65 Aleksandar Stamboliyski Str., 8000 Bourgas, Bulgaria
Tel: +359 56 820027, Fax: +359 56 836700
E-mail: [email protected] ,Web: www.devabroadcast.com
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RF ENVIRONMENT – PRECAUTIONS,
INSTALLATION SPECIFICATIONS AND ANTENNA TYPES
The observation of the RF Environment, in which DB7000 is functioning, is necessary for
ensuring normal and reliable function of the device. In order for the device to work properly, the
best conditions according the standards listed below should be provided.
The usage of an external antenna is required because DB7000 is used for off air re-broadcasting
away from the transmitter site. Proper outdoor FM antenna, antenna location and direction should
be selected. The three most popular antenna types are: omni directional, unidirectional dipole and
directional multi-element array antenna.
Because of the low antenna gain, the bad signal to noise ratio (compared to any directional
antenna) and its high multipath interfaces reception, we do not recommend the usage of omni
directional antenna with DB7000.
The other type antennas: unidirectional dipole and directional multi-element array antenna are
much proper for your needs. We recommend the usage of factory made antenna or antenna system,
manufactured specially for the FM Radio Band 88 – 108 MHz.
The recommended working RF signal strength is in the range of 55-90 dBµV. If the input RF
level is above this range, we recommend using external RF attenuators with attenuation value
between 20 – 90dB.
After selecting the antenna type that will meet your needs, the next step is its installation. The
following important principles should be considered:
- Install the antenna far enough from walls, roofs, buildings or any transmitting equipment;
- The minimum spacing between the antenna and the closest object should be more than 3
meters.
One unit DB7000 re-broadcast receiver is used for re-broadcasting of one transmitter site,
transmitting more than one program. In such cases, the best antenna that can be used is a directional
antenna directed exactly to this site.
DE-EMPHASIS AND PRE-EMPHASIS SELECTION
As well known, there are three different de-emphasis selections. One of the problems with the
high quality VHF FM transmissions is that the increased audio bandwidth means that background
noise can often be perceived. It is particularly noticeable towards the treble end of the audio
spectrum, where it can be heard as a background hiss. To overcome this, it is possible to increase
the level of the treble frequencies at the transmitter. The frequencies are correspondingly attenuated
at the receiver in order to restore the balance. This also has the effect of reducing the treble
background hiss which is generated in the receiver. The process of increasing the treble signals
is called pre-emphasis, and reducing the treble signals in the receiver is called de-emphasis. The
rate of pre-emphasis and de-emphasis is expressed as a time constant. It is the time constant of the
capacitor-resistor network used to give the required level of change.
In UK, Europe and Australia the time constant is 50μs whereas in North America it is 75μs. The
de-emphasis and pre-emphasis should be selected depending on the region you are located or can
be disabled.
Содержание DB7000
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