TV & SAT TV
Community TV installations intended to receive satellite television, often called SMATV systems (Satellite Master Antenna TV), seem to be the most effective and economical method of reception of satellite broadcasts, especially in densely populated urban areas. They allow to avoid a great number of individual dishes on the facades or roof of the building, and when the manager of the building does not permit to use them at all, such an installation is the only possible solution, often realized at the construction stage of the building.
Types of installations:
- with application of multiswitches,
- with application of first IF conversion,
- with application of MPEG to PAL decoders,
- with application of MPEG encapsulation with QAM modulation.
There are described installations with multiswitches and with conversion of first satellite IF in this article.
Installations based on multiswitches
Multiswitch is an electronic device that allows for connection of any output to any of its input. It is a kind of switch that operates in 950-2150 MHz frequency range (or even to up to 2400 MHz). Connection of terrestrial TV signals (46 to 862MHz) is also possible. All the signals are separated at the outputs of subscribers' outlets.
Such an outlet has three outputs - one with satellite TV signal, to connect SAT TV receiver, second for TV set , and the third for FM stereo. The outlet must be transparent to DC voltage and control signals (DiSEqC and 22 kHz tone) from sat TV receiver. The sat receiver recognizes multiswitch as LNB converter and switching between its inputs is performed by changing voltage sent by the receiver (14V or 18V), presence or absence of 22 kHz signal or in the case of 9 inputs multiswitches - by DiSEqC signal.
Multiswitch ensures that subscribers are able to access satellite signal from any polarization, band and satellite. The variety of installations requires different kind of multiswitches. Basic differences between types of multiswitches are:
1) Number of inputs for converters. It determines ability of reception from different satellites or polarizations;
There are multiswitches with:
- 4 inputs (possibility of reception of all programs from one satellite),
- 8 inputs (possibility of reception of all programs from two satellites)
- 12 inputs (possibility of reception of all programs from three satellites)
- 16 inputs (possibility of reception of all programs from four satellites)
Installation with 2- or 4-input multiswitches (4 outputs)
Installation with 4- or 8-input multiswitches (4 outputs)
2) Number of outputs. Multiswitches usually have 4, 8 or sometimes 12 outputs. For larger number of subscribers cascade connection of multiswitches is applied.
Installation with 4-input and 8-output multiswitch
3) Prepared or not for combining terrestrial TV channels - the former kind makes possible to avoid separate cabling for terrestrial TV.
4) Multiswitches designed for cascade connection (enabling to connect increased number of users); thanks to pass-through outputs suitable for terrestrial and satellite TV.
Cascade connection of multiswitches with 4 or 8 inputs (8 outputs)
5) Power supply for converters from external supplier. This solution increases reliability in big installations and allows to use converters with high power requirements.
6) Different methods of supplying converters. There are usually used following means of supplying:
- The same supply voltage on all inputs, or additionally stabilized voltage. In this case one should apply dual, quatro or in some cases fullband converter type;
- On inputs corresponding to vertical polarization - voltage level 14V, and on inputs corresponding to horizontal polarization voltage 18V. Here, besides dual and quatro converters, application of twin and fullband converters is also possible. In the case of signals from upper band one may apply external 22 kHz signal generator;
- As above, but additionally on inputs associated with upper band there is possibility of applying 22kHz signal, so we have free choice of converter types.
One should remember that logical connection of individual inputs to specific polarization is not determined physically, and only informs us to which output the satellite receiver will be connected. Thus the multiswitches inputs there may be connected to any signals.
The rules of designing multiswitch installations are the same as in the case of TV antenna installations, but one should remember that due to higher frequencies adequate coaxial cables have to be applied, and if number of subscribers exceeds 20, or distances between multiswitches exceed 10m, then amplifiers have to be used.
For more than 20 subscribers, it is recommended to divide installation on two independent parts or to divide the signals from converters. Splitters applied for division have to be able to work at 2150 MHz and be transparent for DC (converter's power supply).
Big installation with 4- or 8-input multiswitches (4 outputs), for 40 subscribers
Installations based on conversion of first sat IF.
Converters of first IF frequency, also called IF/IF converters, change signals from individual transponders and equalize their levels. Frequency conversion is realized in a range of the first IF (frequency of signal from converter's output). It enables distribution of signals from different transponders, satellites, polarizations and bands via one coaxial cable (fig.2), without switching devices (multiswitches).
Distribution installation is similar to usual TV installation. It may be built as through or tap installation type. Thanks to frequency conversion, we avoid situation when programs overlap, when frequencies of two or more of programs have similar or the same value, what is possible when they are broadcast from different polarization or satellites. Equalization of levels enables to avoid difficulties related with using adjacent channels, as in the case of Astra broadcasts.
All signals from IF range are amplified in end amplifier and mixed with those coming from terrestrial TV. Signal distribution in this kind of installation requires devices that are able to operate in 5 - 2150 MHz (or even 2400 MHz) frequency range. The equipment consists of taps, splitters, cables, outlets and amplifiers.
fig.1: Examples of conversion and level equalization in IF frequency range
fig.2: An example of IF conversion
Input frequency has to be equal to first IF of the transponder to be received. It is recommended that output frequencies were adjusted according to standard plan (fig.3), that contains range 950-2110 MHz with 40 MHz inter-channel space (with assumption of transponder's bandwidth equal to 40MHz).
This plan is valid for transponders with 27 and 36 MHz bands and allows for distribution of signals from up to 30 of them. In the case of 27 MHz transponder's bandwidth, the space between channels can be decreased up to 30MHz and it still allows for distribution of signals from 40 transponders; those plans enable the subscriber to receive 240 to 320 digital programs adequately.
fig.3: Required inter-channel spaces in dependence of bandwidth
Input frequencies are freely chosen from first IF range (950-2150MHz). Recommended space between transponder's channels is equal to:
Space between channels of transponders
The situation when band reserved for any transponder is narrower than this transponder's bandwidth is unacceptable.
It is advisable to set lower output signal frequencies in the modules installed further from the amplifier and higher output signal frequencies in modules installed close to it. Thus we can achieve better equalization of signal levels for different output signal frequencies. It is also advised to set input frequencies of similar value for one module. In the case of transponders that broadcast analog programs, 27 MHz band should be set, with AFC turned-off.
fig.4: Adjustment of converter's polarization tilt (skew)
Correct setting of polarization tilt enables to obtain minimal interferences between different polarizations (fig.4). It is important to get high quality digital signals, as the tilt is related with polarization angle.
Attenuation of interferences should be at least 20dB, i.e. the ratio of desired and disturbing signal (e.g. when receiving signal of horizontal polarization we mean it desired signal; the signal from vertical polarization is then the interfering, undesired signal).
Before running the set, one should switch ON converter's supply for all modules that the converters are directly connected to (fig.5). The remaining modules one should set in OFF position.
One should check (in the specifications) maximal power consumption of modules, LNBs and other equipment, if used, like amplifiers. Full set with (with eight UC-102 modules) may feed devices with maximal current 750 mA, which is enough to power three to four LNB converters, depending on their power consumption.
fig.5: Power supply of converters
It is very important to ensure reduction of maximal signal level in dependence of number of amplified transponder signals (the amplifier PA
Lowering of maximum output level in dependence of number of channels
The simplest option is distribution of signals of one polarization from one satellite
We recommend to look at Digital program of DIPOL
We also advise to view installation diagrams.
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Sistemas SMATV (10)
Alguma Teoria (14)
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