As DTT, Satellite, DAB and CCTV are now required by most communal developments, the flexibility and coverage of IRS systems are essential in order to deliver these services. Here, Bob Calaz introduces the IRS concept and outlines the services that can be relayed, which is taken from in his new book An Introduction to Domestic Radio TV and Satellite Reception.

Customer Requirements
Customers can now require some or all of the following services:

FM radio
Digital audio broadcasts (DAB)
Analogue terrestrial TV
Digital terrestrial TV (DTT)
BSkyB digital satellite TV
Analogue and digital broadcasts from other satellites
CCTV relayed as UHF TV channels
Satellite programmes relayed as UHF TV channels

An integrated reception system (IRS) can deliver all these services on a single coaxial drop-in cable to each viewing location.

Fundamentals of IRS
The necessary terrestrial services are amplified and combined on to a single coaxial cable.
A Ku band universal “Quattro” LNB gives the following outputs simultaneously:

High band horizontal
High band vertical
Low band horizontal
Low band vertical

These signals are amplified and distributed together with the combined terrestrial signals on separate coaxial cables to one or more “multiswitches” that in turn deliver the signals to each outlet location, as shown in figure 1.

The multiswitch independently selects the appropriate LNB feed for each outlet according to whether the voltage from the satellite receiver is 12V or 18V and whether or not a 22KHz tone is present. The terrestrial signals are mixed with the selected satellite feed and relayed via the drop-in cable to the receiver.

The system is “transparent” is as much that the satellite receiver and TV receiver will function as if they were connected directly to a single universal LNB and terrestrial aerial respectively. The cable links to the multiswitches are called backbone cables (see figure 2)

Early IRS networks utilised two-cable backbone with the terrestrial signals diplexed on to one or both of the satellite IF feeds; most modern systems incorporate a five-cable backbone with the terrestrial signals relayed on a separate coaxial cable to simplify the adjustment of signal levels. If necessary, the backbone cables can be split to feed in two or more directions as shown in figure 3.

It is also possible to extend the IRS concept to provide access to two different satellites using a nine-cable backbone with four cables dedicated to each satellite and the ninth for the terrestrial signals. Each viewer could then access either satellite using DiSEqC commands generated either by the receiver or a stand-alone DiSEqC generator as described in section 27.3 of An Introduction to Domestic Radio TV and Satellite Reception. Three or more satellites could be accommodated using the same principles.

For most systems, the mains power supply is located at the head end location with dc power relayed to the multiswitches on one of the backbone cables. Only for larger systems would mains power be required at other locations.

Careful planning is necessary to keep all the signal levels within their correct windows of operation throughout the system. This is especially true for the digital signals because the C/N ratio will be degraded by each amplifier or multiswitch.

It is particularly important to measure the BER at each outlet location, even if the signal levels and C/N ratio are within their theoretical limits, since this will determine the overall performance of the system.

There are several guidelines to follow when planning and installing a system:

-Incorporate some form of terrestrial filter (such as a cluster leveller) to remove extraneous interference

-The level of the lowest terrestrial digital multiplex into the multiband preamplifier should be at least 42dBµ

-Try to avoid having more than four amplifiers and three multiswitches in cascade.

-Drop-in cables should preferably be not more than 40M long.

-Terminate all the unused outputs of each multiswitch.

It is always necessary to check the required analogue launch level against the amplifier specification when planning a system. Check also that the FM and DAB launch levels do not exceed those of the analogue TV signals.

For larger community systems it could be advantageous to use fibre optic links between remote sites to relay signals from a central head-end location to two or more independent IRS systems, each covering a smaller area.

Services Available
A site survey and field strength test will be required in order to determine the services available at any particular location. It will be necessary to consider the following:

The signal strength of the relevant services. The minimum limits are given in chapter 5 but it is vital to get the best possible signals from the aerial or satellite dish. 2dB more signal means 2dB more C/N ratio throughout the system. A separate satellite dish would be preferred for each orbital location. Possible sources of interference, for example neighbouring TV transmitters, cellphone relay stations and microwave transmitters. This subject is discussed in detail in chapter 23. It may be necessary to consider the use of an aerial with a good front-to-back ratio or a dish location such that a nearby structure will provide screening against interference.

Layout Concept
Aspects to be considered include the following:

Aerial/dish location and aesthetics, the rigidity of the supporting structure and local planning requirements
The head end equipment location, availability of mains power and a system earth
The length of the drop-in cables
The location and number of backbone cables
Number of coaxial cables in each backbone

Future expansion (maybe two multiswitch outlets per viewing location)
The number of multiswitches in cascade
Access to the equipment for maintenance

Equipment and Materials to be used
For all but the smallest systems use equipment where terrestrial TV and satellite signals are relayed on separate cables.

The make of equipment will depend on the size of the system and the facilities required. There are many products on the market for small systems. For larger systems choose equipment with adjustable gain and equalisation. The choice is more limited if more than five backbone cables are required.

Compile a table of parameters for each item of equipment listing the following:

Minimum/maximum input levels
Maximum output levels
Loss or gain at each relevant frequency

Network Plan
Prepare a schematic drawing showing equipment/outlets and cable lengths.
If possible, make several copies these will be useful when calculating the signal levels at various frequencies.

Identify the furthest point (the outlet at which the signal will be weakest). This is not always easy, especially if network taps are to be used.
Mark up one copy of drawing to show gains/losses for terrestrial TV signals and cable losses at 860MHz.

Mark up a second copy of the drawing to show gains/losses for satellite TV signals and cable losses at 2150MHz.

This extract was taken from An Introduction to Domestic Radio TV and Satellite Reception, written by Bob Calaz.