TIPS & GUIDES

CCTV Basics

The first thing you should ask yourself before purchasing a security surveillance system is: What do I need this system to do? Do you need detection of an incident only, or do you need to identify the object in question. Keep your answer in mind when reading this document.

The second thing you should ask yourself before purchasing a system is: What is my application? The most common applications are: security applications, safety applications, and management applications.

Many elements must be considered before designing your surveillance system:

Without proper equipment and studying all these variables, you may be deceived on what really is causing poor imaging. For example, what might look like a poor camera, may be a poor monitor. You can't simply take a high resolution camera and expect its high performance to be visible on a poor monitor or display. Each component in the system will affect the overall performance. Your overall quality is only as good as the weakest component in your system.

Scene and Lighting

The scene refers to the objects or area to be observed as well as the environment in which it will be observed. One important aspect that must be considered is the environment. The environment contains multiple colors, materials, reflective surfaces, and varying degrees of light within the picture. To select the proper equipment you must determine the amount of light present during peak times of operation. The amount of light on the scene determines everything from picture clarity to focus. What is the minimum light that will be available? Will it be more cost effecient to go with a better night-view camera or adding more artifical lighting to the scene?

The Cameras

A camera's performance largely depends on the amount of light present, as well as the imager used. When the level of light changes dramatically, usually a camera equipped with automatic iris control can help ensure consistent image quality. Auto iris enables a camera to open or close its lens accordingly to the varying levels of light, limiting or increasing the amount of light passing onto the sensor. Cameras are available in several imaging formats expressed as 1/2, 1/3, 1/4 inches. These are the sizes of the imager used. Generally speaking, you should match the camera's format with the len's format (ex. 1/3" sensor with 1/3" lens). It is crucial to understand a camera's format, resolution, and corresponding lens focal length when determining what camera will best suit your needs.

The Lens

The lens plays a large role in a system's design. The primary function of the lens is to collect light from a scene, focus the image to produce a sharp image on the camera's imager. Selection of a lens is critical. The lens directly affects the size, shape, and sharpness of the image to be displayed on the imager. Factors such as distance to the scene, focal length, desired field of view, lighting, and format affect the size and clarity of the image.

The field of view (FOV) is the actual picture size (height/width) produced by a specific lens. If the view is not suitable, consider a different lens to increase or decrease the field of view.

Camera lenses are divided into two major categories: fixed and varifocal (manual zoom). A fixed lens obviously has a fixed focal length, while a varifocal lens enables the user to change its focal length to produce a zooming effect (narrowing the FOV). Focal length is the distance from the optical center of the lens to the focal point near the back of the lens. This focal length distance is displayed on the lens (in millimeters). A lens with a focal length of 8mm on a 1/3" camera produces a field of view similar to the view produced by the human eye. A wide-angle lens has a short focal length, while a telephoto lens has a long focal length. In order to change the field of view, you must change the lens.

The ability of a lens to gather light depends on the relationship between the lens opening (aperture) and focal length. This relationship is symbolized as the letter F, also know as F-stop. The lower the F-stop number, the larger the aperture, thus the greater ability to pass light through the lens to the camera's imaging device. For example, a lens with an F-stop of F1.2 can gather much more light than a lens with an F-stop of F4.0. A lens with a low F-stop number is sometimes referred as a "fast" lens.

Depth of field is another consideration when determining the proper lens. Depth of field is the area in focus ahead of and behind the main object. When you focus on a particular object there is an amount of area behind the object and in front of the object that will still be in focus, although not as sharp. Depth of field increases or decreases based on the length of the lens, the len's aperture, and distance from the camera to the subject.

If depth of field is important consider increasing artificial lighting or install cameras with normal angle lenses.

Camera lenses generally come with a C-mount of CS-mount and must be matched appropriately to the camera's mounting requirements. The difference is the distance from the lens' actual optics and the camera's imaging device. The C-mount lens is 17.5mm from the imaging device, whereas the CS-mount lens is 12.5mm away. A C-mount lens can be used on a CS-mount camera only if a 5mm spacer ring is added (this is why some cameras allow C or CS mount lenses, they just include a spacer). Though, a CS-mount lens cannot be used on any C-mount camera.

Video Transmission Methods

There are many transmission methods that exist today. The purpose of the transmission medium is to carry the video signal from the camera to the monitor or other device. The most common mediums include: coaxial cable, fiber optic, CAT5 cabling, phone lines, microwave, and radio frequency. The choice of determining which medium to use depends on many factors including distance, environment, cost, and facility layout.

Coaxial cable is the most popular in the CCTV industry. The cable, preferrablly copper, is shielded to minimize interference from any nearby electronic devices or electrical wires. It is the most economical solution for short run applications. Usually no longer than 300 feet. This type of cable is used for direct connections with no special conversions.

Fiber optic transmission technologies convert an electronic analog signal into a digital signal using a series of light pulses or lasers. The medium that carries these light signals come either in plastic or glass rods. Fiber optic transmission is unaffected from almost any type of interference. Fiber optics have a large signal capacity (bandwidth) and have no possibly for spark. Fiber optic cabling offers a cost-effective method for sending large amounts of data over long distances (miles). Special conversions and devices are needed to facilitate this type of media transmission.

Telephone line is a standard twisted pair of wires that can transmit signal up to 1 kilometer. It is possible to use standard telephone lines for video transmissions with the use of specialized transmission and receiver equipment.

If already in place, microwave can be a very efficient and cost-effective method of delivering black & white or color video.  Microwave turns the video and data signals into high radio frequency signals and transmits them from one point to another via free air and space.  A receiver then converts the transmission back into the video and data signals and displays the scene on a monitor.  Good quality transmission can be achieved over a line-of-sight path.  Microwave technology offers a large bandwidth to carry video, however, it can be affected by environmental conditions.  It is a practical option when a wire path between the camera and monitor locations cannot be established or is prohibitively expensive.  Microwave transmission is regulated by the FCC, and a license is required.

Radio Frequency (RF) is a reliable, but short distance, line-of-sight video transmission technology.  It is becoming increasingly popular where hardwiring methods are either impossible or impractical, and has been used successfully to reduce cabling costs even within large buildings.  Environmental conditions or other RF in the area can affect it.

The Monitor

The monitor receives the transmitted electronic video signal from the camera and paints it across a cathode ray tube (CRT) to display an image to a viewer.  Although similar in function to a TV set, a CCTV monitor provides higher lines of resolution (better picture quality) and accepts only video signals rather than RF/antenna signals.

Lines of resolution refers to the total number of horizontal lines the camera or monitor is able to reproduce.  The more lines on a screen, the better or sharper the video picture will appear.  CCTV monitors can provide up to 1000 lines of resolution compared to an average of 300 lines provided by television sets.

Peripherals

There are many devices on the market today to bring multiple video signals and channel them through one device, either enabling multiple channel viewing on a single display, sequencing multiple channels on a single display, recording capabilites, and many other features. The most common include quad processors or splitters, digital video recorders, and time lapse VCRs.

Quad processors allow up to four cameras to be displayed on a single screen. Each camera will occupy a quarter of the screen and a single camera can be selected to display full screen. If a VCR or recording device is attached to the quad, then on playback you will see all four screens. Some quad processors have addtional features, such as motion detection that triggers a recording device to record only when motion has occured within the images.

Most CCTV systems use VCRs to record video images from the dedicated and/or call-up/switchable monitors.  Recordings make it possible to view events that may have gone unnoticed at the time they occurred or that may require close scrutiny later.  Technological advances now make it possible to record images in digital form on a computer disk.  VCRs presently are the most prevalent recording method. The number of current systems upgrading to digital is rapidly increasing due to the advanced features available and the increased recorded image quality associated with Digital Video Recorders (DVRs). When digital systems are compared side-by-side to VHS format TLRs most clients opt for the superior quality and features of todays DVRs. The latest DVRs are now able to record up to 480 frames per second divided by the number of cameras used in the system.

VCRs designed for CCTV can record video images in either real-time or time-lapse modes.  In the real-time recording mode, the tape moves at the same speed as home VCRs (2 to 6 hours) and captures 30 pictures per second.  This produces high quality recordings, but requires operators to change tapes every two to six hours.  The 24-hour real time VCR will record 24 hours of video on a single tape at 20 pictures per second. It is considered a real time recorder because 20 pictures per second approximates the ability of the human eye to easily distinguish moving images.

Time-lapse recording makes it possible to record video over long periods of time on a relatively small amount of videotape.  Time-lapse recording can capture from 12 to 960 hours of video on one T-120 tape.  However, the number of pictures recorded per second in time-lapse mode decreases significantly as the recording time increases.  As fewer pictures are recorded per second, critical images may not appear on tape, and movement (e.g. a car traveling across a parking lot) may appear jerky.

Another way to capture video on tape is through alarm recording.  With this method, the VCR usually runs in time-lapse mode until an alarm occurs.  The VCR then switches from time-lapse mode to real time mode, capturing video images at a rate of 30 pictures per second.  After the alarm resets, the VCR returns to time-lapse mode to conserve tape.  The CCTV system will need an alarm switching mechanism in order to perform this function.

When a time-lapse recording is played back at normal playback speed, the playback will present events at a speed faster than real time.  It is common to play back a time-lapse recording in real time mode to speed the time necessary to review the tape.  If necessary, the tape can be slowed to review those events that require greater attention.