Requirements To Produce A Good Quality Picture
All cables, no matter what their length or quality, produce problems when used for the transmission of video signals, . the main problem being related to the wide bandwidth requirements of a video signal. All cables produce a loss of signal that is dependent primarily on the frequency, the higher the frequency, the higher the loss. This means that as a video signal travels along a cable it loses its high frequency components faster than its low frequency components. The result of this is a loss of the fine detail (definition) in the picture.
The human eye is very tolerant of errors of this type; a significant loss of detail is not usually objectionable unless the loss is very large. This is fortunate, as the losses of the high frequency components are very high on the types of cables usually used in CCTV systems. For instance, using the common coaxial cables URM70 or RG59, 50% of the signal at 5MHz is lost in 200 metres of cable. To compensate for these losses, special amplifiers may be used. These provide the ability to amplify selectively the high frequency components of the video signal to overcome the cable losses.
There are two main types of cable used for transmitting video signals, which are: Unbalanced (coaxial) and balanced (twisted pair). The construction of each is shown in diagrams 2 and 3. An unbalanced signal is one in which the signal level is a voltage referenced to ground. For instance a video signal from the camera is between 0.3 and 1.0 volts above zero (ground level). The shield is the ground level.
A balanced signal is a video signal that has been converted for transmission along a medium other than coaxial cable. Here the signal voltage is the difference between the voltage in each conductor.
External interference is picked up by all types of cable. Rejection of this interference is effected in different ways. Coaxial cable relies on the centre conductor being well screened by the outer copper braid. There are many types of coaxial cable and care should be taken to select one with a 95% braid. In the case of a twisted pair cable, interference is picked up by both conductors in the same direction equally. The video signal is travelling in opposite directions in the two conductors. The interference can then be balanced out by using the correct type of amplifier. This only responds to the signal difference in the two conductors and is known as a differential amplifier.
Unbalanced (Coaxial) Cables
This type of cable is made in many different impedances. In this case impedance is measured between the inner conductor and the outer sheath. 75 Ohm impedance cable is the standard used in CCTV systems. Most video equipment is designed to operate at this imped+ance. Coaxial cables with an impedance of 75 Ohms are available in many different mechanical formats, including single wire armoured and irradiated PVC sheathed cable for direct burial. The cables available range in performance from relatively poor to excellent. Performance is normally measured in high frequency loss per 100 metres. The lower this loss figure, the less the distortion to the video signal. Therefore, higher quality cables should be used when transmitting the signal over long distances.
Another factor that should be considered carefully when selecting coaxial cables is the quality of the cable screen. This, as its name suggests, provides protection from interference for the centre core, as once interference enters the cable it is almost impossible to remove.
Balanced (Twisted Pair) Cables
In a twisted pair each pair of cables is twisted with a slow twist of about one to two twists per metre. These cables are made in many different impedances, 100 to 150 Ohms being the most common. Balanced cables have been used for many years in the largest cable networks in the world. Where the circumstances demand, these have advantages over coaxial cables of similar size. Twisted pair cables are frequently used where there would be an unacceptable loss due to a long run of coaxial cable.
The main advantages are:
1) The ability to reject unwanted interference.
2) Lower losses at high frequencies per unit length.
3) Smaller size.
4) Availability of multi-pair cables.
5) Lower cost.
The advantages must be considered in relation to the cost of the equipment required for this type of transmission. A launch amplifier to convert the video signal is needed at the camera end and an equalising amplifier to reconstruct the signal at the control end.
It is extremely important that the impedances of the signal source, cable, and load are all equal. Any mismatch in these will produce unpleasant and unacceptable effects in the displayed picture. These effects can include the production of ghost images and ringing on sharp edges, also the loss or increase in a discrete section of the frequency band within the video signal.
The impedance of a cable is primarily determined by its physical construction, the thickness of the conductors and the spacing between them being the most important factors. The materials used as insulators within the cable also affect this characteristic. Although the signal currents are very low, the sizes of the conductors within the cable are very important. The higher frequency components of the video signal travel only in the surface layer of the conductors.
For maximum power transfer, the load, cable and source impedance must be equal. If there is any mismatch some of the signal will not be absorbed by the load. Instead it will be reflected back along the cable to produce what is commonly known as a ghost image.