Volume 6, Number 3
April 2009

Analog vs. Digital Considerations for Integrated Physical Protection Systems

by David Lambert
INMM Physical Protection Technical Division Chair
Oak Ridge National Laboratory

The debate concerning the use of analog or digital signals in commercial radio and television transmissions has been going on for some time in major countries around the world. In the United States, the official changeover date has been postponed to June 2009 to give consumers more time to complete the transition. Alarm control and access control systems are also undergoing changes caused by the continuing convergence of physical protection and information technology. More and more systems are being networked to, among other things, provide monitoring flexibility and because of the increased storage capacity of digital data.

Technically speaking, analog signals are signals with continuous values. The best way to describe it is that the video and audio signals are modulated directly to a carrier wave, which is then transmitted through the air, cable, via satellite, etc. During this transmission, the carrier and the modulated signal will lose amplitude (power) due to interference from noise introduced to the carrier and its modulated signal. The result will always be a received signal that has a lower quality than the transmitted signal. In contrast, digital signals are signals that are represented by binary numbers, "1" or "0", also used in computer coding. The result is a received signal that is an exact copy of the transmitted signal. A digital system can be implemented with a generic microcontroller, and some quick programming.

Analog signals require hardware receivers and transmitters that are designed to perfectly fit the particular transmission. If modifying a system changes the analog signal, the hardware (transmitters and receivers) have to be completely changed. On the other hand, digital signals can be handled by simple, standardized receivers and transmitters (more like computer processors), and the signal can be then dealt with in software (which is comparatively cheap to change).

Although digital signals are generally associated with the binary electronic digital systems used in modern electronics and computing, digital systems are actually ancient, and need not be binary nor electronic:

• A beacon is perhaps the simplest non-electronic digital signal, with just two states (on and off).
• Smoke signals are one of the oldest examples of a digital signal, where an analog "carrier" (smoke) is modulated with a blanket to generate a digital signal (puffs) that conveys information.
• Morse code uses six digital states—dot, dash, intra-character gap (between each dot or dash), short gap (between each letter), medium gap (between words), and long gap (between sentences)—to send messages via a variety of potential carriers such as electricity or light, for example using an electrical telegraph or a flashing light.

In physical protection systems, there seems to be a sense of urgency to move from analog to digital system configurations. However, many of the valid reasons to transition from analog to a totally digital structure are not factors influencing the effectiveness of currently installed detection and assessment systems.

• More data can be transmitted over a digital system. The security systems installed at many facilities are basically “closed” systems with very little need to expand past current capacities. From a strictly security perspective, protection of critical assets is paramount and there is no need to provide anything more. Many existing analog infrastructures have been designed and upgraded to provide an adequate level of protection, so the expense of transitioning to a totally digital system may be accomplished more cost-effectively over time.

• Data stays more consistent over distance in a digital system. Analog pulses transmitted via wires are typically attenuated by the resistance of the wire, and changed by its capacitance or inductance. Temperature variations can increase or reduce these effects. While digital transmissions via wires are also degraded, slight variations do not matter since they are ignored when the signal is received. With an analog signal, variances cannot be distinguished from the signal and so provide a kind of distortion. In a digital signal, similar variances will not matter, as any signal close enough to a particular value will be interpreted as that value. Most analog alarm control and display systems account for the natural attenuation of signal through the use of in-line amplifiers and other means to maintain signal integrity over relatively short distances within sites.

• Digital signals can carry a variety of data types. There are two basic types of signals that need to be transmitted in a typical alarm control and display system: alarm data and video. Any data not associated with detection and assessment is relatively unimportant to the protection of critical assets. Additionally, analog video signals (which use a considerable amount of bandwidth) are easily converted to digital using software at the main processor to improve archiving, retrieving, and viewing. Such a practice can provide the enhanced storage of digital data without the need for wholesale replacement of existing hardware.

Transitioning from analog to digital signal transmission in physical protection and access control systems should not be confused with analog to Internet Protocol (IP)-addressable network transmission. Analog and digital signals can both use existing twisted-pair wiring and coaxial cables to transmit information to a central alarm control and display system. An IP-addressable system is really a totally digital, local area (or wide area) network (LAN or WAN) requiring installation of CAT 5 or CAT 6 cabling instead of wiring. Network-capable cameras and sensors would also be required to ensure total integration of the system. This could involve a very expensive retrofit of existing systems’ hardware and software.

Changing signals from analog to digital (including video) is easily done through use of commercially available signal converters installed prior to the primary alarm control and display processor. Most security vendors around the world have already demonstrated the ability to modify their systems and provide digital storage of alarm data and video imagery without having to purchase different cameras, sensors, or cabling. Several experts agree that most countries cannot complete switching their broadcast signals from analog to digital until at least 2015, there should be plenty of analog equipment available to support the existing systems until then.