Sep 1, 2001 12:00 PM

Digital technology is on everyone's mind, but sometimes the more technical aspects of digital can be confusing. This article will address common digital queries in an alphabetical format.


An audit trail is a way of authenticating a digital image. It comprises a digital signature made up of a "fingerprint" related to information about the capture of an image. Audit trails are widely used by the police in audio recordings, and an audit trail on a digital recorder can show a combination of vital details on each video frame. These details can include time and date of creation and the camera on which the image was recorded, thus proving beyond doubt that an event happened at a particular time.

Authentication of a digital image may be done in many different ways, including digital signatures, encryption, audit trails and watermarking. Digital signatures are built from information within each image, which are then encrypted and stored with the rest of the image data. Each frame will have its own signature. The image can be copied many times with the signature still embedded, so at any stage, it is possible to verify the image as unchanged. Any alteration of the image would no longer correspond to the signature. Watermarks usually comprise an invisible pattern woven into the structure of the image. They are generally used to prevent and detect copying, but also offer a degree of authentication. Fragile watermarks destroy the image if it is changed in any way (e.g. cropped, zoomed, compressed, etc.) whereas robust watermarks withstand image manipulation. Watermarks do not usually hold audit trail information and some watermarks degrade image quality.

Bandwidth determines the amount of data that can be transmitted in a fixed amount of time. For digital devices, the bandwidth is generally expressed in bits-per-second (bps) or bytes-per-second (Bps). For analog systems, the bandwidth is expressed in cycles-per-second, or Hertz (Hz). Transmission through a low-bandwidth public access line can result in poor image quality, due to a high compression ratio. However, a special digital connection, such as an ATM network, which uses a higher bandwidth, reduces compression ratios, transmission time and provides good quality images, even when transmitted over long distances.

Compression is the task of removing non-essential information from an image in order to reduce storage space. There are many types of digital compression methods, the most commonly used in the security industry are JPEG, MPEG and Wavelet. Compression is expressed as a ratio; current compression methods can remove anything between 90 to 99 percent of an image's information. At a 10:1 compression ratio, 90 percent of information is lost, while at 100:1, 99 percent is lost. With over 600Kbytes of data in a single color full frame, compressing it at a ratio of 10:1 will shrink it to around 60Kbytes ! a considerable difference.

CCIR monitors are commonly used in CCTV applications, as opposed to VGA monitors, which are used in computer-based applications. CCIR monitors display images at a higher resolution than VGA. While CCIR images may be displayed on VGA monitors, the opposite is not true, which means that integrating a PC-based recording system into an existing system may incur hidden costs for replacement of CCIR monitors.

Digital recording is the new wave in the security industry. Just as compact discs have replaced vinyl, and e-mail is taking over from faxes, digital recorders appear ready to take over and replace analog VCRs.

New technology should revolutionize the way the security industry operates, with innovative features available from digital systems. Digital advantages include pre/post event recording, automated event-detection, synchronous record/replay/archive, instant access to images, image security and image transmission. One of the most obvious benefits of digital is its future potential. Digital opens up many exciting possibilities.

Using digital images as evidence in court raises many queries as to their authenticity. Initial fears that digital technology allowed the potential for tampering and image manipulation could lead to missed opportunities, a reluctance to use digital imaging technology and the failure to fully understand the process.

The government encourages the use of authentication techniques such as high-security encryption to enhance image confidence and ensure the image cannot be altered ! criteria crucial for evidential purposes. Once the procedure is explained and the implications of any measures to substantiate authenticity is understood, digital imagery could measure up and indeed exceed existing standards in legal evidence.

Resolution refers to the sharpness and clarity of an image. The term is most often used to describe monitors, printers and bitmapped graphic images. Monitors and other I/O devices are often classified as high- (or full) resolution, medium-resolution or low-resolution. The actual resolution ranges for each of these grades shift as technology improves.

Resolution makes big differences with regards to image quality and storage requirements. Low-resolution digital images look worse than low-resolution analog images due to "pixelation."

Genlocking is a method that enables a composite video machine, such as a television, to accept two signals simultaneously. This is normally achieved with a Time Base Corrector, which locks one set of signals while it processes the second set. It enables you to combine two or more video signals to be combined.

If multiple cameras feed into the same recorder, the time to switch from camera to camera generally halves the available recording rate. Genlocking (or synchronizing) the cameras to overcome this restriction requires additional wiring and can be expensive ! a factor worth bearing in mind when purchasing a digital system.

Hard disk means simply a magnetic disk on which computer data can be stored. Hard disks are initially more useful than CDs or DVDs as they can store and retrieve data faster. A hard disk can store anywhere from 10 megabytes to several gigabytes, whereas CDs hold under one Gbyte and for useful data rates essentially read-only.

Hard Disks can currently store up to about 20 hours of recording at 30 frames-per-second, and almost a month's worth at one frame-per-second time-lapse. After this, the disk must transfer archive information to another medium.

Digital images recorded onto hard disks produce better image quality. They are good for storing large amounts of data in small compact cassettes, are easily manageable, and can be electronically indexed to improve search times.

An argument that analog companies frequently use to dissuade prospective buyers from purchasing a digital system is that digital images are of poorer quality, especially when compressed and transmitted over a long distance. However, digital images do not exhibit the jitters often seen on standard VCRs, nor are fast searches streaked with frame bars.

An advantage of the digital image is the quality remains the same after hundreds or even thousands of viewings. Standard VCR tapes are subject to image degradation.

JPEG (Joint Photographic Expert Group) is a well-established compression method that operates under the ISO (International Standards Organization). It works by isolating and removing spatial redundancies from each still frame, or series of frames. Compression ratios are variable, with highly compressed images not looking as "blocky" and pixelated as images compressed at the same ratio using other methods.

JPEG may also be referred to as M-JPEG, or motion JPEG, which indicates that JPEG principles are applied to moving video, as opposed to still images or single frames. JPEG is also called 'MPEG ! I frame only' by some security manufacturers.

A KByte is a unit of storage in a computer. A single television image field needs about 300 Kbytes of data. One MegaByte equals 1,024 Kbytes.

A LAN or Local Area Networking is a computer network, which is confined to a room, a building, or a group of adjacent buildings. Digital images can be transmitted via ethernet (the most frequently used area network) to anywhere in the near vicinity.

For example, if the director of security wished to check on proceedings in the central control room, images could be accessed in a separate office in the building using LAN. Directors can become actively involved in the security procedure and keep a constant overview anywhere in the building through the application of Local Area Networking.

MPEG (Moving Pictures Expert Group) is a full motion compression method, regulated by the ISO. It uses information from key frames to build a picture of what happened in the frames in between. This taking of information from key frames can introduce compound errors noticeable when examining single frames. It also results in a time delay, from comparing and processing the information between the frames. This delay is only one second when recording in real-time, but is greatly exacerbated when recording in time-lapse mode.

A network is simply a group of two or more computers linked together. Many types of networks exist, but the most common types of networks are local-area networks and wide-area networks. Networking enables the user to access images from any location.

Increasingly, many high-security sites will not only have their captured CCTV images monitored at the actual site itself, but also at a separate control room. Often, due to the nature of the premises being surveyed, the police or other authorities will want to maintain the ability to monitor activity at a remote location. In that kind of situation, the need to transmit video signals over long distances via networking is essential.

The four main technologies in use are telephone-based systems allowing national and international remote monitoring, fiber optics, and infrared or microwave links for shorter ranges.

Offline and Online digital video storage systems store images on hard disk and archive data to digital tape. Images held on disk are online, and are available instantly for replay and review.

Images stored offline are on tape cartridges held in robotic autochangers ! the system selects and loads the correct tape as required. With a manual archive system the user must select the correct tape and load it.

With some digital systems, playing back video footage can affect real-time recording. Image quality can deteriorate and recording may become slower ! in some cases stopping altogether.

New systems feature a full-frame "simultaneous record and replay" functionality which allows previously recorded video footage to be replayed while the system is still recording in real-time. Images can be replayed in continuous playback by hitting rewind or viewing captured image by captured image.

Some companies do not have full-color (24 bit-per-pixel) display hardware. Low-cost hardware stores 8 bits-per-pixel, so it can display a maximum of 256 colors at a time. To display a full-color image, the computer will choose an appropriate set of representative colors and map the image into these colors. This process is referred to as "color quantization" or simply "color reduction".

Color quantization has proven to affect image quality directly. The details of the color algorithm have much more impact on the final image quality than do errors introduced by JPEG.

Since JPEG is a full-color format, displaying a color JPEG image on 8-bit or less hardware involves color quantization. The image quality of a JPEG viewer running on such hardware is largely determined by its quantization algorithm.

The size and quality of an image is expressed by its resolution. This can be full resolution, where a total of 60 fields (30 frames) of information are captured every second. The maximum resolution possible for a frame of video is 640 pixels-per-line, and 480 lines-per-image. CIF resolution contains information from a maximum of 320 pixels-per-line, with 240 lines. All other lines are duplicated to make up the frame. A variation on this is QCIF, which is of a lesser quality, and contains information from just 160 pixels-per-line, with a maximum of 120 lines. Other lines are duplicated to make up the picture. This resolution looks poor when pixelated as a full-screen image. A common method of overcoming this is to show or demonstrate QCIF images in small on-screen windows. In reality, for security applications, this quality-of-image may be suitable for detecting movement within a scene, but in order to recognize faces or vehicle-registration numbers, clearer images are needed.

Security operators can now access stored information instantly while minimizing the risk of missing potentially critical events with the aid of synchronous replay. This surveillance feature is like the equivalent of having a hard drive with two heads ! one head to write the video data onto a disk and the other to read it. This duality makes it possible to move back and forth through a video sequence, access data instantaneously and view events as they happen.

Methods of storing recorded data are flexible within a digital system. Hard disks can currently store nearly 24 hours of recording at 30 images-per-second, and almost a month's worth at one image-per-second time-lapse. This obviously varies depending on factors such as disk size and the compression ratio used.

Digital tapes are good for storing large amounts of data in small compact cassettes, are easily manageable and can be electronically indexed to improve search times. Searching through digital video is quick and simple, with some systems automatically indexing and searching events. This archiving can be achieved using a combination of time, date and camera guides, or by events such as an alarm trigger from a video motion detector.

SCSI, the Small Computer System Interface, is a set of evolving ANSI standard electronic interfaces that enable personal computers to communicate with peripheral hardware such as tape drives, disk drives, CD-ROM drives, scanners and printers with more flexibility than earlier interfaces.

In addition to faster data rates, SCSI is more adaptable than earlier parallel data transfer interfaces. ULTRA-2 SCSI for a 16-bit bus can transfer data at up to 80 megabytes per second, however the latest SCSI standard is ULTRA-3, which increases the maximum burst rate from 80 Mbps to 160 Mbps. New disk drives supporting "ULTRA3" will offer faster data transfer rates. ULTRA3 also includes cyclical redundancy checking for ensuring the integrity of transferred data and domain validation for testing the SCSI network.

Un-synchronized. Most CCTV installations have cameras which are un-synchronized. Some digital video storage systems need the camera inputs synchronized, which involves using special cameras and running a separate "sync" signal to each camera (in addition to the normal video cables).

VHS stands for Video Home Systems and was developed by Panasonic. Conventional analog VCRs, such as VHS, differ from digital in many ways.

If 30-frame recording is required, a videotape will be full in three hours. Three frames-per-second would fill the same tape in 30 hours. If, for example, 16 channels were to be recorded by the same machine over a period of 30 hours, the frame-rate would have to be about 0.2 frames-per-second (a single frame every 5 sec).

If using a new tape in a SVHS recorder, image quality is of the highest standard. However, image degradation occurs if the tapes are used more than 12 times. The system also needs to be well maintained and tape heads should be replaced annually.

There are many different types of Wavelet compression. Wavelet technology works by filtering the entire field or frame at once, removing all obvious redundancy and using only the areas that can be perceived by the human eye.

One form of archive media is Exabyte, a leading supplier of tape storage devices and network storage solutions for the data-intensive applications and database-server markets.

Exabyte's M2 series features a line of libraries that provide extremely high data throughout with uptime performance, along with high-rack density and scalability.

Exabyte is a popular choice for archiving digital data for its reliability and compatibility. Exabyte continues to be selected for use by the world's leading manufacturers, distributors and systems intergrators.

YUV is an alternative to RGB signal coding. It represents the luminance and chrominance components of the signal. Y alone will produce a monochrome image and U and V improve the color quality. Broadcast quality generally transmits luminance and chrominance in equal measure sometimes called 4:2:2, but security applications frequently sample the U and V components less often to save memory and bandwidth. 4:1:1 is typical.

Zone masking helps to save on recording time, storage capacity and limits false alarms to aid in cutting overall costs. It can be seen on both outdoor and indoor motion detectors and is essential when defining select areas of interest in a scene, like perimeters and entrances to buildings.

Different security systems have varied ways of setting up zoning so that an alarm is triggered only if activity is detected in an unmasked area. All events or changes outside of these areas are ignored, with the video motion detector only registering differences in specific areas in the field-of-view, thereby reducing the amount of work. Movement can take place in other parts of the scene, but unauthorized access will still trigger an alarm.

Zone masking is particularly useful if the field-of-view encroaches on a busy footpath or road. Zone masking helps to increase overall surveillance efficiency.

This article was provided by Primary Image, a U.K.-based supplier of digital video systems

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