The information we write into the vertical blanking interval (that wide black line between the fields,) is time, date and camera number. When we play back a tape, this information is what allows us to view a particular camera, or, several cameras. If we want to see only camera #7, the multiplexer displays only that camera. We can view played back images in all the same ways that we can view live images. The speed on playback is limited to what was recorded on tape.

Reviewing Tapes: What do you see when you review a tape? It depends on what was recorded on the tape. If the tape was recorded from the multiplexer VCR output, it will contain alternating images of different cameras. Each image is encoded with the time, date and camera number. When played back through a multiplexer, the information is decoded and you can review the video using the functionality of the multiplexer. Anything that you can do in live mode, you can do on tape review (full screen, zoom, 2x2, 3x3, 4x4, sequence, etc.). The update speed of the played back images will depend on what is recorded on the tape and the playback speed of the VCR. If it was recorded in two hour mode, the updates are relatively fast. If recorded in a longer time lapse mode, they will be updated at increasingly longer intervals.

All VPD multiplexers record in the same format. All tapes can be reviewed on any VPD multiplexer. The record device will determine whether images were recorded in color. The playback device will review according to its capabilities. As a example, if the tape is recorded using an older style simplex multiplexer with limited capabilities, the tape may be reviewed using a duplex multiplexer and all the multi-screen functions are available. If a color tape is reviewed on a black and white multiplexer, no color is available because the reviewing device does not have color capability. If a black and white tape is reviewed on a color multiplexer, no color will display because no color was recorded on the tape. Other than that, all functionality (or, limited functionality) of the playback device will be available.

VCR Playback Compatibility: Tapes must be recorded and played back on similar devices. The reviewing device must be able to recognize the images recorded on the tape. Since different width record heads are used in different types of recording schemes, the playback device must be compatible with the recording technology. The safest course of action is to use the same model VCR wherever there is a possibility of the tapes being reviewed on other devices. Don't mix 24 hour "real-time," high density, time-lapse and commercial VCRs.

We occasionally find someone who wants to record with a home style VCR in the EP, six hour mode, then play back in the SP, two hour mode. Sounds reasonable, who wants to watch the tape for six hours when you can do it in two? The problem is that most consumer VCRs know which speed the tape is recorded in and automatically playback in the same speed. So, you can't play back in the faster speed. The consumer VCR assumes that you are playing back a movie which you recorded. If this assumption is made, playing back in the same speed is a good idea. These devices were not designed with the CCTV industry in mind.

Simplex vs. Duplex: Basically, simplex does one thing at a time. Duplex can do two things at the same time. Different manufacturers have different explanations of simplex and duplex. Most people describe duplex as the ability to view multi-screen images while recording all the cameras. Another definition is to be able to record and playback at the same time. This, of course, requires two VCRs. Our duplex multiplexer can do several things simultaneously. With two monitors and two VCRs, you can view live cameras in any multi-screen format while recording. You can play back a previously recorded tape (this occupies the main monitor,) while still recording current activity. The call monitor is active through all this, viewing full size images. It even switches to the alarmed camera as alarms are received. You could install a third VCR to the call monitor output. You can trigger this VCR to record when an alarm is received. Now you have a tape with only alarmed images.

The point in all this is that different people get different ideas when you mention a particular function. A simplex can usually do all the things that a duplex can do, it just can't do all these things at the same time. A simplex can either encode and send images to tape, or, use the same circuitry to decode and display multi-screen images. It can't do both at the same time because it does not have the hardware to accomplish this. A duplex can do both these things at the same time because it has two sets of circuitry. One in sending images to tape while the other is sending them to the screen.

Activity Detection: The purpose of activity detection is to record more images of active cameras. This is accomplished at the expense of cameras with no activity. The assumption is that camera images with movement are of greater importance than cameras with no movement. A multiplexer typically records cameras in order (1,2,3,4, 1,2,3,4, etc.) Activity detection interleaves active cameras as every other image. Assuming that camera number five had movement, while all others did not, every other image is camera 5 (1,5,2,5,3,5, etc.) If several cameras have movement, they form a motion group (1, 567, 2, 567, 3, 567, etc.) The result is faster updates of the cameras you want to see the most.

Information sent to the monitor is interleaved, as well. You can use this to demonstrate the effect of activity detection. With four cameras installed on a multiplexer, enter the menu and turn activity detection off on all cameras. Select a 2x2 screen. The multiplexer will update all cameras equally. You will be able to observe this on the monitor. Go back into the menu and turn activity detection on one of the cameras. It is best to do this with a demonstration camera aimed at you, however, any camera with constant motion will do. Back at the 2x2 screen you will see that the motion is smoother on the camera with activity detection selected.

Motion Detection: There is much confusion between activity detection and motion detection. Some manufacturers refer to the activity described above as motion detection. Generally, motion detection provides a motion alarm output. Activity detection does not provide any external outputs. Part of the confusion is because, up to a point, activity detection and motion detection do the same thing. They both look for a difference in gray scale (the black and white portion of the video signal,) from one image to the next. When they find a difference, they both interleave the camera with the movement. Activity detection stops there. Motion detection uses a little more hardware and a lot more software to make intelligent decisions with the gray scale difference. How much difference in gray scale is required to cause an alarm output? (Sensitivity.) How many of the motion targets must see motion before an alarm is issued? (Size.) How long must the motion take place before an alarm? (Delay.) Several other variables can be used to zero in on motion in a particular area.

Motion detection provides all the alarm indications you would normally expect with a hard wired alarm. It does interleave the cameras to tape, like activity detection. It also lights the associated camera LED on the front panel, sounds the alarm tone, displays the alarm graphic, switches the call monitor to the alarmed camera and switches the VCR to real-time mode. All this from movement in the camera image.