Expert Advice

Contributor: Paul Broome
Company: Sensormatic VPD

Multiplexer Applications & Troubleshooting Guide

Typical Multiplexer Applications

Multiplexers are one of the building blocks used in designing a CCTV system. The type and number of multiplexers will vary with the application. The questions to ask yourself are:
  1. What are you trying to accomplish? Define success so that you will recognize it when you see it.
  2. What is the primary objective? What really needs to be accomplished?
  3. What additional functionality would be nice to have, if the price was right?
  4. What additional functionality is likely to be needed in the future?
    • Once this is established...
  5. What components do we need to assemble to provide an acceptable solution for the application?
Sometimes it is relatively easy to add additional hardware later to enhance system capabilities. Sometimes it is not. You need to consider this in designing the system. If you have an existing simplex multiplexer and now need duplex features, you must make an expensive change. Working closely with the customer during the initial system design will help avoid an unexpected surprise in the future. A duplex multiplexer might be the right initial choice even though all the features are not needed immediately.

Remember that multiplexers are one of the building blocks in a system. Sometimes more than one multiplexer is appropriate. VPD multiplexers come in 4, 9 and 16 input versions. Why not 32? The law of diminishing returns comes into play at some point. We are time sharing (multiplexing) camera images to tape. These are played back with a time delay between updates of each image, based on the number of cameras in the cycle. At some point, that time delay makes the whole idea of recording all the cameras on one VCR seem less attractive. It just takes too long to see anything happen. It is more practical to use a second multiplexer and VCR.

Are you limited by the functionality of the multiplexer, or, are you limited by the budget of the customer? Don't limit your thinking to what a multiplexer can do. Design a solution to the problem first. You can compromise on price versus functionality later. It could take a second multiplexer to do all the things that the customer wants to do at one time. However, all the items might be performed one at a time by a single multiplexer. An example:

We are often asked why you can't view all cameras live on one monitor while reviewing a tape of all the cameras on the second monitor. If you paid attention to the simplex/duplex descriptions in this manual, you already know the answer, there just isn't enough hardware for one multiplexer to do this. Could we redesign a multiplexer to make it view multi-screen live while playing back multi-screen? Perhaps. However, we wouldn't have enough hardware left to do the recording. This would not be a practical device as all customers would have to pay for this additional functionality when only a few would use it. Could you assemble such a solution now? Sure you could.

You can view 16 cameras live, record the same 16 cameras and review a tape of the 16 cameras, all at the same time. This requires two monitors, two VCRs and two multiplexers. Use a duplex multiplexer to view live and record. Use the second VCR to playback to the simplex. You have two monitors side by side, both multi-screen, one live, one playback. Want to see two different views of the live cameras? Loop the cameras from the duplex to the simplex. You have two independent live displays when you are not using the simplex for tape review. You can use two additional monitors as call monitors. Want a tape with only alarm images recorded? Use a third VCR on the loop out of the duplex call monitor. Leave the VCR in idle and have the multiplexer trigger it to record on alarm.

Don't limit your thinking. Your design ability and the customer budget are the real limits. Most multiplexers are installed into applications with 16, or, fewer cameras. Few are as elaborate as the situation described in the previous paragraph. Although almost any combination of devices is appropriate in installations with a large number of cameras and particular customer needs, most small systems using multiplexers fall into the following categories:

  1. View Only.
  2. Record Only
  3. Review only.
  4. View and Record.
  5. View and Record and Review.
View Only:(Config #1). The objective with this configuration is to allow the operator to manage the cameras available in the system. Minimum requirements to accomplish this objective are; cameras, a simplex multiplexer and a monitor. As long as there is no need to record the cameras, this system is complete. You can enhance attendant capabilities by adding a Call Monitor and installing alarm inputs to alert the operator to important events.

Record Only:(Config. #2). The objective with this configuration is to record images for later review. Minimum requirements to accomplish this objective are; cameras, a simplex multiplexer and a VCR. A monitor is not necessary, however, it is a good idea to have some viewing capability to ensure yourself that the system is actually working. Many record only Systems are locked in closets and available to authorized personnel only. Often the tapes are routinely picked up and taken to another location for storage and review.

Review Only:(Con fig. #3). The objective with this configuration is to review tapes which were recorded at other locations. Minimum requirements to accomplish this objective are; a simplex multiplexer, a VCR and a monitor. The type of VCR used must be compatible with the VCRS used at the record locations.

View and Record:(Config. #4). The objective with this configuration is to allow an operator full control of live viewing of cameras while all cameras are recorded to tape. Minimum requirements to accomplish this objective are; cameras, a duplex multiplexer, a monitor and a VCR. Attendant capabilities may be enhanced by adding a Call Monitor and alarm inputs to alert the operator to important events. This configuration is typically used to review tapes, as well, using the same VCR. Of course, while tapes are reviewed, the VCR is not available for recording. Care must be taken to disable the VCR alarm input to prevent accidental recording while a tape is reviewed. It is better to use two VCRs, one to record, a second for playback.

View and Record and Review:(Config. #5). The objective with this configuration is to allow an operator full control of live cameras, record all cameras to tape and to review previously recorded tapes, all at the same time. Minimum requirements to accomplish this task are, cameras, two monitors, two VCRs, and two multiplexers (a simplex and a duplex). Often people ask for this functionality, then settle for the View and Record configuration. It depends on how serious the application is for reviewing tapes. If the previous days tape must be reviewed while the current information is being viewed and recorded, you really need two systems (View and Record, and, Review Only). Using both configurations in the same application allows you to observe two different multi-screen displays of live video when the playback system is not used to review a tape. For instance, you can watch a 4x4 display on one monitor and a 2x2 on another, or, two 2x2 screens with the lower quadrant(s) sequencing. Alarms will show on the call monitor.