Basic feature of these devices is making possible to build a system with more than one camera. They allow connecting a few cameras to one monitor or VCR. However we should remember that the price of it is certain decreasing in surveillance quality.

There are following groups of these devices: switchers and quads, multiplexers and video matrix.

These devices increasingly act only as complementary devices in new installations. It is caused by rapid fall in prices of digital video recording devices - these devices offer features associated with video channels switching and processing - all in one.

Switchers.
The switcher is the device that connects, in defined time period, video signal from one selected camera to monitor output. This connection is realized manually, after switching button with corresponding camera, or automatically. Each switcher can have regulated dwell time, after which camera is connected. Mostly this dwell time can be selected from period from 0.5 s to 20 s. Also there can be programmed complex sequencing of channel switching together with camera name and number displayed. Some more advanced switchers have built in motion detection system.

When some of video inputs is not in use, we can set skipping of it during display sequencing.

Usually switcher has from a few to a dozen of video inputs (the most common are 2, 4, 8, 12 and 16 input versions) and one or two outputs connected to a monitor. Second output is usually used to connect additional monitor, for observation from one, selected camera. Similarly to monitors, switchers also have commonly so called loopthrough outputs. This way each camera input has its corresponding output, where the same video signal may be obtained.

Some of switchers are equipped with alarm inputs and outputs, compatible with standard used in alarm switchboards and PIR sensors. Depending on number of cameras, the sockets D-Sub 25 or D-Sub 9 are used. We can connect to alarm input, for example, PIR sensor, then any motion detection will force switcher to change image to the camera corresponding with that alarm input instantly. Additionally, on alarm output appears signal which can be used for activation of alarm procedure. The advantage of switchers is their price - for the reason that it's popular electronic switching circuit, thus the price is usually the lowest in comparison with other groups of devices.

8-input switcher with audio PV8/2A code M4060 - front view. Visible power switch on the left, knob of automatic work mode, infrared receiver (for remote controller IR BQS-405 code M4065), buttons1-8 for direct image selection of individual camera.

Quads.
Quad (or divider) is a device which allows observation of image from four cameras on one monitor at the same time. If such device allows for simultaneous observation of bigger number of images (cameras), "quad" name is no longer right; we should call it divider then.

To have possibility of simultaneous observation from a few cameras, digital processing has to be performed. Video signal is being digitalized and then its size is reduced, e.g. by selection of every second line and every second pixel in line. After that, images of individual cameras are merged in one that is converted to analog form. Typically, 8-bit processing is used, which can adopt 256 levels of grayscale for B/W switcher. Resolution can be e.g. 1024x512 points for full screen. The use of image processing decreases image quality.

We should note that color and B/W switchers are manufactured. After we connect color camera to B/W switcher it won't transmit color information. Additionally, when we buy color switcher we should check if it is suitable to work in color video system that is used by the rest of equipment (e.g. PAL in Europe).

Dividers usually have 4, 8 or 16 inputs, however there are also double dividers, where, using the same keyboard we can control both of dividers and selection of quad is made with the use of switch. It is worth to remember that most of 8ch quads offer only 4ch simultaneous viewing. It means that 8ch viewing is realized alternately, one 4ch first and then another 4ch. There are also dividers enabling 8ch simultaneous viewing, however their disadvantage is rather high price. Besides that, at 8ch viewing, distinguishing of details isn't satisfying.
Dividers with simultaneous number of displayed images bigger than 4 are usually used only for motion confirmation in viewed area.

Besides the work in the mode of simultaneous viewing, the most of dividers can also work as switchers, mostly with possibility of programming any switching sequence. Higher class dividers have alarm inputs and outputs with the same features as in switchers. The sockets D-Sub 25 and D-Sub 9 are also used here, as well as simple lead frame. Due to such frame, control of additional functions like switching on room lighting or IR illuminators is possible.

Some of dividers have built in motion detection system, what may allow not installing additional alarm detectors (PIR, door opening) in some of surveillance systems. Thanks to it, switching on the light or IR illuminator can be exercised.

Nowadays, such devices as switchers or quads are commonly used at ends of the systems, in which images from a few cameras should be displayed. Below we show an example of such solution - with the use of switcher and divider.

On the scheme three posts are showed, marked with adequate colors:

- orange - main surveillance post ( images from all cameras are registered);- blue - viewing and registration of images from a few cameras;- grey - only viewing of images from a few cameras.

Such a division of surveillance system to a few smaller subsystems allows limitation of access for unauthorized people to viewing a part of camera images. At the main post we have access to all cameras and we can do almost everything with the system. At guards post only a part of cameras is available (additionally registered on local recorder). In turn, at third post we have access only to a few cameras that are used as real time viewing. Practical use: car showroom - orange zone (owners / a man responsible for the system), blue zone (security and guards post), grey zone - viewing for customers (i.e. presentation of showroom and workshop)

Multiplexers.
Nowadays multiplexers have been supplanted by digital video recorders. In older installations they were used for signal processing before recording on video cassette recorders. Multiplexer integrates all features of switchers and dividers, however advanced image processing functions offer a number of features unavailable in above mentioned devices.
To watch signals processed by multiplexer we have to use it again. If we tried to watch it simply on monitor we would see effect of displaying single frames form different cameras displayed alternately.

To watch signals processed by multiplexer we have to use it again. If we tried to watch it simply on monitor we would see effect of displaying single frames form different cameras displayed alternately.
DVRs offer much more functions than multiplexers. Due to their importance in modern CCTV systems they will be described in a separate chapter.

Video matrix.
Video matrix is combination of all previously described image signal processing and switching devices, hence matrix abilities in field of image signal processing and switching are the strongest. This device has many inputs and many outputs; it allows connecting any input with any output. Basic matrix advantage is ability of simultaneous work of a few operators, from whom every operator can watch camera image from each camera, naturally basing on rights of access. Matrix work in big surveillance systems, integrating CCTV with other systems e.g. alarm installation.