The Advantages of Digital Recording
by Graham Tull

Introduction -

Driven by a demand for better picture quality and faster access to recorded video, digital recording has now become a mainstay of the commercial security market. In addition, as the capability of computing technology has increased with the rapid expansion in PC use, so have the capabilities of digital recorders.

The use of digital recording, transmission and integration has now effectively been endorsed through the successful installation of a number of major systems in significant sites by more established companies such as IES.

Subsequent to these successes, there has been a noticeable rise in the number of manufacturers seeking to gain entry to this market and compete alongside those of us who have been developing and perfecting digital CCTV systems over a number of years. With design manufacturers turning increasingly to digital technology as the basis for the new generation of CCTV systems, it is now accepted that digital recording and transmission devices will provide the standard operating format for all CCTV equipment in the future.

It is easy to see why these systems are proving so attractive; in addition to offering users far greater sophistication in image capture, storage and handling capabilities, digital-based systems also provide the base architecture for multiple system integration whereby monitoring and response functions, for example, can be conducted from a single central station.

For the first time, the security manager is truly able to benefit from access to a totally integrated security ‘system’.

Undoubtedly, the digital medium offers a significant number of advantages over its analogue counterpart and this document attempts to set out some of these advantages in a simple to understand manner and includes a glossary of terms.

Automation

One of the key advantages to digital recording is the ability to automate a number of actions that would normally be undertaken by an operator.

• Fully programmable camera patrols.
  
  
• Fully programmable Video Motion Detection.
  
  
• Fully automated movement of cameras to the source of an alarm.
  
  
• Fully automated indexing of any recorded events, allowing an operator to locate and review them in an instant.
  
  
• Automated video signal equalisation, to simplify set-up procedures.
  
  
• Video pixelation software enables the user to protect privacy and comply in the UK with the Data Protection Act, simply and cost-effectively.

Picture quality

We’ve all seen the grainy black and white CCTV images on TV shows and these are representative of many of the VHS systems in use today. Indeed, the copy that we see on TV may well be a second or even third generation copy, reducing the picture quality even further.

• With digital systems there is no picture degradation – unlike VHS tape based recording systems where consecutive copies are of poorer quality than previous. Also, with VHS tapes there is a UK Home Office recommendation of 12 uses before replacement, as the tapes themselves will wear.
  
  
• Ability to enhance or zoom into parts of the image.
  
  
• Having a choice of compression algorithms that can be used simultaneously gives the user the ability to apply larger file sizes to each image under alarm conditions thereby giving a higher quality output whilst making the best use of the remaining image storage capacity.

Security

The issue of security is fundamental to any CCTV system, from security of output to security of access to the control room and security of transmitted images over the Internet or phone lines.

• Digital recording gives the ability to apply hugely complex encryption algorithms to images that are needed for evidential use on CDRW or for transmission over the Internet.
  
  
• It is important that there is no unauthorised access to Windows – preventing malicious or accidental compromise and so multi-level security password protection is essential.

Scalability

The digital recorder is basically a very clever PC and thus the ability to network units together exists, creating the potential for a much larger CCTV network. If you imagine that a standard digital recorder can accommodate, say 16 camera inputs, then by networking this unit to another you have a 32 camera system and so on.

• Enables the use of many hundreds of cameras.
• Enables the use of cameras on multiple sites globally.
• Enables remote operation / review of footage and images.
• Enables remote set-up, upgrade and diagnostics.
• Enables use of existing IT network infrastructure.

Admissibility as evidence in a court of law

Most CCTV systems have the potential to have their images used for evidential purposes. Because digital images have the potential to be manipulated it is important to create a means of time stamping images and to encrypt any output so that there can be a very high degree of confidence in the evidence.

• Watermarking, time stamping and creation of an audit trail are all possible with digital recording. The creation of an audit trail for evidential use is crucial in determining the provenance of a given recording.
  
  
• Different compression methods offer different levels of evidential viability.
  
  
• For instance, many systems use compression methods utilising a conditional refresh algorithm e.g. MPEG. At the start of a given recording, MPEG effectively takes a reference frame from which it will draw information for subsequent frames.
  
  
• As an example, if a man stands in front of a brick wall, conditional refresh records the whole scene in the first frame – the wall and the man. The wall is unlikely to ever move; conditional refresh confirms this in each frame and uses the original frame’s picture of the wall for subsequent frames. If the man’s legs remain stationary, they too will be taken from the original frame. If the man now waves his arms around, the recorded frame will show the original picture of the wall, the original picture of the legs with new pictures of his arms as they move around.
  
  
• Conditional refresh is used in order to reduce file sizes and therefore get more files (pictures) on a given hard drive. It does this very successfully but at the potential expense of picture quality and evidential viability. It can be argued that the picture presented in court is, in fact, a composite (separate legs, torso and arms against a separate background) and therefore does not offer a true representation of the defendant or his actions.
  
  
• Where MPEG is of significant benefit is when it is used in remote viewing of files over a telephone, data line or the Internet. The smaller file sizes are ideal for speeding up transmitted images to give a much more ‘live’ feel to viewed footage.
  
  
• Other systems offer compression methods using a full-frame refresh algorithm e.g. M-JPEG. This method of compression takes a fresh picture in each frame i.e. there are no reference frames and consequently the file sizes are much larger. It is therefore much more difficult to argue against the evidence in court as the pictures are more accurate and are not composites.

Variety of output formats

Output format requirements vary from user to user, dependent on their purpose. The images are held on the HDD until such time as they are required to be archived or copied, at which time a choice of output media is available.

• CDRW with playback on any multimedia PC.
• DAT / AIT
• VHS
• Standard Windows printer

Simple integration with other security equipment

A digital system has the advantage that it can communicate with the other components in the security system. For instance it is possible for your CCTV system to react to alarms generated by:

• Access control systems
• Intruder alarms
• EPOS systems
• Facial recognition
• Fire alarms
• Vehicle number plate recognition etc.
• X-ray baggage machines

Speed of access to recorded events

Digital recording offers the operator the ability to access recorded events as one might locate files on a PC, that is by providing a list of events that can be accessed immediately.

• No more trawling through hours of footage to find a recorded event.
  
  
• Digital recording gives an indexed list of recorded events and permits immediate access to each recording.
  
  
• Ability to view record video index textually or graphically.

Transmission of images

Digital images are very easily transmitted securely between sites. Digital recording and transmission systems are capable of the following:

• Remote monitoring of live images
• Remote replay of recorded images
• Remote camera control (telemetry)
• Connections via Internet, PSTN, ISDN, GSM and broadband

Simultaneous operation

From a flexibility point of view, the use of digital recording offers a major advantage over analogue VHS systems in that it has the ability to carry out several tasks simultaneously:

• View live and archived footage without interrupting recording.
• View footage from remote sites without interrupting recording.

Built in high-speed video multiplexing

• Allows multiple cameras to be recorded to single recorders
• Saving cost and physical space

Pre-event buffer

Pre-event creates a temporary recording of a pre-defined length for selected cameras, which is only preserved if an alarm event occurs. In this way it is possible to see what happened prior to an alarm activation along with the post alarm video. The best systems allow for selection or variable pre-event periods per camera and even variable frame rates per camera. There is no equivalent ability within analogue systems.

Video Motion Detection

Video motion detection is another useful software module. It permits an operator to overlay a number of detection zones on each camera view. The purpose of these zones is to either record or ignore changing information within them. For instance you might wish to record any person approaching a certain door or conversely you might wish to ignore a bush that moves in the wind, which would otherwise make the system record every time the wind blew. Better VMD packages will allow you to assign multiple adjustable detection zones per camera.

Reduction in cost

• Less money spent on VHS tapes.
  
  
• Less storage area required for new / used tapes.
  
  
• No secure-disposal costs for old VHS tapes.
  
  
• Video pixelation software removes time-consuming manual editing or external cost for same.

Better use of operator time

• Less time trawling for recorded events.
• Less time spent erasing tapes.
• Less time spent replacing tapes.
• More time spent in observation.

Glossary of terms

Analogue signal - Representation of audio or video information through electrical signals with continuously varying levels which are proportional to the original loudness, tone, brightness, colour etc.

Archive - Long-term off-line storage.

Bandwidth – This is the measure of the maximum amount of information that can be sent through a transmission medium over a given period of time.

CCTV – Closed Circuit Television i.e. a private video surveillance system.

CDRW - Compact Disk Read Write - A CD that can have information stored on it by the user.

Chrominance – Refers to the colour information component of a video signal.

CODEC - Code/Decode. An encoder plus a decoder is an electronic device that compresses and decompresses digital signals usually from analogue to digital and vice versa.

Compression - The method by which complex video images are converted from analogue to digital formats. Different compression techniques result in different file sizes, picture quality and evidential admissibility.

DAT / AIT - Digital cassette tape formats - Digital Audio Tape / Advanced Intelligent Tape. Digital recording - The means of recording analogue video images and audio onto a digital medium such as a computer hard disk.

DPA – UK Data Protection Act governs the use of images recorded in public areas.

EIDE - Enhanced IDE, a newer version of the IDE mass storage device interface standard. EIDE has replaced SCSI in many areas because of lower cost.

Encryption - The rearrangement of information such that it is unrecognisable to third parties other than the intended recipient who can restore the information via use of a suitable authorisation ‘key’. Can be used for voice, video and other communications signals.

Field - Refers to one-half of the TV frame that is composed of either all odd or even lines. In CCIR systems each field is composed of 625/2 = 312.5 lines, in EIA systems 525/2 = 262.5 lines. There are 50 fields/second in CCIR/PAL, and 60 in the EIA/NTSC TV system.

FPS – Frames per second - the recording 'speed' of a CCTV system.

Frame - (See also Field). Refers to a composition of lines that make one TV frame. In
CCIR/PAL TV system one frame is composed of 625 lines, while in EIA/NTSC TV system of 525 lines. There are 25 frames/second in the CCIR/PAL and 30 in the EIA/NTSC TV system.

GUI - Graphical user interface –the on screen controls and displays provided for the system operators.

HDD - Hard Disk Drive - the internal storage medium in a computer or digital recorder.

Impedance - A property of all metallic and electrical conductors that describes the total opposition to current flow in an electrical circuit. Impedance is often referred to with the letter Z. It is measured in ohms, whose symbol is the Greek letter omega - ?.

Interlaced scanning - A technique of combining two television fields in order to produce a full frame.

IP Address - An identifier for a computer or device on a TCP/IP network. Networks using the TCP/IP protocol route messages based on the IP address of the destination.

IR light - Infrared light, invisible to the human eye. It usually refers to wavelengths longer than 700 nm.

ISDN - Integrated Services Digital Network. The newer generation telephone network.

Luminance - Refers to the brightness information component of a video signal.

Lux [lx] - Unit for measuring level of illumination.

M JPEG - A compression algorithm. Stands for Motion - Joint Photographic Experts Group.

Matrix switcher - A device for switching more than one camera, VCR, video printer etc, to more than one monitor, VCR, video printer and similar. Much more complex and more powerful than video switchers.

Matrix - A logical network configured in a rectangular array of intersections of input/output channels.

MPEG - A compression algorithm. Stands for Moving Pictures Experts Group.
Multiplexer – multiplexing (MUXing): The combining of two or more information channels onto a common transmission medium.

NTSC - National Television Standards Committee - the US television standard
ODBC - Open DataBase Connectivity, a standard database access method. The goal of ODBC is to make it possible to access any data from any application, regardless of which database management system (DBMS) is handling the data.

PAL - Phase Alternating Line - the European television standard.

Pixelation - Computer screen images are made up of 'Pixels' or individual dots of colour that form part of the whole image. Also refers to the process by which video is obscured to hide identity.

PSTN - Public switched telephone network. Usually refers to the plain old telephone service, also known as POTS.

PTZ - Pan Tilt Zoom – a mounting device for CCTV cameras, which can swivel horizontally and vertically via electric motors

RAID - Redundant arrays of independent disks. This technology connects a number of hard drives into one mass storage device, which can be used, among other things, for digital recording of video images.

Resolution - A measure of the ability of a camera or television system to reproduce detail. The number of picture elements that can be reproduced with good definition.

RS-232 - A format of digital communication where only two wires are required. It is also known as a serial data communication.

RS-485 - This is an advanced format of digital communications compared to RS-422. The major improvement is in the number of receivers that can be driven with this format, and this is up to 32.

SAN – Storage Area Network. Is a high-speed special-purpose network (or sub-network) that interconnects different kinds of data storage devices with associated data servers on behalf of a larger network of users. A storage area network can be local or remotely networked.

SCSI - Small Computer System Interface. Pronounced "scuzzy," SCSI is a parallel interface standard used by Apple Macintosh computers, PCs, and many UNIX systems for attaching peripheral devices to computers.

Serial port - A computer I/O (input/output) port through which the computer communicates with the external world. The standard serial port is RS-232 based and allows bi-directional communication on a simple wire connection as data flow serially.

TCP/IP - Transmission Control Protocol/Internet Protocol, the suite of communications protocols used to connect hosts on the Internet.

Telemetry - Digitally encoded data, intended to remotely control pan, tilt, zoom, focus, preset positions, wash, wipe and similar.

Time lapse VCR (TL VCR). A video recorder, most often in VHS format, that can prolong the video recording on a single tape up to 960 hours (this refers to a 180 min tape). This type of VCR has historically been used in CCTV systems.

Time division multiplexing - The technique of recording several cameras onto one time lapse VCR by sequentially encoding and sending camera pictures with a timed interval delay to match the time lapse mode selected on the recorder.

Transmission - Sending of images to a remote location, usually via a computer data network or by telephone (fixed or mobile).

UPS - Uninterruptible power supply. These are power supplies used in the majority of high security systems, whose purpose is to back-up the system for at least 10 minutes without mains power.

VHS - Video home system. A video recording format used most often in homes but also in CCTV. Its limitations include the speed of recording, the magnetic tapes used and the colour separation technique. Most of the CCTV equipment today supersedes VHS resolution.

VMD - Video Motion Detection – A software device that senses movement in a field of view, which then sets off an operation on the recorder, usually commencing recording.

Wavelet - A type of video compression.