This is the fourth paper in the CCTV series, in which we look at Video Switching. The simplest form of CCTV system is to have one camera connected to one monitor, which is fine for a basic system, but what if several cameras are required in a particular application?
The answer is either to have a bank of monitors, one for each camera, or use some form of switching so that any one camera can be selected by the monitor. Realistically, it is more practical to have a switching device, as a large number of monitors would be expensive and take up a lot of room. This is the basic concept of video switching: to provide an interface between the monitor and a number of cameras.
VIDEO SWITCHING PRINCIPLES
A number of camera input signals are fed into the switching unit. The number of inputs on a unit will depend on the type, but standard input configurations are 2, 4, 8 or 16.
The switching is carried out in Integrated Circuits (ICs), which have internal switches known as logic gates. These gates are controlled to select a specific camera, operated either manually or automatically. Most video switchers have a selection switch that allows the operator to select which.
Manual Selection This is carried out by the operator selecting a camera control button. The button selection closes the switch contact causing the selection control circuits for that camera to operate. This allows the signal to flow from the selected camera to the switcher output.
Automatic Selection This is achieved in the same way as manual selection except that the selection circuits are activated automatically. An internal timing circuit causes the camera selection circuits to operate in rotation. The speed of this rotation is known as the 'dwell time', referring to the length of time the unit will select each camera to display for). This can normally be pre-set by the operator and may offer options from five seconds to one minute.
One of the advantages of automatic camera selection is the help it offers busy operators by sequencing the pictures without any need for manual intervention.
SWITCHER TYPES
There are many different types of switchers available with different input and output specifications as well as varying control techniques. For details on the individual types and specifications it may be necessary to refer to the manufacturers' data, but initially we can divide video switching into two groups: integrated and segregated.
Integrated Switching: This incorporates the video switching unit with the operator control into one unit. This means that all the camera inputs and monitor outputs are connected into one unit that also has camera selection and control built in. Integrated switchers appear in a number of formats, such as:
Segregated Switching This differs from integrated switching in that the video switching process is separate from the operator controls. The switcher units come in two distinct parts: the video processing unit, normally referred to as the remote unit; and the selection unit, referred to as the controller.
The segregation means that there must be a link between the two units so that switching information can be sent from the controller to the video switching circuits. There are two ways in which this link is made, using either the signal cable between the switcher and the monitor, or a lightweight multi-core cable.
SIGNAL CONTROL CABLE
Using this system, the operator's controller is fitted on to the signal line between the monitor and the remote switching unit.
The control information is sent from the controller to the remote unit in the form of pulses down the existing signal line. The pulses are a code that will provide the information to allow the selection circuits to operate and switch to the next camera. With this type of unit, camera selection is normally a stepping process rather than direct access to a particular camera input.
This does mean that the units need a minimum of wiring with the camera inputs fed into the remote unit and a single signal cable from the remote unit to the monitor via the operator controller.
SEPARATE CONTROL CABLE
This system uses a separate multicore cable linked between the remote unit and the operator control. The cable usually carries the control information and a low voltage supply to operate the controller. The advantage of using this system lies in the selection of cameras.
Manual selection of cameras can be random and immediate as the system goes directly to the required input rather than stepping through. These switchers will have the camera input signals fed directly into the remote unit.
Installation of these units is a little more complex than the single cable units and it is important to ensure that all the control cable connections are secure and clean.
These may terminate in a connector such as a DIN plug and it is important to ensure that each connection is suitable. (*This is an important point because many problems that occur in CCTV are because of poor terminations.)
MONITOR OUTPUTS
Up to now we have looked only at the switchers as having a number of camera inputs with just one output to the monitor, whereas modern switchers tend to offer several, or multiple, outputs.
The basic design is the same, but for each output there is a separate switching circuit, each deriving its input from the same source. In effect, this gives us a number of individual switchers in one unit, enabling a number of monitor points to be independently run off the one switcher unit.
As each output is, in effect, a separate switcher, the control for each is independent from the others. This means that camera selection can be different for each output.
The number of camera inputs and monitor outputs vary according to the design and manufacture, so it may be necessary to look around for the most suitable unit for a particular application. This type of switcher offers many advantages to both installer and user and is common in use.
Well, that's all for this month. Next time round we'll be taking a look at multiplexing.
Source
Security Installer
