Cam-tech Security Systems Limited

If sufficient bandwidth is provided a system can carry the standard 25 frames per second required for real-time monitoring and instant camera control.

If the bandwidth is less than that needed to carry 25 frames per second then there has to be some compromise in the quality and refresh rate of the image provided and in the ability to control the cameras. However, the effect of reduced bandwidth or “information carrying capacity” may be somewhat redressed by attempting to reduce the amount of data that needs to be transmitted in the first place. Such a reduction (or “compression”) may be achieved by using digital signal processing, to selectively remove picture information that is repeated or “deemed unnecessary”. The latter is clearly very subjective.

Different compression techniques make different choices as to what to delete from the original picture, and because of this, not all are best suited to the specialised requirements of CCTV. It has to be remembered that the recorded material may be needed for evidential purposes and so the compression system chosen must be unambiguous and uncompromisable.

The main CCTV signal transmission systems currently available are described below, with a brief description of the advantages, disadvantages and typical applications of each system. They are presented approximately in order of decreasing information carrying capacity or frequency bandwidth.

The choice of transmission method will depend on the operational requirement for the scheme, the size and nature of the area to be covered, and the available budget. A combination of systems may be required to meet the needs of the scheme.

Co-axial, Unshielded Twisted Pair or Fibre Optic cable

Very high bandwidth allows excellent image quality and instant camera response. Secure from interference or unauthorised access. May require cables to be laid causing delay and disruption. Inflexible – camera positions cannot easily be changed. Costs rise with distance covered. Fibre rental charges can be very high, although competition among suppliers may ease this. It may be better to purchase if capital funding is available.

Microwave systems

 Very high bandwidth. Flexible and very reliable, provided suitable sites are available for mounting cameras and relays. Purchase costs can be high, but avoid cable rental charges. The range between transmitter and receiver (or a repeater) is approximately 20 km, but must be “ line of sight ”. The signals will be obstructed by hills or high buildings, requiring additional signal repeaters. Very occasionally weather interference may occur. No signal encryption is generally necessary to guard against unauthorised access by third parties as the radio beam is so directional A cheaper alternative to cabling in town centres. Used to link main schemes with isolated areas such as housing estates. It is very important to check that the equipment is correctly licensed.

Short-range omni-directional radio systems

Very high bandwidth. Low cost purchase and operation. Signals can be affected by terrain. Systems may require an individual radio licence, depending on frequency, range and power. Range limited to a few km by licence conditions. License exempt systems are legal, but offer no protection against interference from other radio users. Being omni-directional these systems require security encryption of signals. Usually involve a permanent second link such as fibre or microwave.

 Infra-red systems

Although not radio frequency, these systems have a similar functionality to microwave links. However, they are prone suffer to a greater degree from atmospheric interference such as snow and rain, and are only suitable for very short local transmission distances.

Telephone transmission

 Unlimited range, subject to availability of suitable lines. Flexible, secure – no encryption required. The lower bandwidth reduces the rate at which data can be transmitted, and hence the picture size, image quality, and response time available. Systems are unable to achieve the 25 frames per second refresh rate required for real-time viewing. To increase the frame rate some manufacturers trade-off, with varying degrees of success, some aspects of picture quality using video compression techniques. Cameras need to be ‘dialled-up’ through the telephone system, causing potential delay. Line rental costs can be high, depending on the service provider. These can be reduced by competition. Used where transmission is required over long distances, and where an immediate response is of less importance. Useful for monitoring remote sites and for alarm verification.

Global System for Mobile telephone links

Unlimited range, subject to availability of signal. Relatively inexpensive. Flexible, secure, no encryption required. The very low bandwidth means a slow refresh rate and camera response time. Video data compression techniques are employed to overcome these shortcomings, but the price paid may be inferior recognition standards. A partial solution is to hold real-time, higher-quality images on site for a period for evidential purposes. There are potential problems of network provision and capacity in some geographical areas. Limited use so far for public space CCTV makes it difficult to identify potential applications. Forthcoming increases in GSM bandwidth provision and planned new technologies in this area will help to improve frequency and quality of images.

Internet based systems The limiting factors are the telephone transmission system and the Internet protocols. These factors make current Internet based systems relatively slow, but they can be used successfully for remote monitoring of sites, where only an occasional camera image is required.