Selection of TV signals transmission lines is very important in realizations of diverse projects, because the quality of receivable TV image depends on the communication lines specifications. There are several signal transfer mediums: twisted cables, coaxial and fiber-optic cables, wireless and telephone communication lines, computer networks. Each one will be overviewed.
Coaxial cable is the most widespread transmission facility of TV signals. It belongs to the nonsymmetrical communication line category and contains a center conductor, dielectric material and armour. The signal transmitted over the central conductor, and the armour is a shield. External interferences, induced by the armour, are grounded. This way, the level of cables external interferences amenability depends on the armour stiffness. Coaxial cables armour can be twisted or foiled. The cable with twisted shield usually used in security and video control systems, because it is more resistant to external interferences.
Foiled coaxial cable doesn’t used in the security TV, so it is susceptible to low-frequency interferences. This kind of cables are wide used in cable TV, where the signal transmits by subcarrier frequency. During a coaxial cable assembly, it is necessary to remember about a probability of leakages appearance in places, where the electric cables lay close enough. It is also needed to take into account that the foreign ground currents may appear while signals passing through the center conductor. It happens when the receiving and transmitting equipments are connected to diverse electric supply phases or grounding points. This problem comes out in image deformation as a consequence of background interferences attack. There also may be an image loss of synchronization.
Background interferences can be removed in a very easy way – by isolation transformer installation. It is set into a coaxial line break on the receiving side. The effect evolves because of a break in a interference circuit in the center conductor and a shielded part. Foreign ground current discontinues and background interferences disappear. Though, the isolation transformers have a disadvantage: the signal bandwidth is unsteady. That’s why the most effective way of the frequency interference removing is galvanic isolation devices installing, which are used on the base of an optoelectronic transducer. Besides, the devices enable to neutralize electromagnetic blasts with network wide frequency of 50 Hz.