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Twisted Pair
An alternative to the use of coaxial cable is the ‘twisted pair cable’.A twisted pair
cable is known as a ‘balanced’ cable and can be similar to telephone wires. Most of
the same considerations apply to twisted pair as apply to the coaxial but there are
some expectations. The twisted pair system will accept the standard 75 ohms, 1
volt composite video signal and convert it to a balanced video signal of 2 volts
with an output impedance of 50 to 150 ohms to suit the particular twisted pair
being used.
The approach is relatively inexpensive and can transmit over longer distances
than conventional coaxial cable. The major disadvantage is that transmitter and
receiver units are required for every video signal source. This is because units such
as monitors which process base band video signals will not accept the balanced
twisted pair type of transmission.
Fibre Optics
Fibre optic transmission of video signals is now widely used within the CCTV
industry, most particularly on large projects such as town centres. Optical fibres are
fine strands of glass with a high quality of optical transparency which act as wave
guides for light beams. They have lower transmission losses than conventional
cables and are virtually immune to electrical interference.
Developments in laser technology have made optical fibre a useful means of
video transmission. The video signal modulates the laser beam, which is then
transmitted through the fibre. Picture quality remains excellent over distances
greater than 50km.
ISDN Transmission
An increasingly popular method of transmitting video signals is to use ISDN. This is
a digital phone line which transmits video signals at 128 kilo bits per second. The
signals are encoded at site, transmitted across the telephone network, and then
decoded at the monitoring centre. These systems are event driven which means
that they only transmit video when an event has occurred – for example a break-in
at a property activates a PIR which then causes the transmitter to dial the
monitoring centre which can then take appropriate action.
These systems do not provide live video images because of bandwidth
constraints but some systems, like the Sony PMS-500 Codec, can transmit up to 15
frames per second. The PMS-500 can also transmit and receive full duplex audio,
together with data signals for control of telemetry and other remote devices via the
RS232 and RS485 communication ports.
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For more information contact the Sony Business Information Centre: 01932 816340
Transmission of Video Signals