1851 SITO/City & Guilds masterclass [part 33] CCTV [paper 9] - Controlling cameras

Direct wiring is where the individual camera function is linked directly to the operator control and the supply voltage is sent directly down the cable (see fig 1). This method should use low voltage units and is not recommended where there are long cable runs or a large number of cameras to control, but is ideal for the smaller installation.

This system will utilise a multi-core cable and each function will have its own feed. For panning there will be three cables – pan left, pan right and neutral. The same applies for tilt, although the neutral may be common to both. This is linked to a control unit which will consist of a number of switches which, when closed, send the supply down to the function required.

In some control units the individual switches for pan and tilt may be replaced with a joystick. This has the same effect, closing the required microswitch according to the position of the joystick but it is easier to operate.

Relay interface

Low voltage units may not be powerful enough for a heavy duty application and 240v units may be required. In this situation direct wiring is not recommended. If multi-function wiring is still to be used it will be better to use a relay interface. This system can also be used where control of cameras from more than one point is required.

The operator control and interwiring is the same as described for direct wiring but an additional set of relays are added at the camera control point. A separate power supply provides the drive for the units which is switched via the relays. Power to switch the relays is fed from a low voltage supply at the control but the neutral is switched through the operator's control.

Control of the camera functions through this method allows a second or third point to be added. To prevent two operators clashing, an inhabit device is usually fitted to allow only one unit to operate at a time.

Telemetry

The third method of control is through a system known as telemetry. While initially more expensive than wiring, it is easier to install and can be used over large distances – and can prove more reliable. Instead of sending an operating or switching voltage down a multi-core cable, telemetry sends a coded signal. This coded signal is activated by the operator selecting a function key. This prompts an encoder in the transmitter to produce a series of pulses. Each function has a different set of pulses.

This signal is then sent by the controller to a receiver unit located near to the camera. The receiver decodes the signal, interprets the command and switches the required relay supplying a drive voltage to the appropriate function. Lens functions may be driven directly off the receiver board, not through the relays.

Because we are sending a coded signal we do not need a multi-core cable and the signal can be sent down an ordinary twisted pair cable which is wired as a current loop. On some systems the signal could be sent down the video signal cable. This is achieved by sending the control pulses up the cable during the frame blanking period. That is the time between picture frames when no video is being transmitted.

Single channel or multi-channel telemetry systems are available, the single channel unit consisting of one transmitter control unit and one receiver. The multi-channel units consist of a video switcher combined with a transmitter control unit. The selection of the required camera switches the telemetry transmitter to that channel. This means that the one unit can be used to drive a number of receivers, with one receiver at each camera position (see fig 2).

Safety

Many of the units we have discussed operate at 240v AC potential. Care must be taken when working on these units to ensure that they are isolated from the supply first. Special care must be taken when operating on the site where units are located above ground level and subject to wet conditions. Always ensure all equipment is earthed correctly in accordance with the current regulations.

The ability to be able to stamp the picture with the time and date of recording can be useful in two ways. Firstly, it can be helpful when searching the tape in play-back. Secondly, it can provide valuable proof of recording times when incidents are captured. If the tape is likely to be used to substantiate evidence, then a time and date overlay is essential. Some equipment such as time lapse recorders and multiplexers may have time and date generators built into them – in that situation a separate unit is not necessary.

When time and date is required and does not already exist it can be added very simply by fitting one of the In-Line time and date generators available. The video signal to the video recorder or monitor is fed into the unit. A special integrated circuit (IC) known as a clock chip generates a synchronised pulse. This is driven by a very accurate oscillator controlled by a crystal which ensures the pulse timing is exact.

The pulse from the clock chip is fed into a counter, which outputs a signal to the code word generator. This is usually a ROM (read only memory) chip which produces a set coded message. The code word generator sends an updated code to a RAM (random access memory) chip every time a digit is to be updated.

This is stored in the RAM until a new code word is received. The RAM then sends the code word to a character generator that will produce the character required. This is produced as a video signal which is then mixed with the input from the camera or switcher.

Video motion detectors

While CCTV has many advantages it is basically a passive system. This means constant monitoring and checking which is labour intensive and open to human error. To reduce these problems several systems have been developed which provide an active element to the CCTV system. The first is video motion detection. Most motion detectors are fitted in line between the signal source and the recorder or monitor because it used the incoming video signal. This means they can easily be added to the system after installation.

If we imagine we are looking at a camera picture being displayed on a monitor, the scene is likely to be made up of several elements, some of which need protecting, while others have free access. The first stage of motion detection is to separate the areas where movement detection is required from those where movement is acceptable. This is normally achieved by the operator using a built-in grid or dictating areas of the screen to be monitored. The signal from the camera is fed into the video motion detector.

A sample of the area to be monitored is converted from its analogue form to a digital signal and stored in a frame store as a series of pixels (picture elements). The incoming signal is checked against the stored data by sampling the signal converting it to digital data and comparing it to the stored sample. Where there is movement across the area being monitored the content of the signal will change as the light levels vary. If we look at an example of the window scene used in the camera unit, (if someone walks across the window this would cause a change in the output signal from the camera as the light level changed. It is the change in the signal that is recognised by the VMD (see fig 3).