MATV Systems – Know more about them
Master Antenna Television systems, or MATV systems for short, is a system that allows multiple receivers (including TV and FM Radio) to receive signals from one antenna, the master antenna, as opposed to a single antenna for each dwelling.
An MATV system is made out of two separate areas, the Head End and the Distribution System. By properly designing and implementing both areas of the system, using the appropriate MATV equipment and installation techniques, the signals will be distributed throughout the system without a loss of signal quality.
An example of an MATV system
When a signal passes through the MATV system’s components and cable it will have a depreciated level. You need to make sure your levels don’t depreciate too much you will lose your high quality of the signal.
Signal quality is built up of signal level, system noise and headroom. The system noise needs to be kept low but have a high signal level. The caveat to this is that the signal level can’t be too high as it can overdrive the equipment.
To simplify MATV design, noise and signal level’s can be expressed in Decibels.
The signal level from a tv antenna is measured in microvolts. As calculations in microvolts are difficult, MATV systems calculations are carried out in decibels.
Decibels are added and subtracted, not multiplied and divided. A decibel was derived originally from a formula used by telephone engineers and is 1/10 of a bel.
The decibel is a way to indicate how many times larger or smaller a quantity is from the reference level. This relationship between dB levels is logarithmic and not linear. For example:
10 dB = 3.2 x reference level
20 dB = 10 x reference level
30 dB = 32 x reference level
40 dB = 100 x reference level
50 dB = 316 x reference level
The zero reference level is 1 microvolts measured across 75 Ohms of impedance. The minimum signal of 1,000 microvolts is required to produce an acceptable picture on the television which is approximately equal to 50dB?V.
The dB figure is represented as dBms (a reference to 1 millivolt), or dB?V (a reference to 1 microvolt).
To determine an amplifier output and any system losses, decibels are added and subtracted. Values that expressed in dB are:
- MATV amplifiers gains
- cable losses
- insertion losses and,
For a good quality signal, the minimum is typically expressed as 60dB?V (though most televisions will work with a signal as small as 50dB?V). Working to the level of 0sBmV will provide a tolerance to small variations in signal. When the signal is then fed into a television, it should be kept below 70dB?V.
For audio and video signals to be transmitted over a portion of the available bandwidth, the signals need to be modulated onto carrier signals within a fixed bandwidth. In Australia for instance, the standard bandwidth is 7MHz, known more commonly as a channel.
When a TV is tuned to a particular channel. it is not tuned into a single frequency but instead a 7MHz bandwidth where the audio and video information is retrieved.
The Head End
The Head End of the MATV system consists of an antenna which receives broadcast signals, processing equipment to filter all the signals and a distribution amplifier to increase the signals to compensate for any distribution loss.
The Head End contains:
The MATV Antenna
The reception of the TV can be no better than the quality of the signal from the antenna.
You need to make sure you have the correct antenna for the intended location if you want the best quality.
Antenna manufacturers produce detailed maps and the preferred antenna types based on the location. A proper MATV systems designer and installer carry test equipment to ensure that antenna placement and orientation is optimal.
The strength and quality of any signal is determined by:
- Proximity to the transmission tower
- Power of the transmitter
- Quality of the transmission
- Line of sight to the transmission tower
- Weather conditions
- Interference from power lines
- Directional characteristics and orientation of the antenna
- Level of gain of the antenna
An antenna install should put out at least 60dB (or 0dBmV) of picture signal per channel at the amplifier input.
In good signal areas, this will be relatively easy to obtain. In weaker signal areas, a larger antenna that has high gain will be a usual necessity.
The directivity of the antenna is important, as it is a measure of how well an antenna will reject signals from other direction other than the front. A front-to-back ratio is one way, putting the signal received from the front against the signal received from the back. A highly directional antenna will usually have a high front-to-back ratio.
Determining signal levels is one of the most vital parts of a Head-End design and performing a signal survey before installing the system can avoid a lot of problems later down the line.
Outfitted with an antenna, sections of mast, field strength meter and portable TV a tech can carry out a signal survey. With the field strength meter, the amount of signal received, on each channel, can be measured. Take time in selecting the antenna as it can also do a lot to overcome certain types of interference. With the portable tv, the tech is able to determine the end quality of the signal received will be.
In the case of a weaker signal area, you may need to amplify the signal, using a distribution amplifier. This will make sure a signal, with enough strength and acceptable, quality is received. This is done with a Masthead Amplifier.
The key to the right masthead amplifier is a low noise figure. The noise figure of the masthead amplifier starts with the noise figure of the entire system, and the amplifier should always increase the signal more than it increases the noise.
To guarantee an adequate signal at every receiver connected, the distribution system needs to be properly designed. The distribution system of the MATV is a combination of splitters and taps.
Splitters and taps have a set signal level and maintain the correct impedance to each of the outputs. Calculation of losses associated with splitters, drop taps and cable are required in order to keep a quality signal.
Each outlet has a defined amount of required signal level with a recommendation for maximum cable runs. Splitters and taps provide the same signal quality to each outlet, as long as design recommendations are implemented.
If you receive a poor signal from the antenna, the output to your tv will also be poor, so in order to get a good signal to your tv, you will need to start by having a good signal from your antenna.
With many different signals received by the antenna, there can be a varying range of signal levels. To get the same picture quality on all channels, these signal levels need to be equalised. This prevents the stronger signals from overriding the weaker signals. This type of equalisation happens when you use attenuators which can reduce the stronger signals by specified amounts.
Attenuators can be fixed or variable, with the fixed attenuators designed for one specific level and the variable being switchable so that the signals can be reduced in increments to the required level.
Attenuators reduce the signals that pass through it by the same amount. Frequencies that need reducing have to be separated from the rest of the signals, so only the stronger signals are getting passed.
Attenuators decrease the strength of signals, Amplifiers increase the strength of signals received to a level greater than the losses in the distribution system. The amplifier gain determines the level of signal increase, which should be high enough to provide an acceptable signal level to all televisions in the system.
As much as the amplifier’s gain is important, the output capability is just as important. Checking the amplifiers specifications to ensure the output level is sufficient to feed the system and that the strength of the input signal plus the gain of the amplifier doesn’t exceed the amplifier’s rated output capability. Exceeding the output capability will result in overloading, cross modulation distortion, and overall signal deterioration.
A certain amount of signal will be lost as it travels through the coaxial cable. This loss depends on the type of cable used and the frequency of the signal being carried.
Losses are greater at higher frequencies, the greatest loss occurring at channel 69 in UHF/VHF systems. The cable loss should always be calculated at the highest frequency received or the highest frequency received or the highest frequency to be received in the future.
When a two-way splitter is inserted in-line, the signal in each leg will be about 3.5dB?V less than that of the main line. If a 4-way splitter is interested in-line, the signal in each leg is 6.5dB?V less than that in the main line. The signal sent to each branch of the system will be equal to the signal sent into the splitter minus the splitter loss. That is, an input of 70dB?V minus 3.5dB?V splitter losses, or 66.5 dB?V to each branch of the system.
All tap-off devices inserted into the distribution system create signal loss. This type of loss is called insertion loss, (sometimes called feed-through loss). On the line, the insertion loss of each tap-off is subtracted from the signal carried by that line. When estimating the total system losses, the insertion loss of each unit is added together to find the total insertion loss for that system. For example, if there are 10 tap-offs on the line, and each tap-off has an insertion loss of 0.5 dB, the total insertion loss is 5 dB.
For the initial calculation, the tap-off values and the insertion losses are estimated as the output of the amplifier and will influence the final selection.
Each tap-off attenuates the signal by a specified number of dB to prevent one set from interfering with another. For example, if there is an 80dB?V signal in the line, and a 20dB drop tap is inserted in the line, the signal available at the tap-off would be 60dB?V. The 20dB loss is called ‘Thru’ loss. In computing the total distribution system losses, calculate the Thru Loss of the last tap-off only. Since the system design requires a minimum of 60dB?V to each set, the lowest tap value should be used. For most MATV tap-offs this value is either 10 or 12dB.
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