interlaced output to see a DVD picture on your old analog set.)
Σάββατο 1 Σεπτεμβρίου 2007
Digital Television (DTV)
Digital Television (DTV) is a form of television broadcast that utilizes digital data and signals to transmit audio and video programming to viewers through cable and satellite transmission mechanisms. Digital television uses compressed digital data for faster transmission. The compressed data must be decoded by the television set, receiver or TV-enabled computer.
DTV takes up less bandwidth than analog TV, enabling broadcasters to provide a greater number of channels, multicasting, digital media guides, interactive services, pay services, and high-definition TV service.
DTV takes up less bandwidth than analog TV, enabling broadcasters to provide a greater number of channels, multicasting, digital media guides, interactive services, pay services, and high-definition TV service.
Interactive television (ITV)
Interactive television (ITV) can be defined as “TV that is controlled by the viewer, whether this implies interprogram or intraprogram decision making capabilities� (Hodge 1995:3). There has been talk of interactive television of some sort or another, and even though there has been a slow progress, it is not as radical as analysts predicted. The remote control, VCR, pay-per-view, early teletext services like the British Ceefax, and now digital TV and PVRs (such as TiVo) has each contributed to make television more convergent and interactive- one such company specialising in this specific field is Open TV. One issue concerning many large media companies is where this interactive television will leave advertisers? Viewers having the power to watch only what they want to watch, TiVo letting people record favourite programs minus the ads? The advertising industry will change. It's just a question of how quickly and to what extent. The television is, in these days, also increasingly used as a monitor for other media such as game consoles (PlayStation, Xbox, Game cube etc.), DVDs and VCRs. Furthermore, video on demand (VOD) has a high degree of interactivity between the viewer and the material being viewed by having the potential of providing individual television viewers virtually on the spot access to a wide range of recorded movies, video programs, games, information and other services.
history of television
When we look at the history of television it is a topic well documented, but in the search for television’s convergent history, it is a subject less written about. Maybe the first real step towards a more integrated television set came from teletext technology. Teletext is a one-way non-interactive information retrieval service. A fixed number of information pages are repetitively broadcast on unused portions of a TV channel bandwidth. With these broadcasts a decoder at the television set is used to select and display pages. The ability to send simple Text and Graphics as part of a television signal was discovered by BBC engineers in the mid 1970's. Teletext was first screened on Australian and New Zealand Television on the 1st of February 1984. Although Teletext was originally intended as a service for the deaf and hearing impaired, the popularity of the service has grown, and today there are more than 7 million viewers in Australia and New Zealand alone. The UK is recording figures as high as 18 million people using the teletext service every week, a figure surprising for a technology originally intended for the hearing impaired.
Now with television taking the walk down digital lane, it has begun to converge with other mediums in leaps and bounds. This path from analogue to digital is a bumpy one being smoothed by the set-top box. A set-top box is a device that enables a television set to become a user interface to the Internet and also enables a television set to receive and decode digital television (DTV) broadcasts. DTV set-top boxes are sometimes called receivers. A set-top box is necessary to television viewers who wish to use their current analogue television sets to receive digital broadcasts. To some extent, set-top boxes are lessening the digital divide widened by the transition to a digital television system. Instead of a person having to buy a new digital television set to remain able to receive a television broadcast, one can now simply buy a much less expensive set-top box and convert the digital signal to their analogue television set. 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.
Now with television taking the walk down digital lane, it has begun to converge with other mediums in leaps and bounds. This path from analogue to digital is a bumpy one being smoothed by the set-top box. A set-top box is a device that enables a television set to become a user interface to the Internet and also enables a television set to receive and decode digital television (DTV) broadcasts. DTV set-top boxes are sometimes called receivers. A set-top box is necessary to television viewers who wish to use their current analogue television sets to receive digital broadcasts. To some extent, set-top boxes are lessening the digital divide widened by the transition to a digital television system. Instead of a person having to buy a new digital television set to remain able to receive a television broadcast, one can now simply buy a much less expensive set-top box and convert the digital signal to their analogue television set. 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.
Formats and bandwidth
[edit] Formats and bandwidth
In current practice, HDTV 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 60Hz progressive scan frame rate—known as 1080p60—but many broadcasters are not able to transmit these signals over the air at acceptable quality, due to economic bandwidth constraints.)
Standard definition TV, 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 704 × 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 704 × 576 (rescaled to 1024 × 576) is used in PAL countries. 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, 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". 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 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.
[edit] 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 with guaranteed quality of service (QoS). Finally, an alternative way is to receive TV signals via the open Internet infra-structure, usually referred to as Internet TV.
Today, regardless of how viewers receive DTV, most will pick up digital television via a set-top box, which decodes the digital signals into signals that analog televisions can understand—thus using the television purely as a monitor. However, a growing number of TV sets with integrated receivers are available—these are known as iDTVs.
Some signals carry encryption and specify use conditions (such as "may not be recorded" or "may not be viewed on displays larger than 1m 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.
In current practice, HDTV 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 60Hz progressive scan frame rate—known as 1080p60—but many broadcasters are not able to transmit these signals over the air at acceptable quality, due to economic bandwidth constraints.)
Standard definition TV, 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 704 × 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 704 × 576 (rescaled to 1024 × 576) is used in PAL countries. 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, 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". 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 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.
[edit] 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 with guaranteed quality of service (QoS). Finally, an alternative way is to receive TV signals via the open Internet infra-structure, usually referred to as Internet TV.
Today, regardless of how viewers receive DTV, most will pick up digital television via a set-top box, which decodes the digital signals into signals that analog televisions can understand—thus using the television purely as a monitor. However, a growing number of TV sets with integrated receivers are available—these are known as iDTVs.
Some signals carry encryption and specify use conditions (such as "may not be recorded" or "may not be viewed on displays larger than 1m 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.
Παρασκευή 29 Ιουνίου 2007
Digital television pedia
) is a telecommunication system for broadcasting and receiving moving pictures and sound by means of digital signals, in contrast to analog signals used by analog (traditional) TV. DTV uses digital modulation data, which is digitally compressed and requires decoding by a specially designed television set, or a standard receiver with a set-top box, or a PC fitted with a television card. Introduced in the late 1990s, this technology appealed to the television broadcasting business and consumer electronics industries as offering new financial opportunities.
Digital television has several advantages over traditional analog TV, the most significant being that digital channels take up less bandwidth space. This means that digital broadcasters can provide more digital channels in the same space, provide High-Definition digital service, or provide other non-television services such as pay-multimedia services or interactive services. Digital television also permits special services such as multicasting (more than one program on the same channel) and electronic program guides. The sale of non-television services may provide an additional revenue source. As well, digital television often has a superior image, improved audio quality, and better reception than analog.
However, digital television picture technology is still in its early stages. Digital television images have some picture defects that are not present on analog television or motion picture cinema, due to present-day limitations of bandwidth and the compression algorithms such as MPEG-2. When a compressed digital image is compared with the original program source, such as a 35mm motion-picture film print, some digital image sequences may have distortion or degradation such as quantization noise, incorrect color, blockiness when high-speed motion is depicted, or a blurred, shimmering haze.
Digital television has several advantages over traditional analog TV, the most significant being that digital channels take up less bandwidth space. This means that digital broadcasters can provide more digital channels in the same space, provide High-Definition digital service, or provide other non-television services such as pay-multimedia services or interactive services. Digital television also permits special services such as multicasting (more than one program on the same channel) and electronic program guides. The sale of non-television services may provide an additional revenue source. As well, digital television often has a superior image, improved audio quality, and better reception than analog.
However, digital television picture technology is still in its early stages. Digital television images have some picture defects that are not present on analog television or motion picture cinema, due to present-day limitations of bandwidth and the compression algorithms such as MPEG-2. When a compressed digital image is compared with the original program source, such as a 35mm motion-picture film print, some digital image sequences may have distortion or degradation such as quantization noise, incorrect color, blockiness when high-speed motion is depicted, or a blurred, shimmering haze.
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