Specifications

[
StarServe Installers Guide
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page 27
StarServe Video
Distribution
The StarServe Video Distribution Units or ‘VDUs’ distribute
broadband video signals to multiple locations within the home
and are designed to simplify MATV design. The 8053/8VHPIR
VDU supplied with the StarServe ‘Video’ and ‘Pro’ allows for
distribution of locally generated video signals in addition to
free to air TV.
StarServe is essentially the distribution end of a MATV system.
Therefore, there is a requirement for a basic understanding of
MATV systems in order to implement StarServe correctly.
The major consideration is the ‘Head-End’ of the system. This
comprises of a combination of antennas, baluns, masthead
amplifiers, filters and couplers. The StarServe VDU’s distribute
the signal provided by the ‘Head-End’ of the system. If this
signal is too weak, noisy or arrived from multiple paths
(‘ghosting’), the signal at each distribution point will be the same.
The Basics of MATV
MATV stands for Master Antenna Television. MATV systems
allow multiple receivers (TV & FM) to receive signals from a
single (Master) antenna, as opposed to individual antennas
for each receiver.
MATV systems are separated into two portions, the ‘Head
End’ and the ‘Distribution System’. When these two portions
are planned and engineered using suitable MATV equipment
and the appropriate installation techniques, signals will be
distributed without loss of signal quality.
Any signal passing through system components, including
the cable, will be attenuated (i.e., have its level diminished).
The level of this attenuation is important, as it will be a factor
in signal quality.
Signal quality within the system is related to signal level,
system noise and headroom. The system needs to maintain
a low noise level and a high signal level. However, the signal
must not be too high, as this can overdrive the equipment.
In order to simplify MATV design, the relationship between
noise and signal levels is measured (in Decibels).
Decibels
The signal level received on a television antenna is measured
in microvolts. Calculations in microvolts are difficult, therefore
MATV calculations are carried out in decibels.
Decibels are added and subtracted, as opposed to being
multiplied and divided. The decibel is 1/10 of a bel and is
derived from a formula originally used by telephone engineers.
The decibel indicates how many times greater or smaller a
quantity is from a pre-established reference level. The
relationship between dB levels is logarithmic, not linear.
Therefore 40 dB is not twice as much as 20 dB. 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
In the MATV industry, the zero reference level is 1,000
microvolts measured across 75 ohms of impedance. The
reference level determines that a minimum signal of 1,000
microvolts is required to produce an acceptable picture.
The dB figure is represented as dBmV (a reference to 1
millivolt), or dBµV (a reference to 1 microvolt).
MATV amplifier gains, cable losses, insertion losses and
isolation values are all expressed in dB. To determine an
amplifier output and any system losses, decibels are added
and subtracted.
The following dB conversion chart highlights minimum to
maximum signal strengths for free to air television signals.
The minimum signal for a good quality, noise free picture is
typically stated as 0dBmV although most televisions will work
with signals as small as -6dBmV. Working to the 0dBmV level
will provide a tolerance to slight signal variations. The signal
level fed into a television should be kept below 20dBmV so
the tuner is not overdriven.