TV – Methods of transmission
There are several types of TV broadcast systems:
- Analogue Terrestrial TV
- Systems for sound transmission
- Digital Satellite TV
- Cable TV: analogue and digital systems
- New technologies:
- Digital terrestrial TV (DTTV)
- High Definition Television (HDTV)
- Web TV
ANALOGUE TERRESTRIAL TV
Terrestrial television is a term which refers to modes of television broadcasting which do not involve satellite transmission or via underground cables.
Terrestrial television broadcasting dates back to the very beginnings of television as a medium itself and there was virtually no other method of television delivery until the 1950s with the beginnings of cable television, or community antenna television (CATV).
The first non-terrestrial method of delivering television signals that in no way depended on a signal originating from a traditional terrestrial source began with the use of communications satellites during the 1960s and 1970s of the twentieth century.
Analogue TV encodes the image and sound information and transmits them as an analogue signal in which the message transmitted by the broadcasting signal is composed of amplitude and/or frequency variations and modulated into a VHF or UHF carrier.
The analogue television picture is “drawn” several times on the screen (25 in PAL system) as a whole each time, as in a motion picture film, regardless of the content of the image.
DIGITAL SATELLITE TV
Satellite television is television signals delivered by means of communications satellites and received by satellite dishes and set-top boxes. In many areas of the world it provides a wide range of channels and services, often to areas that are not serviced by terrestrial or cable providers.
Satellite television, like other communications relayed by satellite, starts with a transmitting antenna located at an uplink facility which have very large uplink satellite dishes, as much as 9 to 12 meters (30 to 40 feet) in diameter what results in more accurate aiming and increased signal strength at the satellite.
The uplink dish is pointed toward a specific satellite and the uplinked signals are transmitted within a specific frequency range, so as to be received by one of the transponders tuned to that frequency range aboard that satellite, which ‘retransmits’ the signals back to Earth but at a different frequency band, a process known as “translation”, used to avoid interference with the uplink signal, typically in the C-band (4–8 GHz) or Ku-band (12–18 GHz) or both.
The downlinked satellite signal, quite weak after traveling the great distance, is collected by a parabolic receiving dish, which reflects the weak signal to the dish’s focal point where is a “downconverter” device called LNB (low-noise block) that is essentially a waveguide that gathers the signals, amplifies the relatively weak signals, filters the block of frequencies in which the satellite TV signals are transmitted, and converts it to a lower frequency range in the L-band range.
The evolution of LNB was a need, so the designs for microstrip based converters were adapted for the C-Band taking advantage of its central design that was the concept of a block for down conversion of a range of frequencies to a lower, and technologically more easily handled block of frequencies, the IF – intermediate frequency.
The advantages of using an LNB are that cheaper cable could be used to connect the indoor receiver with the satellite TV dish and LNB, and that the technology for handling the signal at L-Band and UHF was far cheaper than that for handling the signal at C-Band frequencies.
The shift to cheaper technology from the 50 Ohm impedance cable and N-Connectors of the early C-Band systems to the 75 Ohm technology and F-Connectors allowed the early satellite TV receivers to use what were in reality modified UHF TV tuners which selected the satellite television channel for down conversion to another lower intermediate frequency centered on 70 MHz where it was demodulated. This shift allowed the satellite television industry to change to a far more commercial mass production one.
The satellite receiver demodulates and converts the signals to the desired form (outputs for television, audio, data, etc.) and sometimes, the receiver includes the capability to unscramble or decrypt; the receiver is then called an Integrated Receiver/Decoder or IRD.
The cable connecting the receiver to the LNB must be of the “low loss” type, RG-6 or RG-11 and should not be used the standard RG-59 cable.
Cable Television or Community Antenna Television (CATV) is a system for distribution of audiovisual content for television, FM radio and other services to consumers through fixed coaxial cables, avoiding the traditional system of radio broadcasting antennas (broadcast television) and have widespread use, mainly through the pay-TV services.
Technically, the cable TV involves the distribution of a number of television channels received and processed in a central location (known as head-end) to subscribers within a community through a network of optical fibre and/or coaxial cables and broadband amplifiers.
The use of different frequencies allows many channels to be distributed through the same cable, without separate wires for each, and the tuner of the TV or Radio selects the desired channel from among all transmitted.
A cable television system begins at the head end, where the program is received (and sometimes originated), amplified, and then transmitted over a coaxial cable network.
The architecture of the network takes the form of a tree, with the “trunk” that carries the signals in the streets, the “branches” carrying the signals for buildings and, finally, the “arms” carrying the signals to individual homes.
The coaxial cable has a bandwidth capable of carrying a hundred television channels with six megahertz of bandwidth each, but the signals decay quickly with distance, hence the need to use amplifiers to “renew” the signals periodically to boost them.
Backbone trunks in a local cable network frequently use optical fibre to minimize noise and eliminate the need for amplifiers as optical fibre has considerably more capacity than coaxial cable and allows more programs to be carried without signal lost or noise adding.
Most of the TV tuners are able to directly receive the cable channels, which are usually transmitted in the RF (radio frequency) band, however, many programs are encrypted and subject to a tariff itself and in such cases, you must install a converter between the cable and the receiver.
DIGITAL TERRESTRIAL TV
Digital Terrestrial Television (DTTV or DTT) is an implementation of digital television technology to provide a greater number of channels and/or better quality of picture and sound using aerial broadcasts to a conventional antenna (or aerial) instead of a satellite dish or cable connection.
The technology used in Europe is DVB-T that is immune to multipath distortion.
DTTV is transmitted on radio frequencies through the airwaves that are similar to standard analogue television, with the primary difference being the use of multiplex transmitters to allow reception of multiple channels on a single frequency range (such as a UHF or VHF channel).
The amount of data that can be transmitted (and therefore the number of channels) is directly affected by the modulation method of the channel.
The modulation method in DVB-T is COFDM with either 64 or 16 state Quadrature Amplitude Modulation (QAM). In general a 64QAM channel is capable of transmitting a greater bit rate, but is more susceptible to interference. 16 and 64QAM can be combined in a single multiplex, providing a controllable degradation for more important programme streams. This is called hierarchical modulation.
New developments in compression have resulted in the MPEG-4/AVC standard which will enable two high definition services to be coded into a 24 Mbit/s European terrestrial transmission channel.
DTTV is received via a digital set-top box, or integrated receiving device, that decodes the signal received via a standard aerial antenna, however, due to frequency planning issues, an aerial with a different group (usually a wideband) may be required if the DTTV multiplexes lie outside the bandwidth of the originally installed aerial.
In Portugal, as detailed in the information published by ANACOM in February 2008, Set Top Boxes (STB) or TV receivers must be capable of decoding MPEG-4, H.264 AVC coded transmissions and also be suitable to display HD signals in at least 720p format, as this is the format to be broadcast on the country.
In the case of STB’s, ANACOM advises that an HDMI connection should also be available and that it should be version 1.3 and that the box should of course decode the transmitted HDTV format.
The high-definition television, also known as HDTV (High Definition Television) is a television system with a resolution significantly higher than in the traditional formats (NTSC, SECAM, PAL).
The HDTV is transmitted digitally and therefore its implementation generally coincides with the introduction of digital television (DTV), technology that was launched during the 1990s.
Although several patterns of high-definition television have been proposed or implemented, the current HDTV standards are defined by ITU-R BT.709 as 1080i (interlaced), 1080p (progressive) or 720p using the 16:9 screen format.
The term “high definition” can refer to the specification of the resolution itself or, more generally, the mídia capable of such a definition as the video mídia support or the television set.
What will be of interest in the near future is high definition video, through the successors of the DVD, HD DVD and Blu-Ray (is expected that the last one will be adopted as a standard) and, consequently, the projectors and LCD and plasma televisions sets as well as retro projectors and video recorders with 1080p resolution/definition.
High-definition television (HDTV) yields a better-quality image than standard television does, because it has a greater number of line resolution.
The visual information is some 2 to 5 times sharper because the gaps between the scan lines are narrower or invisible to the naked eye.
The larger the size of the television the HD picture is viewed on, the greater the improvement in picture quality. On smaller televisions there may be no noticeable improvement in picture quality.
The lower-case “i” appended to the numbers denotes interlaced; the lower-case “p” denotes progressive: With the interlaced scanning method, the 1,080 lines of resolution are divided into pairs, the first 540 alternate lines are painted on a frame and then the second 540 lines are painted on a second frame; the progressive scanning method simultaneously displays all 1,080 lines on every frame, requiring a greater bandwidth.
Pay-per-view (often abbreviated PPV) offers a system by which a television audience can purchase events to view on TV-monitors via private telecast of that event to their homes.
The broadcaster shows the event at the same time to everyone ordering it (as opposed to video-on-demand systems, which allow viewers to see the event at any time) and can be purchased using an on-screen guide, an automated telephone system, or through a live customer service representative.
Events often include feature films, sporting events, adult content movies and “special” events.
Video-on-Demand (VoD) or Audio-Video-on-Demand (AVoD) systems allow users to select and watch/listen to video or audio content on demand.
VoD systems either stream content through a set-top-box, allowing viewing in real time, or download it to a device such as a computer, digital video recorder, personal video recorder or portable media player for viewing at any time.
Download and streaming video-on-demand systems provide the user with a large subset of VCR functionality including pause, fast forward, fast rewind, slow forward, slow rewind, jump to previous/future frame etc., these functions are called trick modes.
For disk-based streaming systems which store and stream programs from hard disk drive, trick modes require additional processing and storage on the part of the server, because separate files for fast forward and rewind must be stored.
Memory-based VoD streaming systems have the advantage of being able to perform trick modes directly from RAM, which requires no additional storage or CPU cycles on the part of the processor.
It is possible to put video servers on LANs, in which case they can provide very rapid response to users. Streaming video servers can also serve a wider community via a WAN, in which case the responsiveness may be reduced. Download VoD services are practical to homes equipped with cable modems or DSL connections.
Web TV, TVIP, or TV on the Internet is the transmission of a programming grid through the Internet. It can be known “normal” TV channels or channels specifically designed for the Internet.
Web TV, in a simplified form, is nothing more than the provision of video and audio over the Internet; and the way to assist the transmission varies from the monitor of a computer through the use of an iPod or a mobile phone to the TV set if one have the decoder.
IPTV (TV over Internet Protocol)
The recent introduction of Television over Internet Protocol technology, commonly known as IPTV, made a revolution on the distribution networks for TV signals, allowing eliminate many of the problems associated with a distribution network based on coaxial cables, in particular those related with the degradation of signal, interference, signal levels, and capacity of the transmission of the channel’s band.
Moreover, thanks to IP (Internet Protocol), will be possible the combination of several interfaces in a multi-service unit and the broadcast and distribution of diverse and varied services on the same network, which previously required differentiated infrastructure, including: TV signals, telephone service and broadband Internet access, setting a platform we know today as Triple Play.
In essence, the triple play concept is not entirely new because, in terms of services, there are some years ago that are available some solutions combining a mix of TV services, telephony and Internet access.
Studies show that the churn rate (voluntary abandonment of service) of the offer triple play subscribers is substantially lower than that observed when the voice, data and TV are sold on a non-convergent way.
Another factor is the progress in access technologies and platforms for packet telephony and video. A variant of ADSL (asymmetrical digital subscriber line), known as ADSL2+, represents a change in the effective performance of Internet connection on the original format, not to mention the more recent developments, such as VDSL (very-high-bit-rate DSL).
The access over optical fibre in its more popular form, known as PON (passive optical network), reflects an even more daring way, resulted in significant investments in that technology, seeking for high-speed Internet access, voice and multi-channel of high-definition TV union.
Progress in video distribution systems is on the way too. In recent years, a number of innovations and developments in the industry of hardware and software systems for the TV industry have started to TV over IP (also known as IPTV).
The main driver is integrated platforms consisting of set-top-boxes, servers and video content protection system (DRM – digital rights management), together with appropriate tools, middleware and billing, allow the provision of a variety of TV services in several formats, such as streaming, video on demand and time-shifted TV, based on a combination of underlying IP networks and DSL or optical access systems.
In this context, the sophistication of algorithms for compression of video signals has a relevant role. Techniques such as MPEG-4 AVC (advanced video coding), for example, enable the transmission of signals in high definition TV over IP networks.
The search for a strategy to offer multiple play-based (dual, triple, quadruple etc.) is an irreversible phenomenon in the communications industry but at the same time it impose enormous challenges – particularly in terms of selection of technology platforms, control and regulation – opens a huge horizon of possibilities, both supply and demand.