DTH Technology in India is repeating the performance of cable TV 15 years ago. Although, Zee group’s Dish TV was launched two and a half year back, the real impetus to DTH has been given by the Free-To-Air package of Doordarshan’s DD Direct. Although it has only 40 TV channels and a few radio channels, the very fact that there is no subscription involved, this service has enmassed a huge viewership of more than 10 million, within one and a half of its launch.
Recently, there are indications in the market that Space TV (joint venture of Tata and Star) may also get their clearance for DTH very soon. Most of their hurdles are overcome now. If that happens soon, both Space TV as well as Dish TV can have all the existing channels will start. There will be a number of exclusive channels on these platforms acting as drivers and subscribers will have to install more than one Ku band dish or use elliptical dish with dual-feed or dual-LNB configuration.
Then there will be problems of fixing these dish antennas in the balconies or rooftops in multidwelling buildings as one may not get a line of sight for the dish to get a good signal fr0m the satellite. This article will help DTH subscribers as well as installers to understand the technology and its application in different conditions.
Individual Reception Systems
Figure 1 gives a basic configuration of a DTH system. The IRD shown is an advanced one which can drive a television, VCR/VCD/DVD, telephone etc. There are many methods by which we can change this configuration to receive multiple satellites on the same dish or to drive multiple set-top-boxes. Five DTH reception architectures are shown in Figure1.
> Figure 2(a) is a single-feed system for reception only fr0m a single satellite, like NSS 6 at 95ºE. It utilises a single “universal” dual-band LNB to receive all signals in the range 10.7 to 12.75 GHz on any one of the two linear polarisations (vertical & horizontal). Frequency band switching and polarisation switching is performed within the LNB. Figure 2(b), Figure 2(c), Figure 2(d) and Figure 2(e) are systems for reception from two orbital positions via a dual-feed assembly. Single antenna architectures such as these are recommended for DTH systems receiving fr0m two orbital positions in order to avoid the cost and environmental impact associated with installing two separate antenna subsystems. Systems with a single mechanically steerable antenna would also suit this purpose, with a longer response time when switching between orbital positions. The architectures of Figure 2(d), Figure 2(c), Figure 2(d) and Figure 2(e) differ in their cabling and signalling requirements for the connection between the IRD and the antenna subsystem.
In Figure 2(b) two dual-band LNBs share a common reflector. The system is intended for simultaneous reception from two different satellites, e.g. one NSS-6 at 95º E and a second orbital location close by (typically within ±6º). Frequency band switching and polarisation switching is performed independently within the LNB for each orbital position. Two cables are required for connection to the IRD (indoor unit) which must be equipeed with two LNB inputs that can be independently controlled.
Figure 2(c) is an alternative dual-feed architecture, which additionally allows switching between the two LNBs (orbital positions) to be performed at the antenna under the control of the IRD. Only one cable is required for the interconnection between the antenna subsystem and the IRD and the latter need only be equipped with a single LNB input.
Figure 2(e) shows a mono-block universal LNB for reception from two satellites spaced six degrees apart. Reception is possible across the full frequency band of 10.7 to 12.75 GHz and on both polarisations from each orbital position. Such devices are simpler to install than equivalent systems employing two physically separate LNBs to achieve the same purpose (as shown in Figure 2(b), Figure 2(c) and Figure 2(d). They are designed to allow existing antennas receiving from a single satellite position to be easily upgraded for reception from two orbital positions. A single cable is required for the interconnection between the LNB and the IRD.
In general, it is desirable to minimise the number of connections to be made at the outdoor unit in order to reduce the possibility of equipment failure due to faulty cabling or water ingress. Of the four dual-feed architectures indentified in Figure 2, the mono-block LNB is the best solution in this respect, because it requires only a single connection to be made and waterproofed at the LNB. All architectures of Figure 2require signalling fr0m the IRD so that the appropriate frequency band and polarisation can be selected at the antenna subsystem. Depending upon the architecture, this can be done using either conventional tone signalling methods or by means of a compatible communi-cations protocol called DiSEqCTM. The use of DiSEqCTM-compatible equipment is recommended.
Figure 2(e) is the preferred solution for dual-feed reception systems for environmental, cost and practical reasons (under the assumption that simultaneous reception fr0m both orbital locations is not required).
Reception System Control Capabilities
Conventional Signalling Method: Receiving systems, regardless of their architecture, need to be able to operate over the full frequency range of 0.7 - 12.75 GHz and on both linear polarisations in order to be physically capable of receiving all of the services available fr0m the satellite’s orbital position. The simplest form of receive architecture consists of an antenna equipped with a single feed (LNB) and connected by a single cable to an IRD (i.e. the DTH reception architecture ofFigure 1). In this architecture, the single LNB needs to be capable of working over the full frequency range and on both polarisations.
The bandwidth employed by the satellite broadcasting system is too large to directly implemented in the LNB. Conventional practice is to receive in one of two frequency bands: in the “low” frequency band (10.7-1.7 GHz) or in the “high” frequency band (11.7-12.75 GHz). The LNB is switched to receive in the frequency band that is appropriate for the desired satellite channel.
The LNB also receives on one of two linear polarisations: horizontal (X) or vertical (Y). The LNB is switched to receive in the polarisation that is appropriate for the desired satellite channel.
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