Digital television
It typically uses a widescreen aspect ratio (commonly 16:9) in contrast to the narrower format (4:3) of analog TV.
[9] In the mid-1980s, Toshiba released a television set with digital capabilities, using integrated circuit chips such as a microprocessor to convert analog television broadcast signals to digital video signals, enabling features such as freezing pictures and showing two channels at once.
In 1986, Sony and NEC Home Electronics announced their own similar TV sets with digital video capabilities.
[8] In the mid-1980s, as Japanese consumer electronics firms forged ahead with the development of HDTV technology, and as the MUSE analog format was proposed by Japan's public broadcaster NHK as a worldwide standard.
Between 1988 and 1991, several European organizations were working on DCT-based digital video coding standards for both SDTV and HDTV.
[9][12] An American company, General Instrument, also demonstrated the feasibility of a digital television signal in 1990.
It also argued that progressive scanning enables easier connections with the Internet and is more cheaply converted to interlaced formats than vice versa.
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.
A broadcaster may opt to use a standard-definition (SDTV) digital signal instead of an HDTV signal, because current convention allows the bandwidth of a DTV channel (or "multiplex") to be subdivided into multiple digital subchannels, (similar to what most FM radio stations offer with HD Radio), providing multiple feeds of entirely different television programming on the same channel.
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 bit rate and make reception easier for more distant or mobile viewers.
One of the oldest means of receiving DTV (and TV in general) is from terrestrial transmitters using an antenna (known as an aerial in some countries).
In some countries where transmissions of TV signals are normally achieved by microwaves, digital multichannel multipoint distribution service is used.
Modern DTV systems sometimes use a return path providing feedback from the end user to the broadcaster.
Some of these systems support video on demand using a communication channel localized to a neighborhood rather than a city (terrestrial) or an even larger area (satellite).
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 compression artifacts during fast motion where the eye cannot track and resolve them as easily and, conversely, minimizing artifacts in still backgrounds that, because time allows, may be closely examined in a scene.
Broadcast, cable, satellite and Internet DTV operators control the picture quality of television signal encoders using sophisticated, neuroscience-based algorithms, such as the structural similarity index measure (SSIM) video quality measurement tool.
Another tool called visual information fidelity (VIF), is used in the Netflix VMAF video quality monitoring system.
Typically, a very flat scene, such as a cloudless sky, will exhibit visible steps across its expanse, often appearing as concentric circles or ellipses.
These transitions may be smooth, or may show a scattering effect as the digital processing dithers and is unable to consistently allocate a value of either absolute black or the next step up the greyscale.
When analog broadcasting over the air ceases, users of sets with analog-only tuners may use other sources of programming (e.g., cable, recorded media) or may purchase set-top converter boxes to tune in the digital signals.
In the United States, a government-sponsored coupon was available to offset the cost of an external converter box.
The digital television transition began around the late 1990s and has been completed on a country-by-country basis in most parts of the world.
One superintendent of public works was quoted in 2009 saying; "some of the studies I’ve read in the trade magazines say up to a quarter of American households could be throwing a TV out in the next two years following the regulation change.
In 2009, an estimated 99 million analog TV receivers were sitting unused in homes 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.
