Contributor: Paul Broome
Company: Sensormatic VPD

Multiplexer Applications & Troubleshooting Guide

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Lookout!

This section alerts you to things that go bump in the night. When your equipment interfaces with a variety of equipment supplied by a variety of manufacturers, you can bet there will be situations where each will exhibit a mind of its own.

Lookout! Do you have a device triggering your VCR into the record mode, like an alarm input through a multiplexer, quad, or, another device? There are situations where you could lose valuable data recorded on a tape.

Most time lapse VCRs, while recording, will accept an alarm input which will trigger the VCR into "real time" speed for some period of time. Most VCRs disable this feature while the tape is being played back. After all, you are probably playing back the tape in the same VCR you recorded on, install in the same system. An incoming alarm which triggers the recorder to record, could write over potentially valuable images already recorded on the tape.

The danger arises while the tape is stopped. Some VCRs will still accept that alarm input and start the VCR recording, with potentially negative results. Make sure you protect your recorded data by disabling the alarm input to the VCR while reviewing a tape. Using two VCRs, one for record and another for playback is the safest way (the playback VCR will not have an alarm input wired to it.) A switch wired into the alarm lead is very effective for single VCR applications. Breaking the write protect tab on the valuable tape is a good safety precaution.

Being aware of the potential danger is crucial. Know what your VCR is going to do when an alarm is encountered. The VCR and multiplexer must work together. Only the VCR knows for sure whether it is recording, playing back, or, sitting idle.

Things to look out for - Cameras: We accept most cameras used in the industry. We are looking for a one volt peak to peak composite video signal with no DC offset. Most modern cameras follow this convention, however, you can run into lots of strange situations with existing installations.

We are looking for a signal with a very low voltage level. The standard video signal is a composite signal which contains video, horizontal sync pulses and vertical sync pulses. This is also called base band video, which is the NTSC standard with out the carrier frequencies and lots of other things used in broadcast video. These signals are combined so they can be sent through a single coax cable. The device at the other end separates the signals and uses them accordingly.

The horizontal sync pulse is a negative going pulse, centered around a zero volt reference. If the reference raises very far above the zero volt level, the sync pulse is no longer negative (below 0 volts.) If it is too high a reference it may damage the input of the multiplexer. High, but less than danger level high, will result in loss of sync and a continually rolling picture. There is a fringe area where the picture will roll occasionally, or, continually jump up and down. Try to use cameras you are familiar with. When installing the multiplexer in a system with existing cameras, be careful. Why would a camera have a DC offset? There are a few cameras which provide power through the same coax as the composite video signal. Whatever you do, don't use these cameras with the multiplexer. Improper grounding could provide any number of problems.

Things to look out for - Monitors: Generally, monitors are more forgiving than most devices. Monitor manufacturers know that they are going to see a wide variety of input levels from old, new and soon to be developed cameras. A camera with a DC offset, which will not work through a multiplexer, will probably work fine when connected directly to a monitor. Is it a monitor, or, a television? Televisions are showing up more and more often in CCTV systems. There are several things to consider. Price is certainly is one consideration. You can probably buy a nice 31" TV for the same price as a dinky little monitor. So, size and price are certainly on the side of the TV. Before you run out to the local home appliance store, consider this:

• A TV will probably have RCA connectors for video. This requires an adapter to BNC and they can fall out.
• You will have to set the TV to a "line," or, "direct" mode to accept composite video. Otherwise, you will have to install a modulator on the video input.
• TVs over scan (the edges of the picture run off the viewable area.) This may cause the time/date to run off the top of the screen and/or the camera titling to run off the edge of the screen. Typically, there are no screen size adjustments on televisions.
• TV resolution is generally lower than a monitor.
• The larger the monitor, the poorer the picture (you are displaying the same number of scan lines on a larger surface.)

Multiplexers sample each scan line and convert the results into a pixel matrix which is processed, then converted back to standard video for display. The resultant video will have a black line around the edge of the viewable picture. Such a black line on your home TV would certainly be considered a problem. This is done intentionally with the multiplexer. This allows full viewing of each camera in multi-screen modes. If we over scanned the image, we would lose a large portion of the images around the perimeter of a 4x4 screen.