Video switching

From CCTV Information
Jump to: navigation, search


There are few CCTV systems that have only a single camera apart from door entry or vehicle rear view systems, etc. Most systems incorporate more than one camera and therefore have the need to select the view from any camera on to a monitor. This chapter covers the main types of video switcher and their applications.

Principles of Video Switching

It would be possible to switch video signals using simple toggle switches but this would introduce several undesirable results. The switching could cause severe interference on the screen due to the induced noise on to the signal. There would be a lot of picture roll until the monitor became synchronised to the next camera. The picture might be unstable until the monitor is synchronised correctly.

Modern video switchers incorporate electronic switches and a technique known as ‘vertical interval’ switching. When a new camera is selected, the electronic circuits wait a fraction of a second until the field sync pulse of the video signal is detected and then switch over. This allows the monitor to lock immediately on to the new line sync pulse and the new picture is displayed without any rolling. This assumes that all the cameras in the system are compatible and on the same phase of the supply. The elimination of picture bounce is the main reason for specifying that all cameras are on the same phase of the supply. There are cases where it is not possible to connect all cameras to the same phase such as large industrial sites or systems having cameras in several buildings. There are cameras available with ‘phase adjustment’ controls. This allows the video signal to be transmitted out of phase from the local supply and in phase with the other cameras. In many cases, the adjustment is too coarse for accurate alignment and the result would be a small amount of ‘bounce’ but not a complete roll of the picture. The measurement should be carried out at the monitor using a dual trace oscilloscope. One trace would show the local mains sine wave. The other would show the camera output and its relationship to the supply.

The Basic Video Switcher

The simplest switcher is one that includes the features mentioned previously and where the coaxial cables are connected directly into the rear via BNC plugs. These switchers usually have a number of buttons according to the number of cameras in the system. They are mainly 2, 4, 6, and 8 way units. This type of switcher is usually known as a manual switcher where the keys directly switch the cameras.

Switchers are usually terminated with a 75-ohm resistor, as is the monitor. In the case of the system shown in diagram 6.1 the terminations at both the switcher and the monitor should be left at 75 ohms.

Most switchers have two other controls, one to set the cameras to sequence automatically, the other to adjust the dwell time between switching from one camera to the next. The dwell time will be the same for each camera in the system.

Diagram 6.1 System with Simple Manual Switcher

Looping Switchers

On occasions, it may be required to loop one or more cameras to part of the system or another switcher, for dual control. Here a switcher with loop through facility would be used. This type of switcher will have two rows of BNC connectors, one above the other. There will also be a switch adjacent to each camera input, the purpose of which is to set the 75-ohm termination on or off. One position of the switch will usually be marked ‘high,’ the other ‘low’ or 75 ohm. The camera inputs are normally the top row of connectors with a corresponding loop through connector below. The camera signals that are required to carry on to another location would be taken off the output connectors via BNC plugs. The termination switch next to each looped through camera should be set to ‘high.’ The signal should then be terminated at 75 ohms at its destination. Some switchers with looping outputs do not have a termination switch. Instead the resistance is set to ‘high’ and plugs with a built-in 75-ohm resistor are provided to fit in unused outputs.

It is not acceptable to loop through a video signal by using a BNC ‘tee’ connector. If this is the only way available then the internal 75-ohm resistor inside the unit should be snipped out, The correct termination at the end of the line should be ensured.

Diagram 6.2 Rear Panel of Looping Switcher.

Switchers with Additional Features

Switchers are available with two monitor outputs. Normally one monitor can be set to sequence through the cameras and the other used as a selectable spot monitor.

Another feature available on many switchers is the capability to accept alarm inputs. There is usually one alarm input to each camera input. If there is an input from an alarm, the switcher will automatically switch the monitor to the associated camera. An alarm input will override a sequence if it is set up and hold the selected camera on the monitor. In the case of a switcher with dual monitor outputs one monitor will switch to the alarmed camera while the other continues to sequence.

Remote Switchers

Often it may be inconvenient or difficult to route all the coaxial cables to a desktop switcher. This is especially the case if there are eight, sixteen or more cameras in the system. A remote switcher is one where the camera cables are connected into a panel containing all the switching electronics. This box can be situated anywhere convenient for routing the cables. The desktop control unit is then connected to the remote panel by a small two or four core cable or sometimes a single coaxial cable. The coaxial cable to the monitor(s) is connected to the remote panel.

Diagram 6.3 System with Remote Switcher.

Remote switchers can generally be more sophisticated than the desktop type and can incorporate more features. There can be up to six or eight monitor outputs and more versatile handling of alarm inputs. In addition, several keyboards may be incorporated into one system. This allows selection of cameras from more than one control position. The controls in this type of system are generally of the master and slave type, which means that the controls are not totally independent. Where greater flexibility is required then the choice would be to use a matrix switcher as described in the following section.

For a system with more than four cameras, remote switchers can achieve significant savings in installation costs.

This article is an extract from chapter 6 of 'The Principles & Practice of CCTV' which is recognised as the benchmark for CCTV installation in the UK.