Contributor: Paul Broome
Company: Sensormatic Video Products Division

Quads vs. Multiplexers

What is the difference between a Quad and a Multiplexer? They both allow us to display and record multiple cameras. Which is better? The answer depends on the application. A little history will help us to understand how both products arrived on the market.

History:

In the beginning, a CCTV system consisted of a camera, a cable and a monitor. If you want to see a second image, get another camera, another cable and another monitor. It is easy to see that multiple camera systems can get expensive. You need as many monitors as you have cameras. If you want to record the camera images, you need a VCR for each camera. The total cost includes the equipment, the space to install the equipment and the manpower to watch the equipment. A simpler solution was needed. How do we solve the problem of displaying multiple cameras on one monitor and recording them on one VCR?

SWITCHERS: A switcher is a black box which allows you to take the inputs from several cameras, perform some electronic wizardry, then display each camera for a period of time on a single monitor. You can record the same image on a VCR. Switchers typically display each camera sequentially (that's why they are called sequential switchers). The length of time each camera is displayed is controlled through front panel controls, or, internal jumpers. Switchers have been around for a long time and are still widely used in many applications.

SWITCHER ADVANTAGES: You get to see several cameras displayed on one monitor. You can record the same images on one VCR. You save installation space and monitor and VCR costs.

SWITCHER DISADVANTAGES: While one camera is displayed and recorded, the other cameras are not visible. Watching the sequential display for long periods of time could be a cure for insomnia.

SPLITTERS: Splitters allow you to display two cameras on one monitor. The same image can be recorded on one VCR. A single camera image is displayed on the full monitor screen. A smaller insert of a second camera is overlaid on the first image. This can range from a small picture in a picture to an almost equal split of the monitor showing the two images either vertically, or, horizontally. Splitters have been around for a long time and are still used in selected applications, usually where economy is most important.

SPLITTER ADVANTAGES: You get to see two camera images on one monitor. You can record the two images on one VCR. You save on cost of equipment and the space to house it.

SPLITTER DISADVANTAGES: The full camera images are not available for viewing as one image conceals the other. Some provide an elongated image, either two tall thin images, or, two short wide images, rather than the traditional image (ratio of 4 wide by 3 high).

OTHER DEVICES: A variety of devices provide system enhancements. These include devices for AGC (automatic gain control) to provide equal viewing intensity for cameras, video loss devices to indicate when the video signal is interrupted and alarm devices which associate hard wired alarms to a particular camera, to mention a few. Text inserters allow you to display titles on camera images. Motion detectors provide alarm outputs when movement is present in a camera image. More sophisticated devices combine several of the items mentioned so far into a single package.

QUADS: Quads take the images from four cameras, digitize and resize them to display on a single monitor and record on a single VCR. Some Quads incorporate additional features into a single package. Sophisticated Quads allow you to view individual cameras, sequence through the cameras and display all four images at the same time. There is often an alarm input for each camera, video loss indication and camera titling. Remote operation may be available. There are "two page" Quads which take eight camera inputs and display individually, sequenced and four cameras at a time (1-4, or, 5-8). Quads have been around for ten years, or, longer. They are still widely used in a wide variety of applications. There are lots of manufacturers making Quads.

QUAD ADVANTAGES: Quads combine several features into a single device, often allowing small system control from a single front panel. Four cameras are displayed on one monitor and recorded on one VCR (8 with dual page Quads). With standard Quads, all cameras are visible at the same time and recorded at the same time. There is considerable savings on equipment and space. You have better control of small systems allowing comfortable viewing for a single operator.