As the U.S. transition from analog television nears completion, most residents in the nation now have totally digital television service. While some people prefer digital stations broadcast over the air or through satellites, digital cable is the most popular type of service today. Most cable providers have already transitioned to an all-digital format, but not everyone understands what this means or how it differs from the older analog service.
Development of Digital Cable
Cable service is not much different than other types of digital TV. The only factor that makes it unique is that it is delivered to subscribers through coaxial or fiberoptic cables. Digital TV is widely regarded as the most significant enhancement to television since color programming came into use in the 1950s. This system allows for several features that many people have come to expect from cable TV, including high-definition (HD) service, electronic program guides (EPGs) and video on demand (VOD).
The first tests of digital TV were performed by General Instrument, now a subsidiary of Motorola, in 1989. General Instrument proved that analog cable signals could be converted to digital signals and then be converted back to analog before being displayed on analog televisions. In the 1990s, satellite providers quickly adopted digital technology to support satellite TV broadcasts, and cable companies were forced to invest in digital upgrades to stay competitive. As the nation moved into the 21st century, nearly all cable companies were offering digital service alongside analog service, and many subscribers were happy to experience the features that were possible through digital TV.
Features of Digital Cable
Digital TV service allows for several features that are not possible through analog service. These features include an increase in available channels, interactive menus and guides, and VOD. What makes these features possible are that digital signals use less bandwidth than analog and can be used for two-way communications.
The digital compression of television signals allows several channels to be carried in the bandwidth required for a single analog channel. These added channels are being used by broadcast stations to increase their offerings, and they are recognized by digital TV tuners through decimal identifiers. For example, a station broadcasting on a frequency recognized as channel 10 can now transmit up to 1,024 signals that are recognized as sub-channels 10.0 to 10.1023. However, because each channel has a limited amount of total bandwidth, broadcast stations are effectively limited to two or three sub-channels. Instead of offering completely different programming on each sub-channel, many stations use one sub-channel for standard-definition (SD) TV and one for HDTV.
Channels and sub-channels included in digital-cable packages are remapped into a number of virtual channels so that the decimals can be stripped and associated with whole numbers. For instance, sub-channels broadcast as 10.0 and 10.1 can be remapped as channels 45 and 502. Because cable companies can deliver over 1,000 channels to subscribers, HD programming is often remapped into a specific range of channels, such as 500 to 700.
Digital signals also have the benefit of two-way transmission. This means that signals can be sent from cable companies to subscribers, and outgoing signals can be sent by subscribers to their cable companies. This makes it possible for subscribers to choose VOD programs from a menu, and it allows them to control VOD playback by pausing, rewinding or fast-forwarding programs.
Electronic program guides (EPGs) also make use of two-way digital communications to provide subscribers with information about current and upcoming programs. With these guides, subscribers can perform searches, schedule reminders and choose favorite channels.
Digital Cable Resolution
Although early digital transmissions were converted from analog recordings or broadcasts, nearly all programs today are digitally encoded before reaching the cable companies. Digital programs can be encoded in four different resolutions. SD programs have a resolution of 480p. The 480 represents the number of pixels in each vertical line of resolution, and the p stands for progressive, which means that all rows of pixels are refreshed in each cycle.
The other three resolutions that may be used for digital programs are the HD resolutions. The lowest of these is 720p. At this resolution, programs are displayed in a grid of 1,280 pixels by 720 pixels. A step above 720p is 1080i. This resolution is 1,920 pixels by 1,080 pixels, but as an interlaced resolution, only half of the pixels are refreshed in each cycle. The highest quality of digital resolution is 1080p, which uses the same number of pixels as 1080i. Very few types of media deliver a resolution of 1080p, and most cable companies deliver resolutions of 720p or 1080i for HD programs.
Digital Cable Is Here to Stay
Digital cable has become an indispensable aspect of the lives of many television viewers. The benefits of the system are numerous, and the drawbacks are virtually nonexistent. As digital TV continues to develop, even more features may become available. Although consumers can no longer choose between digital and analog TV, a choice remains on how to receive the signals, and cable is still on top.