Sample Chapter: 15 Transmission, cable
From CCTV Information
Sample chapter from Mike Constant's book: The Principles & Practice of CCTV
Transmission of Video Signals
This is not meant to be a textbook on transmission but is intended to remove some of the mystery associated with various methods of transmission. Many approximations and simplifications have been used in writing this guide. This is to make the subject more understandable to those people not familiar with the theories. For general application in the design of CCTV systems it should be more than adequate and at least point the way to the main questions that must be addressed. The manufacturers of transmission equipment will usually be only too keen to help in final design.
This first part deals with the transmission of video signals by cables. Part 2 deals with the transmission of video signals by other methods such as microwave, telephone systems, etc.
Diagram 15.1 Methods of Transmitting A Video Signal
Diagram 15.1 illustrates the many methods of getting a picture from a camera to a monitor. The choice will often be dictated by circumstances on the location of cameras and controls. Often there will be more than one option for types of transmission. In these cases there will possibly be trade offs between quality and security of signal against cost.
General Principles
Video Signal
The essential components of the video signal are covered in Chapters two and three. Certain aspects that are related to the effective transmission of those signals are repeated in this chapter where it is necessary to save continuous cross-reference.
Synchronising
The video signal from a TV camera has to provide a variety of information at the monitor for a correct TV picture to be displayed. This information can be divided into: Synchronising pulses that tell the monitor when to start a line and a frame; video information that tells the monitor how bright a particular point in the picture should be; chrominance that tells the monitor what colours a particular part of the picture should be (colour cameras only).
Bandwidth
The composite video output from the average CCTV camera covers a bandwidth ranging from 5Hz to many MHz. The upper frequency is primarily determined by the resolution of the camera and whether it is monochrome or colour. For every 100 lines of resolution, a bandwidth of 1MHz approximately is required. Therefore, a camera with 600 lines resolution gives out a video signal with a bandwidth of approximately 6MHz. This principle applies to both colour and monochrome cameras. However, colour cameras also have to produce a colour signal (chrominance), as well as a monochrome output (luminance). The chrominance signal is modulated on a 4.43MHz carrier wave in the PAL system therefore a colour signal, regardless of definition, has a bandwidth of at least 5MHz.
Requirements to Produce a Good Quality Picture
From the above it will be obvious that to produce a good quality picture on a monitor, the video signal must be applied to the monitor with little or no distortion of any of its elements, i.e. the time relationship of the various signals and amplitude of these signals. However in CCTV systems, the camera has to be connected to a monitor by a cable or another means, such as Fibre Optic or microwave link. This interconnection requires special equipment to interface the video signal to the transmission medium. In cable transmission, special amplifiers may be required to compensate for the cable losses that are frequency dependent.
Cable Transmission
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.
Cable Types
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 13.2 and 13.3. 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 impedance. 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.
Diagram 15.2 Unbalanced Cable