Software helps with MATV technology

Monday, 05 January, 2004

The ever-increasing demand for distribution of more channels is resulting in pushing the upper limits of coaxial cable usage to 1000 MHz.

Greater availability of Transmodulators makes it feasible, particularly for larger installations, to reticulate satellite signals on a single coaxial cable simplifying the cabling and providing a simple channel selection for viewers.

MATV (which stands for Master Antenna TV) installations are using 75 ohm coaxial cables with good shielding properties to prevent interference, low losses and good structural return loss (reflecting uniformity of production processes).

They need to be properly terminated. This is usually done by connectors specifically designed for the cable and using proper tools.

Throughout the project implementation good engineering and installation practices should be employed. The complexities of designing MATV stem from the following:

  • Different levels of the signal at the input
  • Losses of the coaxial cables increase with increase of frequency
  • Passives components have different losses at VHF and UHF frequencies
  • Amplification of signals introduces distortion, which has to be strictly controlled and kept to a minimum
  • To operate satisfactorily wide-band amplifiers require at their input (possibly) equal signals at all channels (for either pure analog or digital systems)
  • Maximum output of an amplifier (hence its useful gain) depends on its 'rating' that is for how many channels it is designed and how many amplifiers are cascaded
  • All outlets should receive signals above the required minimum and within a specified range of values
  • Need to keep signal reflexions within the installation to a minimum.

The requirements for more channels, and therefore higher frequencies, are adding to the complexities involved in designing and successfully implementing larger MATV projects.

The issues requiring consideration relate particularly to two areas - the choice of topology (the way the networks are constructed) and the certainty that designs and installation meet requirements of the nominated specification on all channels.

The choice of topology has a great influence not only on the cost of installation and eventual maintenance, but also on the complexity of a design.

Implementation of pure Star topology (requires dedicated, individual cable connection between head-end and the user) results in a most expensive installation but is easier to design.

Tree topology (the head-end connects by cables to a distribution network; each part of the network may connect to a spur network; users are connected to the spur network;) and this leads to a less expensive installation but it is much harder to design. A mixed star/tree topology.

To ensure the quality of received signals and hence the quality of pictures and sound, a specification provides information on many design performance requirements. The main ones include:

  • The maximum number of reticulated channels
  • Listing of channels and their sources and specifying if they are analogue or digital
  • Minimum signal levels at the outlets
  • Maximum variation of signal levels at different outlets
  • Maximum signal levels allowed to be reticulated
  • Maximum allowed signal imperfections
  • Maximum allowed radiation from the system
  • The characteristic impedance of the cables and components
  • The quality of picture and sound

Safety considerations

For a design to ensure that the requirements of the nominated specification will be met it needs to provide:

  • Information on setting the head-end amplifiers on all channels
  • Details of all active components including derating of amplifiers, if required
  • Details of all passive components
  • Details of all cable segments
  • Details of equalisation including their frequency response
  • Details of signal levels at all critical points
  • Checking at design stage that performance requirements are met at all outlets at all channels.

In addition to these, it is preferable to provide alternative designs, all meeting the specification and indicating the relevant merits and costs before finally selecting a design.

These tasks are very labour intensive and costly when using conventional methods and so some of them are skipped, which can result in costly problem identification and modifications.

Fortunately the use of computers and software makes the task of design and technical management much easier, more efficient and less costly.

A recent release of one software package called 'MATV Calculator Version 3.0' provides a tool to satisfy all these tasks and requirements. An installation is presented in a number of 'snapshots' giving all design details from the head-end to the user outlets.

Each 'snapshot' may be viewed as a schematic, with all important information available even up to terminal numbers to which sections of the design are connected.

Part of a snapshot is presented as a matrix, which shows clearly that the specification is met at the lowest and highest frequency.

An 'integrity checker' ensures that all snapshots are consistent and compatible and during design stages provides information on details if and when inconsistencies are encountered.

A feature of the software lies in 'quick change', which allows automatic checking of any 'snapshot' meeting requirements of the specification for up to 150 frequencies between the nominated lowest and highest frequencies.

The same quick change is also used to provide information on gain settings for head-end single-channel amplifiers or signal processors and detail requirements of equalisers (if any).

The quick change can also provide this information within the required range on any one stipulated frequency.

The package provides a listing of components used in an installation in the form of a 'report'. As the report is available only for a valid design it follows that the validation of a complete design requires simply subjecting each snapshot to a satisfactory quick change test and obtaining a report.

The whole design using the software requires a small fraction of time needed for any other methodology. It provides a wealth of information which is simply not available otherwise.

To illustrate differences in material requirements for different topologies an analysis was made of two designs for an identical building using the software. The building had 35 floors and the total number of outlets was 528.

One design was for a tree topology and the second for a modified star design where each floor with two wings was supplied by a two-way directional coupler connected to a spur network.

The table shows the different material use for two topologies safe.

The data are stored in Excel worksheets making it easy to evaluate the cost of materials. The table shows that a design using modified star topology uses more material than one using tree topology.

As the labour costs are related to the quantity and hence cost of materials one can expect that an installation based on 'modified star' design will also incur higher labour costs.

Apart from implementation costs of a modified star design being more expensive, maintenance costs may also be higher as there are more components that can develop a fault.

The advantage of a 'modified star' lies in the likelihood that fewer users may be affected by a faulty component.

The software enforces the specification. The design of a new installation using it is made easy by existing library folder, as well as extensive help-on-screen.

During the installation stage of a project, Calculator allows progressive checking of the cabling and parts of an installation. If there is any deviation from the original plans this can be quickly evaluated and design modified to prevent problems occurring later at the commissioning stage.

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