Digital Cable
Digital television (DTV) 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. 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.
All digital TV variants can carry both standard-definition television (SDTV) and high-definition television (HDTV). All early SDTV television standards were analog in nature, and SDTV digital television systems derive much of their structure from the need to be compatible with analog television. In particular, the interlaced scan is a legacy of analog television. Attempts were made during the development of digital television to prevent a repeat of the fragmentation of the global market into different standards (that is, PAL, SECAM, NTSC). However, once again, the world could not agree on a single standard, and, hence, there are three major standards in existence: the European DVB system and the U.S. ATSC system, plus the Japanese system ISDB. For cable, in addition to ATSC standards, the SCTE standard is used to describe cable out-of-band metadata. Most countries in the world have adopted DVB, but several have followed the U.S. in adopting ATSC instead (Canada, Mexico, South Korea). Korea has adopted S-DMB for satellite mobile broadcasting. On June 29, 2006, after long debate, Brazil officially adopted the Japanese system (ISDB-T) mixed up with the SBTVD (short for "Sistema Brasileiro de Televisão Digital", or Brazilian Digital Television System). China has announced yet another standard, called DMB-T/H (GB 20600-2006), which itself consists of two other standards: ADTB-T (similar to ATSC-T) and a variant of T-DMB. There could be other specialised high-resolution digital video formats in the future for markets other than home entertainment. Ultra High Definition Video (UHDV) is a format proposed by NHK of Japan that provides a resolution 16 times greater than HDTV.