CCTV: back to basics (part three)

When examining recording and viewing equipment, it is important to understand the basics of the perception of vision. The human eye retains an impression of what it sees on the retina for about 40 milliseconds after it has disappeared. This equates to 1/25th of a second. Therefore, in 'real time' we need to see images at 25 pictures per second so that the displayed image appears as a natural moving picture with no flicker. This is the same for both CCTV and broadcast practices, and is known as 3-hour recording.

The object of recording equipment is to provide supporting evidence of monitored events. The recording equipment would normally be used in conjunction with camera switching or multiplexing equipment so that the image selection from a number of cameras can be displayed on a single monitor screen with perhaps slave monitors at other locations.

The options are to switch between cameras in a sequential pattern or to produce the images in a multiple display on a monitor/s, using a quad or multiplexer. We will look at the ways in which we achieve camera/image selection, and then at the recording equipment generally available to us (See Table 1 overpage).

The recording of the camera selection processes by the traditional analogue method is in conjunction with a video cassette recorder (VCR) available in time lapse or event recorder form. They can be linked to time and date generators and video printers (See Table 2 overpage). Note that domestic VCRs are not intended for industrial or commercial use.

It should also be noted that owing to the poor colour image reproduction on VHS recorders the new Super VHS (SVHS) format was introduced, offering a much higher picture resolution. SVHS enhances colour, but not black and white – and it cannot be played on VHS machines.

Inputs, outputs, and telemetry

All recording equipment has the ability to receive alarm inputs and operate alarm outputs. Therefore, these devices can respond to inputs from any activity generated manually or automatically, such as a door opening, a card being swiped, or an alarm detection device.

Outputs are used to start up ancillary parts or equipment. Once in alarm, the recorder can be programmed to stay in alarm for a set period of time, or until a reset signal is applied.

Stand alone video motion detection (VMD) units or MUX devices with VMD incorporated can often be used as an alternative to detection devices. These devices analyse the camera picture content to look for changes in the scene so as to start a recording sequence.

Alongside the camera control and recording equipment at the command centre, there is often a need for telemetry, which is effectively a signalling system used to control functions at the camera head such as pan, tilt, zoom and wash/wipe functions. The control data is sent from the controller to a receiver or receivers, which interpret the signal being generated and then operate the appropriate relay to drive the particular device. The cabling between the controller and receiver may be coaxial or twisted pair, depending on the system used.

Appointing a monitor

A CCTV monitor is similar to a television receiver, except that it does not include the tuning circuits. Although new generation alternatives to cathode ray tube screens are now available, the CRT remains the dominant monitor in all CCTV consoles. The resolution of the equipment is its ability to resolve detail, and is measured in TV lines (TVL). The higher the resolution, the sharper and more lifelike is the image that is produced. It is measured at the centre of the monitor screen where it is at its highest. When selecting a monitor, a range of equipment will be found with resolutions from 300 TVL through to 1000 TVL. The actual CRT monitor size is determined by a diagonal measurement.

Most monitors have two BNC connectors. One is provided to accept the composite video to display the images on the screen. The other provides a facility to loop through to an additional monitor (slave) or to connect a further item of equipment.

A two position switch is provided to ensure the correct termination setting. It is essential that CCTV systems are correctly terminated.

At the point the video picture terminates, generally at the viewing monitor, the switch should be set at 75 Ohms. This ensures the signal is correctly terminated. When supplementary equipment to view the same scenes is to be fed from the monitor, the switch should be set to the HI Z (high impedance) position. Z is the symbol for impedance measured in ohms.

When setting the brightness and contrast, the adjustments should be made so that the viewer can see as many picture details as possible. By reducing the contrast, the picture sharpness will improve and the CRT's lifetime is prolonged.

Remember that when setting up equipment and carrying out surveillance that under Home Office guidelines there is a need to fulfil the 'Operational Requirement'. This is required for evidential purposes and refers to the monitoring, recognition, and identification of the target on the monitor screen, as seen in Table 3.

Digital recording

This series of articles has been presented at foundation level to cover the fundamentals of CCTV for security installers. It is fitting to conclude by looking at the basics of digital recording to recognise where the future surely lies.Digital recording techniques enable video images to be stored on PC-style hard disk drives. They offer a speed of retrieval of information far superior to that of analogue, with consistent and superior picture quality, enabling archived footage to be accessed by PC driven menus.

However, at present it remains at a disadvantage, because to store the same number of pictures digitally is considerably more expensive than storing traditional VCR analogue format images. This is because of the storage media memory and hardware required to write it.

The digital video recorder (DVR) takes on the role of both the traditional VCR and multiplexer, and is used to record CCTV images digitally onto a built-in hard drive from one or a number of cameras. In the same way as analogue, the recording may be continuous or event driven, in response to an activity. The stored data can then be replayed locally at the DVR through a CCTV monitor, on a PC connected to the DVR through a local area network (LAN), or via the internet by means of an ADSL connection.

Additional features allow critical information to be downloaded from the DVR to a networked PC so that the computer’s CD or DVD writer can copy the files to a writable CD or DVD, or to an external hard drive for larger amounts of data. Alternatively, the DVR may include an integrated CD-RW drive. The digital future will certainly be influenced by the increased functionality of DVRs, governed by the industry's growing needs for more complex operating systems with the use of internet protocol (IP) working.

At the risk of sounding rather dramatic, the future of CCTV, even at foundation level, seems to have a future offering endless potential for the security installer. At present it may well be analogue, but in the long term it will certainly be digital with enhanced communications.

• Gerard Honey is the leading writer of electronic security industry textbooks including the best selling Intruder Alarms, published by Newnes, an imprint of Elsevier Science