DTV has several advantages over
analog television, • More efficient bandwidth usage provide more digital channels in the same space and/or provide
high-definition television service • Digital TV signals require less transmission power than analog TV signals to be broadcast and received satisfactorily. • Flexible bandwidth allocations are flexible depending on the level of compression and resolution of the transmitted image. • More sound channels. Analog TV began with monophonic sound and later developed
multichannel television sound with two independent audio signal channels. DTV allows up to 5 audio signal channels plus a
subwoofer bass channel, producing broadcasts similar in quality to movie theaters and DVDs. • Can provide for sale of non-television services such as multimedia on demand or interactive purchasing. • Permits special services such as multiplexing (more than one program on the same channel), electronic program guides and additional languages (spoken or subtitled). Digital and analog signals react to interference differently. For example, common problems with analog television include
ghosting of images, noise from weak signals and other problems that degrade the quality of the image and sound, although the program material may still be watchable. With digital television, because of the
cliff effect, reception of the digital signal must be very nearly complete; otherwise, neither audio nor video will be usable.
Compression artifacts, picture quality monitoring and allocated bandwidth DTV images have some picture defects not present on analog television or motion picture cinema, because of present-day limitations of bit rate and compression algorithms such as
MPEG-2. This defect is sometimes referred to as
mosquito noise. 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
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. Quantising effects can create contours—rather than smooth gradations—on areas with small graduations in amplitude. Typically, a very
flat scene, such as a cloudless sky, will exhibit visible
steps across its expanse, often appearing as concentric circles or ellipses. This is known as
color banding. Similar effects can be seen in very dark scenes, where true black backgrounds are overlaid by dark gray areas. 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.
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 perfectly decodable video initially, until the receiving equipment starts picking up interference that overpowers the desired signal or if the signal is too weak to decode. Some equipment will show a garbled picture with significant damage, while other devices may go directly from perfectly decodable video to no video at all or lock up. This phenomenon is known as the digital cliff effect.
Block errors may occur when transmission is done with compressed images. A block error in a single frame often results in black boxes in several subsequent frames, making viewing difficult. For remote locations, distant channels that, as analog signals, were previously usable in a snowy and degraded state may, as digital signals, be perfectly decodable or may become completely unavailable. The use of higher frequencies add to these problems, especially in cases where a clear line-of-sight from the receiving antenna to the transmitter is not available because usually higher frequency signals can't pass through obstacles as easily.
Effect on old analog technology Television sets with only analog tuners cannot decode digital transmissions. 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.
Disappearance of TV-audio receivers Prior to the conversion to digital TV, analog television broadcast audio for TV channels on a separate FM
carrier signal from the video signal. This FM audio signal could be heard using standard radios equipped with the appropriate tuning circuits. However, after the
digital television transition, no portable radio manufacturer has yet developed an alternative method for portable radios to play just the audio signal of digital TV channels;
DTV radio is not the same thing.
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. 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 Michigan in 2009, one recycler estimated that as many as one household in four would dispose of or recycle a TV set in the following year. The digital television transition, migration to
high-definition television receivers and the replacement of CRTs with flat screens are all factors in the increasing number of discarded analog CRT-based television receivers. 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. == See also ==