The change to the new modality was in late 1998. The system remained in force until then, was established in the forties and fifties by the National Television Systems (NTSC). The change has followed a slow and often controversial.
Officials from the Federal Communications Commission (FCC), television broadcasting channel, manufacturers and academics trying to create a digital standard that does not immediately cease existing televisions obsolete.
The new system operates mostly in the range from 470 to 890 MHz (channels 14 to 83) and UHF frequencies). The old and the new system will coexist until 2006, when emissions should cease NTSC signals, both in the band from 54 to 216 MHz (channels 2 to 13), as in UHF and VHF frequencies.
The FCC then reallocates those channels to digital television
The FCC established in 1987 an advisory committee on advanced television services, the ACATS, which was to advise the FCC on advanced television service in the U.S., including the preparation of a technical standard.
In 1988, the ACATS requested by industry, academia and laboratories to propose standards for advanced television.
In March 1990 the FCC took a major step. Decided that the advanced television service would be given under simulcasting (simulcast) with the conventional service and not regime compatible receivers (the latter was the approach adopted to introduce color television, in which the signal should be possible to see both in color television sets and black and white). In the regime of compatibility of receivers, the signal of high definition television (HDTV) could be captured and displayed in conventional current receivers. But the HDTV signal requires much more information than a color signal, so the receiver would require an additional channel to enter additional information (another 6 MHz channel).
This poses several problems:
- The HDTV signal transmitted by NTSC channel will have a very efficient system, some modern and unprofitable.
- You have to assign a new channel for each existing NTSC channel.
For these reasons, is why I opted for the simulcast approach. The HDTV signal is transmitted by a 6 MHz channel itself regardless of the NTSC signal (instead of using compatible receivers, where the HDTV signal is obtained from the NTSC signal and the information that goes into the supplemental channel). That could be equipped modern transmission system for full HDTV signal.
However there is still a drawback that existing televisions can not receive an HDTV signal. To prevent these TVs are suddenly useless, the FCC assigned a new channel for service to each of the 1,500 U.S. stations, upon their request. During a transition period, the FCC would require the same program was broadcast simultaneously (or with little delay) both for HDTV, for NTSC (later abolished this requirement). When a large part of the country already use the new TV, NTSC service would be deleted, and the portion of spectrum occupied by new channels would be used for HDTV or other services.
This decision had a decisive impact on the development of a standard for HDTV.
Shortly afterwards they began to receive proposals for HDTV systems, and the ACATS and the FCC decided to submit five proposals evaluation techniques: one analog and four digital. These technical proposals were discussed at the Center for Advanced Television Testing Alexandria, while the image quality was assessed at the Laboratory for Advanced Television Evaluation Ottawa.
In February 1993, after reviewing the ACATS results concluded that the four digital systems to analog exceeded in performance. In turn, each of the four excelled in various aspects. So the ACATS encouraged the promoters to organize into one system the best of the four elements and evaluated.
In May 1993, formed the Grand Alliance, a consortium of AT & T, Zenith, the research center David Sarnoff, General Instrument Corporation, Massachusetts Institute of Technology (MIT), Philips Electronics North America, and France's Thomson Consumer Electronics .
Between 1993 and 1994, the Grand Alliance introduced improvements on the best technical elements of the four systems and created a prototype. Based on this prototype HDTV, the commission of Advanced Television Systems (industry consortium) created a technical standard.
To be transported in a 6 MHz channel (about 20 Mbps) all information of a high definition image data to be compressed (if required to compress the order of Gbps). The Grand Alliance proposal was based on the MPEG2 system.
The key system according to the MPEG compression is not to send larger images (as in NTSC) but only the changes between these images. The result is that much less data are needed to update an image. The compressed video data, audio and other are multiplexed to form a single string of bits. This sequence of bits modulates a signal that is transmitted by terrestrial broadcasting.
On receiving the signal is captured by an antenna and sent to a receiver, which demodulate the signal to obtain the original bit sequence. These bits are demultiplex and recover the compressed data to pass to decompress below.
In November 1995 the FCC ACATS recommended that the standard developed by the Committee on Advanced Television Systems, and this was accepted in 1996 except for one thing. Eased restrictions on the rule that is limited to 18 formats of video resolution approved.
In early 1997, the FCC added other provisions to support the new technical standard, such as allocating canales.El digital television system based on this standard is very flexible, allowing for example a 6MHz channel can provide images high resolution multichannel surround sound, or transmit several television programs of comparable quality to the current programs. This flexibility has made to replace the acronym for High Definition Television (HDTV) for Digital Television (DTV). Besides the standard is open so that you can incorporate future technical improvements.
Historical Evolution in Europe
Digital TV services by satellite began in 1996, with uneven development as the implementation strategy followed, with the most dramatic results in France, with three platforms and more than a million subscribers as a whole.
The terrestrial broadcasting started before the end of 1998 in the United Kingdom and Sweden. The DVB is promoting its system outside Europe DVB-T for terrestrial broadcasting as a flexible system, capable of HDTV, adaptable to different channels of bandwidth and capable of being used in frequency network coverage of an entire country. This promotion is geared mainly to China, Southeast Asia, Australia, Brazil and Argentina.
The UK has taken a leading role in Europe for the development of digital terrestrial TV (DTT). In the United Kingdom have enabled 6 multiplexes which have been distributed among the existing broadcasters and a trading platform. The broadcasters have been given half multiplex capacity equivalent to 2 programs for existing analog channel. Thus, the BBC has obtained a full multiplex, ITV, Channel 4 and Channel 5, half multiplexes each respectively, and S4C, the Welsh regional broadcaster, otherwise multiplex with exclusive coverage of Wales. For its part, the BDB trading platform, consisting of groups Carlton and Granada, has won three national coverage multiplex and, moreover, half multiplex coverage across the UK, except Wales, has been awarded to the consortium SDN, formed by S4C, NTL and United News and Media.
In parallel with the launch of DTT, there is the beginning of the digital services of BSkyB. The situation is extremely interesting, because this simultaneous launch of digital terrestrial TV services and satellite are added some cable operators are proceeding to digitize their networks. This means that the end user will have to choose which channel of distribution prefers that influence not only the intrinsic merits of each form of distribution, but also the attractiveness of content and forms of grant of user or boxes receivers that offer the different platforms.
Sweden also launched DTT services in early 1999, with two multiplexes and a coverage of 50% of the population. The services were implemented on a common network, while providing the content is brought to the competition.