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The development of timelapse VCRs and video multiplexers enabled the recording of the output of multi-cameras (typically up to 16) on to a single videotape and the display of a number of these cameras on to a monitor. This enabled an operator to view several areas simultaneously and then select an individual view for more detailed analysis.

How Do They Work?

CCTV cameras do not produce continuous picture flows, in fact they give 50 pictures (PAL, CCIR) or 60 pictures (NTSC, EIA) per second. A multiplexer acts like a very fast switcher taking individual pictures from each camera in turn. If 10 cameras are connected to a conventional VCR only 1/10^th of the information from each camera will be recorded. This is one on the reasons why replayed multiplexer pictures have a stroboscope appearance.

Developments

The use of multiplexers has expanded rapidly and developed into two main streams:

Premium feature and performance multiplexers incorporating sophisticated picture management and high fidelity recordings that are generally used in larger or critical surveillance;

Entry level machines that provide a more basic range of features and fulfil the requirement of cost effective, non-critical surveillance.

Simplex, Duplex or Triplex?

Simplex – machines operate in record only, replay only or display live multi-screens. They are commonly used in unmanned systems where the recordings of historical events are required, e.g. dud cheque was passed at a certain time or a theft occurred.

Duplex – the most common use for duplex machines is to continuously record information while and operator monitors the live multi-screen display.

Triplex – if an additional VCR is used then a triplex machine can record and display a mixture of images simultaneously.

Other Features

Activity Detection

Advanced multiplexers are able to analyse picture content from individual cameras and detect scene changes such as people movement.

This movement detection can be used to prioritise which cameras will be recorded and how frequently. For example, if there is movement in camera 3, the multiplexer can be programmed to record camera 3 for a longer period, or change the recording sequence from 1, 2, 3, 4, 5 … to 3, 1, 3, 2, 4, 3, 5, 3 … This makes the multiplexer a more effective tool for recording critical evidence.

Alarm Handling

Most multiplexers accept alarm inputs from other devices such as intruder alarms, door contact and fire alarms to call up cameras relevant to that event. Advanced multiplexers have sophisticated software packages to allow bespoke requirements to be fulfilled.

Built-in Telemetry

Advanced machines may incorporate telemetry control software enabling pan/tilt/zoom cameras and domes to be operated from the multiplexer, making the unit the heart of a sophisticated CCTV system as well as reducing both the cost and clutter in the control room.

Networking

Complex sites may include large numbers of cameras, multiple control points, significant distances across the site and incorporate high-speed domes and video transmission.

Advanced multiplexers incorporate RS232 and/or RS485 data ports for remote control and system integration.

Email: Info@Securasound.co.uk T 01948 667600