The Peripherals
As the number of cameras and monitors increase, simple system designs eventually give way to more complex designs that require peripheral components. These peripheral components may include switchers, VCRs, multiplexers, quad splitters, video printers and time date generators.

Switchers
A video switcher enables different cameras to be switched to different call-up monitors. In a smaller, cost-conscious application, a manual switcher allows users to select the camera they want to see by pressing a button associated with the camera (see figure 14).

Figure 14: A switcher makes it possible to switch cameras on a call-up monitor.

The most popular type of switcher, a sequential switcher, contains circuitry that will switch one camera to another automatically. The operator can set the length of time (dwell time) that a scene remains on the monitor before sequencing automatically to the next camera. This allows operators to keep tabs on numerous cameras with only one monitor, but also creates a drawback known as "switcher dilemma."

To illustrate switcher dilemma, imagine a system with eight cameras, each programmed to switch after "dwelling" on the monitor for five seconds. In this scenario, a considerable gap will occur between the time the first image is displayed and the time the eighth image is displayed. If the dwell time is shortened, operators may not be able to assimilate each camera image before it switches. The situation worsens when recording the video for review at a later time. On playback, you may see a door opening on camera 1, then suddenly see the video switch to camera 2, followed by camera 3, camera 4, and so on. By the time camera 1 appears again, the door is closed, and you are left wondering who came through the door while cameras 2 through 8 were flashing sequentially on the monitor.

Switcher dilemma can be solved with more sophisticated switchers, more operators or an alarm feature that will display video image automatically when an alarm point is activated.

Matrix Switcher
A matrix switcher is a more complex design enabling the user to switch any video signal to any call-up monitor in a large-scale system. They normally incorporate P/T/Z control and other features such as presets and alarm inputs/outputs.

 

Multiplexers
Unlike conventional recording systems, a video multiplexer collects full-screen pictures from up to 16 cameras and displays them simultaneously on a monitor (see figure 15).

Operators have the option of displaying any camera full-screen or multiple cameras in reduced size.
 

Figure 15: A 16-position multiplexer currently displaying only 7 cameras.

Multiplexers also can record all cameras in the system onto a single videotape. The cameras are recorded sequentially at a high rate of speed. As mentioned earlier, a standard video signal is comprised of 30 separate frames each second. In a video system containing 15 cameras, the multiplexer selects two frames from each camera and records them to a single videotape. The result is an effective frame rate of 2 frames per second, instead of the standard 30.

Most multiplexers today contain a motion detection feature that enables the system to record more frames of video from cameras showing motion than from those not showing any motion. The multiplexer does this by reallocating frames from one camera to another as needed. The net result is higher quality recordings of scenes that are more likely to be important to security personnel.

When a time lapse VCR is used with a multiplexer, the recording mode should be as short as possible to reduce the number of seconds required to record all cameras (remember, cameras are recorded sequentially) (see figure 16).

This is why it is a great advantage to use hi-density or virtual real-time TLR's when using multiplexers. Virtual real-time VCRs record 4 times the frames per second of conventional time-lapse VCRs.

Figure 16: The VCR, working with a multiplexer and several cameras,
recordsfewer frames per second in time-lapse mode.

One of the strongest advantages of using multiplexers is that during playback, the multiplexer decodes the tape allowing investigators to display only selected frames with the same address. This pullout feature saves investigators hours of time reviewing recorded actions. Another advantage is that during playback, any desired camera can be displayed full-screen.

Multiplexers offer system administrators an effective means of managing multi-camera surveillance systems:
 

Quad Splitters
The main feature of a quad splitter is the ability to compress images from four separate cameras and simultaneously display them on a single monitor screen (see figure 17). When four cameras are displayed, each occupies a quarter of the screen. A single camera can be selected and displayed full-screen, as well.

Unlike multiplexer recording, quad splitter recording yields only what appears on the monitor screen. If the VCR is recording a four-camera display, then playback will show four cameras.

Figure 17: Quad splitters can display four cameras on one monitor.

Recording CCTV
Most CCTV systems use VCRs to record video images from the dedicated and/or call-up/switchable monitors (see figure 18). 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. While this technology shows great
promise for the near future, VCRs presently are the most prevalent recording method.

Figure 18. A simple CCTV system with VCR recording.

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 (see figure 19).

Figure 19: Fewer pictures are recorded when using time-lapse mode, causing motion to
appear jerky. Notice how cars "disappear" when fewer frames per second are recorded.

 

The seconds per picture and number of pictures per second rendered by various time- lapse VCR recording modes are shown in the next table:

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.

Digital Recording
A more recently developed method of recording video images is that of Digital Recording( see figure 20).
 

Figure 20: A Basic Digital Recorder System.

Digital recorders compress and store images to a computer hard drive using various compression techniques. These techniques include JPEG, MPEG, Wavelet and a host of other proprietary methods. Images can be stored at a rate of 20 frames per second (fps) to as many as 480fps depending on the software features offered by the manufacturer.

Operating systems offered by manufacturers vary from Linux to Unix to Windows based systems. Although Windows is the most user friendly and familiar system, it tends to be less stable than it's lesser-known counterparts.

Archived recording time varies depending on the size hard drive in the recorder. Hard drive sizes range from 0 (external only) to 400gb internally with option of raid storage for virtually infinite capacity. Hard drive size should be considered conjunctively with the time period necessary to archive.

Many Digital Recorders incorporate multiplexing with inputs varying from 4, 9, 16 and as many as 32 inputs. Similar to the conventional multiplexer/VCR setup, the frame rate will be divided among the cameras being recorded. However, unlike conventional methods, each camera input of a digital recorder can be programmed to record more or less frames per second depending on camera priority (Ex: camera #1 = 10fps, camera # 2 = 5fps, camera # 3 = 12fps, camera # 4 = 3fps using a 4 input recorder with maximum 30fps).

Another very distinct advantage to digital recording is the ease of locating events. No more fast- forward and rewind. Just type in a time, date and camera number and you can playback instant images.

Other options available with DVRs include motion detection, remote viewing via LAN, WAN, or Internet, on board media such as CD Rom, DAT storage or removable hard drives, SCSI and USB ports as well as remote control of pan-tilt devices and the list grows everyday!

Work with your VSS sales representative to determine which digital product is best for your application.

Video Printer
A video printer (see figure 21) produces a hard copy printout of any live or recorded video scene, using thermal or other sensitized paper. The "still" photo can be used for multiple purposes, such as providing suspect identification to police agencies, alerting employees to safety hazards, etc. Photo printers are available in black & white or color.

Figure 21: A photo printer.

Time Date Generator (TDG)
A Time and Date Generator can annotate the video scene with chronological information. Also, a camera identifier is placed on the monitor screen to identify the camera scene being displayed. Today, most VCRs, multiplexers and camera controllers have this function built into the product.