Video Experts - Expert Advice: Multiplexer Applications & Troubleshooting Guide

Multiplexers have all the functionality you would want in most small CCTV applications. They are not designed to do everything. They are building blocks. You will be amazed how much additional functionality can be had by installing a switch.

How do multiplexers display cameras on the monitor? The incoming camera images are digitized and resized to fit into the blocks in the multi-screen display monitor. We will call this the "main monitor." There is a second monitor output from the multiplexer; called the "call monitor." This displays full size images which have not been digitized. The camera inputs are just redirected to the call monitor with no digital processing. Want to see the difference between an image which has been digitized and one which has not? Just put the same full size image on the main and call monitor. If you can see the difference, it's probably because one of the monitors is out of adjustment.

The multiplexer timeshares camera images to the main monitor. If a single full sized image is displayed, it is displayed at a rate of 30 images per second (25, PAL). This is "real time," just like television. We always send 30 images per second to the monitor. If we display a picture in picture format, we still send 30 images to the monitor. Fifteen are of the main image and fifteen are of the insert picture. Somewhere around 22 images per second, our mind views the image as fluid motion. With a 2x2 screen, the 30 images are divided by four for a total of 7.5 images per second of each image. It is now obvious that the images are not "real time." The division holds true in the 3x3 and 4x4 screen formats. So the more images you see on the screen, the slower each image updates.

The digitized (sampled) image is processed in a pixel array with 512 pixels horizontally and 464 pixels vertically. In order to make a full size image fit into one of the 2x2 blocks, we need to get rid of some of the information. Every other horizontal pixel and every other vertical line is discarded. Does the resultant picture look as good as the original? Of course it can't have the same detail, however, the results are surprisingly good. To make the full size image fit in a 4x4 block, we throw out three fourths of the pixels in each direction. No, these small images do not look as good, however they are so small that you hardly notice. How well the image looks with the remaining data depends on how well the program processes the original data. In the 2x2 screen scenario, we discard every other pixel. The program analyzes both the discarded and remaining pixels, then adjusts the remaining pixels accordingly. How elegantly this process is done is more important than the actual number of pixels used.

How do multiplexers record to tape? What goes to tape is unrelated to what is displayed on the monitor. We control the timing of images sent to tape to correspond with what the VCR is expecting. When images are recorded, the VCR is in control. We must accommodate the VCR.

The multiplexer has two issues to be concerned with. In one instance, we can't switch images as fast as the VCR can record them. In the other instance, we must slow down our switching rate to accommodate the VCR in time-lapse mode. When the VCR is recording at a real time rate (2 hour mode,) it is recording 60 fields per second (same as a home VCR.) Ideally, the multiplexer would switch through cameras at this 60 updates per second rate and we would get really fast updates. The problem is that multiplexers can't go that fast. There are limitations in the hardware used. We must capture an image, write coded information into the vertical blanking interval and switch to the next camera. Most multiplexers switch at a rate of 20 times per second. Some newer versions (including our new simplex) can switch at 30 times per second. The higher speed is due to upgrades in circuitry. In the "real time" application, we switch as fast as we can and the VCR records it all. Playback speed will be dependent on the record speed.

In time lapse mode, the VCR is capturing fewer images. The VCR has a field delay, which means that it sits idle for some number of fields, then captures a single field. The mechanism typically steps at each capture, rather than stay in continuous motion. There is no standard for field delay on VCRs. Different manufacturers have different field delays at various time lapse settings. The result is a timing problem. There are three possible ways to accommodate this issue.

1. Work with every imaginable VCR manufacturer and VCR to acquire the field delay information, then write a routine in the multiplexer firmware to evoke this information on demand. This can be a long, hard, difficult task. New models are coming out every day. (Most multiplexer manufacturers do it this way.)

2. Write the multiplexer program to be flexible enough to work with any VCR, regardless of the VCR field delay. This is the method we use. Just set the VCR speed and the multiplexer speed to the same number. This works because it takes a worst case position, ensuring that all cameras will update. The down side is that we could update faster (and do) if we know the VCR field delay and set our time to a more appropriate setting. The up side is that you don't have to think about it. It just works

3. Wire the VCR timing output directly to the multiplexer. This is the ideal situation and future multiplexers will be timed in this manner. In effect, the VCR tells the multiplexer when it switches to another camera and the multiplexer switches with it. Most VCRs now have the switched output available. Some do not, so, the other methods will have to be maintained for these VCRs.

The Basics: What is a multiplexer?