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Video multiplexers can improve operator efficiency and maximise hardware resources. We’ve been examining the Computar DPLEX 16, one of the most advanced models on the market



The steady digitisation of CCTV technology has brought with it a number of major benefits, one of the most significant being video multiplexing. Multiplexers have been around for quite a while but only recently have they become a viable option on small to medium-scale installations, enabling the outputs from several cameras to be simultaneously displayed on a single monitor, and recorded on one VCR. The development of specialised digital video processor microchips has allowed the complex electronics to be shrunk to a convenient size and eliminated the need for gen-locked cameras.


The Computar DPLEX 16 is a clear indication of the rapid progress this technology has made in the past few years. It’s a badge-engineered version of the Robot Research MV216, part of their Optima series of video multiplexers. The headline features are 16 camera inputs, (variants with 4 and 9-inputs are also available), with a multiplexed VCR output and the option to simultaneously display 1, 2, 4, 8 or 16 live colour images. The multiplexer can be used with any type of video camera having a standard PAL/CCIR output, there’s no need for gen-locking as the multiplexer synchronises all inputs internally.


However, the key feature on this model is dynamic time division (DTD) multiplexing, which apportions camera recording time according to the motion content of each image. Pictures with no movement are recorded less often than those showing activity, making the most efficient use of the recording VCR. It works by comparing the differences between successive fields from each camera input. If the systems detects a change between fields it assumes there’s movement within the defined target area. The output from the camera (or cameras) where motion has been detected, are sampled more frequently and fed to the VCR output. If movement is detected in more than one camera the multiplexer creates ‘motion groups’, whilst at the same time continuing to scan the remaining cameras on a less regular basis, provided the images remains static. The end result is less tape time devoted to scenes where nothing is happening and faster motion update on playback. DTD operates in the background, there are no alarm indications, though an alarm output is available on the back panel.


The DPLEX 16 has full duplex operation, for simultaneous recording and display. This means the multiplexer can remain operational, whilst it is also being used to replay tapes from another VCR. It has two monitor outputs, one for the main multi-camera display, the second call-monitor output shows a selected full-screen image.  In addition to normal composite video input and output for the VCR there’s a second set of S-Video connections for a Super VHS (or Hi8) VCR. Other features worth noting include a colour bar test pattern,  to aid monitor alignment, and a comprehensive on-screen display system, showing time, date, alarm status, camera ident, video loss and alarm inputs for each camera. The unit can be remotely controlled from a PC, via an RS232 interface, and it has a non-volatile memory which protects programmable features against data loss during a power failure or interruption.



The DPLEX 15 is housed in a slim black steel case measuring 432 x 311 x 44 millimetres. Front panel layout is very straightforward. On the right side there is a row of 16 camera selection buttons, each with an LED indicator. The main function keys are grouped together on the left side. They include the five display mode controls which double up as menu selection buttons. The back panel is dominated by a bank of 36 BNC sockets, 32 for individual camera inputs and outputs, two for the main monitor and call monitor video outputs, the remaining two are for the VCR input and output. Two mini DIN sockets handle S-Video (Y/C) formatted signals and a pair of 25-pin D sockets carry the alarm connections and remote control signals. Power is supplied by a plug-in mains adaptor.


The on-screen display set-up mode is accessed by pressing the function and zoom buttons together. The first screen is used to adjust the time and date displays. The second screen deals with alarm and VCR settings plus video input impedance. Screens three, four and five cover VCR alarm record and duration times, plus camera dwell. The next screen is used to compose the idents (up to 10 characters long), and set the motion sensing parameters for each camera.  Motion detection set-up, when enabled, overlays each camera input with a matrix of 192 ‘targets’. These can be configured to the scene by switching each target on or off, using the cursor keys and camera selector buttons. The last page of the menu display contains a simple security lock that when enabled, freezes all of the front panel controls, except for those used to access the on-screen display.



The instructions are reasonably comprehensive but a tad heavy-going in places. A few more diagrams or examples of on-screen displays would have been helpful. Nevertheless basic operation is very straightforward. The unit defaults to a 16 camera display and multiplexed VCR output at switch-on. The first of the five display mode buttons selects a full screen display of any selected input. Pressing this button a second time enables a 2X zoom mode. A sub screen is superimposed briefly on the display, showing the area of magnification, this can be changed by moving a screen shaped cursor, using the adjacent function buttons, which act as arrow keys. The second button in this group enables the PIP (picture in picture) mode, showing two selected inputs; pressing it a second time flips the main screen with the inset sub-screen. The remaining three buttons select  2 x 2 display (4 sub-screens), 3 x 3 display (9 screens), and 4x 4 display (16 screens).


Each camera has an associated alarm input, that can be connected to any type of detector having a NO contact or CMOS/TTL alarm output (polarity is switchable). If activated the multiplexer responds by flashing the relevant camera indicator, closing the alarm relay, sounding a buzzer, displaying the alarm output on the call monitor, recording a VISS index marker on the tape and superimposing an alarm text message on the  image. During replay tapes with recorded alarm indicators set the appropriate camera light flashing, superimpose an alarm message on the call monitor and relevant camera display. If any of the camera inputs are lost the unit switches the main monitor to multi-camera display, an on-screen message appears and an alarm buzzer sounds. If an alarm is triggered whilst the unit is in the ‘tape’ (i.e. playback mode) the multiplexer detects and reacts to the event as before, though this time there are no audible or visual indicators.


Once triggered the alarms has to be cancelled manually and although the time and date of the event will be logged by the VCR the multiplexer makes no independent record of activations. The unit performed faultlessly though the plug-in mains power supply module that came with our sample gave some cause for concern as it ran extremely hot, almost too hot to touch. It appeared to be under-rated for this application.



Image stability is excellent and colour accuracy is generally very good. Nevertheless  

full screen display of a live camera input shows some signs of the extensive digital processing that is taking place, with a slight loss of detail and hardening of edges. Image quality suffers even more in the various multi-camera display modes, though it’s not a cause for concern and the pictures still show a lot of detail, even on the 4 x 4 setting. Recordings made on S-VHS equipment contain only slightly more detail than standard VHS recordings, though noise levels are lower and there are fewer colour aberrations.



A most impressive piece of kit, designed and built to a very high standard. Installation and operation are both very simple. The performance is well up to the standard we have come to expect from this kind of equipment in general, and Robot Research in particular.




Video system              PAL colour, CCIR black and white

Screen formats            512 x 512/256 x 256/170 x 170/128 x 128

Colour palette            16 million/216 colours

Video level:                1.0 volt p-p, 75 ohms (composite)

VCR in/out                 composite or S-Video


Alarm             one input per camera (contact closure or TTL/CMOS)

Alarm duration             4 secs default (2-999 selectable)

Alarm output              NO and NC,  2.0A at 30VDC or 1.0A at 125VAC


Connections                camera in/out, monitor & VCR (phono), S-Video VCR in/out (mini DIN), alarm in/out (DB25-S),remote DB25-S)


Power requirements            12VDC 18 watts

Dimensions                 432 x 311 x 44mm

Weight                        4.5kg







Product design             8

Build quality                           9

Electronics quality             9



Ease of installation                  8

Set-up functions                     8

Instructions                            7

Manufacturer’s support            8



Functions                                9

Ease of use                             8



Image quality                         8



Ó R. Maybury 1995 0310



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