SECOND PART
Multiplexer Applications & Troubleshooting Guide
Commercial VCR (home style.) This is the VCR you have at home. It is designed to play and record movies in conjunction with a television. It can range in complexity from record only with no additional functions to a variety of features including pause/still, single step and multiple event recording. They all have one thing in common, they are to difficult for humans to figure out, unless you are a VCR savant. There are typically two, or, three record speeds (they are called speeds, even though they are actually lengths of time.) SP is standard play (2 hour.) LP is long play (4 hour.) EP is extended play (6 hour.) These times are relevant with T120 tape (T = time, 120 = 120 minutes.) The longer record settings use thinner record heads so that more images can be recorded on the same length of tape. Quality suffers as record time increases because the smaller heads provide less video information.
Advantage: Least expensive VCR option. Always records at a "real-time" rate.
Disadvantage: Limited record time (2hrs - 6hrs with T120 tape. Up to 8hrs with T160 tape [NTSC].)
Time-lapse VCR: Time-lapse VCRs typically have several record speeds (again, they are actually elapsed times.) In 2 hour mode, they record and play back just like the home VCR. 6 hour mode is like EP. Longer time settings are time-lapse, which means that they do not record all the available images. They skip images so that the capacity of the tape can accommodate a longer period of time. Want the tape to last longer? Just use a longer time-lapse mode. The images are just as good, however, they are further apart.
Time -lapse VCRs were designed with the security industry in mind. Even though the price is much higher than a standard VCR, the benefits for long term recording make this an economical device for solving problems typically encountered in security. System operation requires that the tape is changed at a routine interval. The attendant can change the tape at the same time every day, during the normal work week, then switch to a longer time record mode to accommodate the weekend.
Advantage: A single VCR tape can last for a specified length of time (typically; 2, 6, 12, 24, 48, 72,168, 480, 960 hours) depending on the VCR selected.
Disadvantage: Fewer images of each camera are recorded as time-lapse time increases. You run the risk of missing the detailed action you want.
24 Hour "Real-time" VCR: This is a compromise between a real-time and a time-lapse VCR. It will record at one third the real-time rate for a period of 24 hours (if you use T160 tape.) There are trade-offs to endure. T160 tape is thinner (to fit on the same reels as T120,) and it will stretch. You need to replace it more often than T120 tape. A thinner record head is used. This allows more images to be recorded in the same space. The resolution of the image is not as good as with the normal width head.
Advantage: Records for 24hrs with T160 tape. Records at a rate of 20 images per second (NTSC.)
Disadvantage: Not quite real-time. No longer record time to accommodate weekends.
Things to look out for - Termination: All good RG59 coax cables must have a 75 ohm resistor installed between the center conductor and ground at each end of the cable run. Cameras know that they will be at one end of the cable, so, they have a resistor installed internally. The multiplexer is at the other end of the cable, some of the time, and somewhere in the middle, at other times. We must be able to install the resistor, or not, depending on the installation. There are three possible options for termination. We usually pick the best of the options and stick with that solution, however, this is a situation where there is no clear best way. We have three different methods, depending on the unit vintage.
Soft-set termination: This is what we currently use on most products. From the menu, you select either 75 ohm, if the multiplexer is the end of the line, or, Hi-Z (open circuit,) if the signals pass on to another device. The default setting is 75 ohm. If the cameras terminate at the multiplexer, you're done, you don't have to think about it. If you're looping out, change to Hi-Z. The disadvantage to this method is that it is "global" (all, or, nothing.) Everything is Hi-Z, or, everything is 75 ohms. If you want to loop out only one camera, you must select Hi-Z and individually terminate all the rest with BNC terminators.
Individual terminators: Our new Simplex multiplexer has eliminated the soft-set termination. Individual terminators are installed and shipped with the units. If you want to loop out some of the cameras, remove the appropriate terminators.
Individual termination switches: The plan for future products is to provide individual switches for each camera input. The default position will be 75 ohms.
What is termination, anyway? We keep talking about a one volt peak to peak video signal. This is the industry standard (NTSC, or, PAL) signal, around which, most CCTV products are designed. Electronically, that signal is driven through a transmission line (the coax cable.) The size of the signal at the other end of the cable changes with the impedance of the cable. We assure that there is a 75 ohm termination resistor at each end of the overall cable so that the signal will be as close as possible to our one volt peak to peak standard. Without the terminating resistor, there is not enough "load" to draw the right amount of current to bring the signal down to the size we want.
You can see the effect of termination for yourself. Most monitors have a termination switch installed near the video input. Move the switch between the 75 ohm and Hi-Z positions and watch the video display get brighter and darker. Another way to see this is to toggle the termination from 75 ohms to Hi-Z in the menu. The call monitor will show the difference on an individual camera.
Things to look out for - Vertical blanking interval: The vertical blanking interval is the area of a frame of video which separates the odd and even field. In NTSC, it is 32 blank lines. It gives the monitor a chance to settle down before it has to deal with the next field. This is the black line you have probably seen on a television with a rolling picture. Since there is a vertical blanking interval immediately following each individual field, this is a convenient place to store information. Most multiplexer manufacturers encode the time, date and camera number in this area. Exactly where it is stored within those 32 lines is anybody's guess, as there is no industry standard for usage in this area.
Manufacturers of other devices use the vertical blanking interval to encode information. In the case of "up the coax" camera control, the VBI is used to send telemetry information to control pan/tilt units. VCRs use this area, as well. Problems may occur when two devices encode information into the same area. If we run into a compatibility problem with a multiplexer and a particular VCR, the first thing to do is turn off the time/date on the VCR as this may conflict with the encoded multiplexer information.
Although there is no standard, I doubt that anyone causes a conflict intentionally. Equipment which is not very compatible does not become very popular because of the problems encountered.
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