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Video multiplexing is now one of the fastest growing sectors of the surveillance market, thanks largely to the economies of scale in the production of specialised digital video processing microchips. Multiplexing technology has actually been around for quite a while but until comparatively recently it has been too costly, too inflexible or both, for anything other than dedicated high-end multi-camera installations.


The principles of video multiplexing are essentially quite simple. Take the outputs from a number of cameras, chop them up into brief time slices, and sequentially record each segment on a VCR.  If the output from one camera becomes more significant -- i.e. a related alarm is triggered -- the multiplexer can give that output priority, either by recording just that camera (or cameras), or increasing it’s time allocation, so that it is recorded more frequently than the others in the sequence.


Vision multiplexing makes the most efficient use of costly recording equipment, but it requires a lot of sophisticated technology. One of the main problems has been synchronising camera outputs, so the video signals can be seamlessely integrated with one another for display and recording purposes. The first systems made use of gen-locked cameras, controlled by a common synchronisation signal, that kept them in step with one another. However,  advances in digital video processing and storage have made it possible to use free-running video inputs, from ‘ordinary’ cameras, significantly reducing the costs involved in setting up a multiplexed multi-camera system.


Digital synchronisation is one of the core functions of the Dedicated Micros Sprite DX 9. It’s part of a family of low-cost multiplexers that include colour and monochrome variants, with 9 or 16 camera inputs, and simplex (SX)  or duplex (DX) operation. The DX 9 is a monochrome, 9-camera duplex unit with a typical end-user price of just under £1800. Dedicated Micros are pitching it towards the lower end of the multi-camera system market, though the range and scope of its functions are comparable with systems that not so long ago would have cost several times as much.     


Features common to the Sprite SX/DX range include the neatly styled cream coloured case with its sloping control panel, and room on top for a monitor (Dedicated Micros can supply suitable matching colour and mono monitors). The only significant visual differences between the various models is the number of camera selector buttons, and the cosmetics of the four selection keys. On the SX colour multiplexers they’re coloured, and shades of grey on the DX monochrome units. Back panel variations follow along similar lines, with 9 or 16 BNC input sockets, depending on the model. The remaining back-panel connections include VCR input and output, and two monitor outputs -- one for the main display, the other is for a spot monitor -- that shows a live sequence of all connected cameras. Alarm inputs and outputs are handled by a 25-pin D connector. All models are powered by external 25 watt plug-in mains adaptor that operates over a range of 100 to 250 volts.  


Basic operation is very straightforward. In the ‘live’ mode each camera select button calls up the associated full-screen live image, pressing the button a second time engages a 2x digital zoom; the zoom area can be moved around the screen using the four selector keys. The on-screen  display for each camera includes a user-definable ident (up to 12 characters), camera number at the top of the screen, and time, date and VCR time-lapse mode along the bottom. All connected cameras can be sequenced on the main monitor, or shown simultaneously as quad or multi-screen displays. The multi-screen options on the DX 9 include a 9-way screen, or 8+2 multi-screen, with the top half of the image divided into eight small sub-screens, whilst the lower half has two quarter-screen pictures, showing a nominated camera output and the next consecutive camera.


Camera sequencing and all other functions are controlled from the menu-driven on-screen display system. This style and presentation of the OSD has been clearly inspired by domestic TV teletext/fastext systems -- the instructions admit as much -  where rapid page-selection options are colour-coded, and called up using coloured keys on a remote handset. In the case of the SX multiplexers on-screen displays and control keys are colour co-ordinated; the DX models have monochrome displays, so instead the four selection keys are delineated in varying shades of grey, and relate to similarly ordered menu items on the display.


From the live mode the first set of options are for full-screen, quad and multi-screen displays, the fourth button calls up the next set of sub-menus. They are camera sequencing, selecting pre-set functions for the first 8 camera buttons, recording mode select, and the final four options for user and installer set-up menus, play mode select and exit back to the first menu.  Both record and play modes bring up further menu selections relating to additional display and set-up options.


It sounds like a good idea but in practice the system can be fiendishly difficult to use. The main problem is the constant change of function of the four selector keys, and this is made even more cumbersome on the DX multiplexers, which do not have the benefit of colour-coding. During set-up one key can easily have half a dozen different functions, one moment it’s moving a cursor, the next it’s an exit button, or selecting a display mode. The net result is that it’s easy to get lost, or mess up a particular set-up routine, which then has to be started over again.


Doubtless it seemed like a good idea at the time but the on-screen display/control system has to cover a lot of ground. In addition to the numerous display, record and playback options there are all of the set-up routines and secondary functions to consider, including one of the system’s most useful features, activity detection.


In the set-up mode the image from each camera is superimposed with an activity grid of 128 (16 x 8) sensor dots. Each one can be switched on or off, to define the area of interest. Once again the on-screen display conspires to make this task much more difficult than it needs to be. The sensitivity of each camera activity input can be altered. There are five settings, optimised for particular sets of conditions, they are: indoor high, indoor low, outdoor high, outdoor low, and very low.


When the grid and sensitivity settings have been selected operation can be checked with the walk test facility, which lights up each sensor dot as it is triggered by movement. The final part of the activity set-up concerns how the camera outputs are recorded, and the dwell-times of activity cameras, when there’s no more movement. The activity-interleave option inserts triggered camera outputs into the programmed sequence; activity-only sends just the triggered camera outputs to the recording VCR and activity-select only records  nominated activity cameras.    


The remaining functions cover alarm operation, housekeeping and security facilities. The DX 9 has 9 user-definable alarm inputs, that can be configured for normally open or closed contacts. There are six alarm menu options. The first one concerns the output contacts, used to control the recording mode on a VCR, shifting it from time-lapse to real-time mode, for example. Option two covers the combination of recorded cameras; menu item three deals with the alarm indications on the monitor; number four is for setting the time-lapse mode on the VCR; item five selects or disables full-screen playback of alarm-tagged recordings and menu option six is for the auxiliary alarm input.


Accidental or deliberate tampering with the installer menu can be prevented using a security code option. This is engaged using a PIN code of up to eight digits. The security code facility on our sample reset to the default off condition when the power supply was interrupted for several hours.      



Operating the unit in basic recording and playback modes presents very few problems. The DX 9 has full duplex operation, so it is possible to playback a recording, without disrupting record functions, which can continue unaffected. (The SX colour multiplexers are simplex, and can only do one thing at a time -- i.e. record or playback. During playback the same display options are available, as for recording, namely full-screen, quad, or multi-screen, additionally there’s a ‘hold’ or freeze-frame facility. Depending on the initial set-up alarm-tagged recordings will be shown full-screen, or as an identified sub-screen. Alarm triggerings are also shown by an LED indicator on the status panel. The status key will also display a range of set-up options and a thumbnail alarm activation and camera failure log.



Evidence of the intensive digital processing going on inside the DX 9 is clearly visible on the live and recorded image. Pictures have a slightly jerky motion (very jerky on playback, due to time compression, and dependent on the number of cameras and recording intervals), and a definite ‘texture’ though it doesn’t have any significant effect on resolution, nor does it add any noise to the picture. In any case picture quality will be degraded by most recording VCRs, irrespective of the quality of the original input. Playback of multiplexed images is very stable and again there’s no increase in noise levels other than the contribution from the recording VCR.



The only real criticism concerns the user-friendliness -- or rather lack of it -- of the on-screen display system, and to some extent, the instruction book as well, which further clouds the already muddy waters. With perseverance and determination it is it is possible to master its intricacies, but we can’t help feeling a conventional display system would have been much easier to use. Performance is very good, and the range of facilities is difficult to fault, once you’ve managed to find them. Overall, though, it is a welcome development, that shows how far and how fast video multiplexing is progressing, and illustrates how a technology that has up to now been largely confined to prestige colour systems, can play a useful and cost-effective role within a humble monochrome multi-camera system.



Design and design features              ***

Circuitry and components                  ****

Ease of installation and wiring    ****   

Range and variety of functions            ****     

Accompanying instructions                   **                                

Technical advice and backup            ****     

Value for money                         ****                           




Ó R.Maybury 1996 0202











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