Perplexing Multiplexing

Perplexing Multiplexing

Multiplexer!
Does that word worry you?
Does it make you tremble just a little?
When you hear that word do you suddenly develop a headache?

If you answered yes to any of these questions read on, as I have the cure for your multiplexer woes.

The modern multiplexer is a solid-state microprocessor controlled marvel. It’s only moving parts are the pushbuttons you press on the face plate. It’s in simplistic terms a switcher, a device that allows you to input multiple cameras and output to a recorder as well as a few monitors. Its beauty is the speed in which it sequences through all the cameras and encodes a signal, hidden in the vertical blanking interval that allows many options of camera selection during playback.

The multiplexer’s sequencing speed varies in relation to the number of cameras connected and the selected speed of the recorder. You can observe this by simply connecting a monitor to the “VCR OUT” jack at the rear of the multiplexer. Then try various recording speed settings (chosen from the multiplexer’s menu) as well as increasing and decreasing the number of cameras connected to the unit.

The record speed selection found in the multiplexer’s menu is to select the speed in which the recorder will be recording. It’s imperative you also select the model of recorder you will be using, since there are many types of recording options (real time, field, frame etc.) the multiplexer must be programmed to correctly synchronize with the selected recorder. If your recorder’s brand is not listed (in the multiplexer’s menu) contact your repair centre and they should be able to cross-reference one of the available choices with your actual unit. However the preferred way is to connect the recorder’s “head switching pulse” directly to the multiplexer, this way the multiplexer’s switching is synchronized exactly with the recorder. This method also comes in handy when you are using alarm inputs to speed up the recorder to capture an alarmed event. As the recorder changes speeds the multiplexer follows, allowing for smooth speed transitions and reducing any probable data loss.

As the multiplexer outputs a control signal hidden within the vertical blanking interval, its imperative that the recorder used is perfectly tuned and is capable of reproducing a strong recorded signal. If your recorder is weak or slightly distorts the vertical blanking interval your multiplexer will not be able to decode it’s own recording. Make sure to test the recorder with one camera connected directly to it, at the speed you will be using for the installation, to confirm that the playback, pause and field/frame advance is perfect (without any noise, jitter or flagging on the screen). Only then and after completely reading the operation manual should you connect the multiplexer.

Another possible problem area is the recorder’s time and date generator and “up the coax” type of telemetry camera control. These share the same vertical blanking interval space and could possibly interfere with the multiplexer’s encoded signals. The recorder’s internal time and date generator can be turned off and “up the coax” control problems can be remedied by connecting the output signal to the multiplexer from the camera controller’s looped output (certain models only).

The next step is to confirm you have the multiplexer fully operating with all of it’s features by performing a reset. This will re-initialize the unit to the factory default settings, so be sure to complete this step before you have performed any programming. If you are going to protect the unit with a password (good idea) be sure not to forget it as it is almost impossible to reset a password in the field and the unit will most likely require a trip to the service centre.

If you are experiencing any strange camera signals after connecting and powering up the multiplexer, it’s probably due to a termination problem. Some multiplexers allow for individual camera termination by either software or hardware (75 Ohm resister), where others require a global software setting which effects all attached cameras. It’s a good idea to always carry a couple of 75 Ohm terminators in your pocket when you are installing a multiplexer, that way you can quickly test any problematic camera without having to access the multiplexer’s menu.

Site Troubleshooting.

Troubleshooting a multiplexer installation is not unlike any other installation problem, it’s best to eliminate the multiplexer from the picture and then see if the problem is still evident. Test the installation and try to make it fail; if you can’t “brake” it have the user show you where the problem is. Along the way question the user and try to find out the basics, such as….
1. Did the system ever work or is this a new problem?
2. Have you attempted to perform this particular operation before?
3. What time of day does the problem occur (if intermittent)?
4. Has anyone been recently working on the system or around the installation?

Another common complaint is the multiplexer can’t decode (playback) the tape. You can check the multiplexer’s decoding ability (assuming you have a duplex unit) by removing the recorder and connecting a short cable between the multiplexer’s VCR input and output connectors. Then select play on the multiplexer and you can review its decoding process. Some of the newer units will decode another brand’s recording, so be sure to check in the multiplexer’s menu to confirm it’s the selected brand.

Recently we have had a rash of new multiplexers coming in with the problem description indicating loss of one camera on the monitor only, during playback the same camera would reappear. When troubleshooting the unit it was discovered that some one had programmed the multiplexer to have a “covert” camera and it was the one selected. That was one of our simpler repairs that could have been easily avoided had the installer familiarized him/herself with the multiplexer’s functions.

At this point you should be able to confirm if there is a problem or not with the multiplexer, try to reset the unit and if that doesn’t clear the problem, there isn’t much more you can do in the field and its time to call for service.

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