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Contributor: Paul Broome
Company: Sensormatic VPD
Multiplexer Applications & Troubleshooting Guide
How's That Again?
This section contains explanations of phenomenons associated with multiplexer technology. There are tradeoffs which must be made in order to record multiple cameras on a single VCR tape.
Are we recording frames, or, fields?
A frame contains twice as much information as a field. A frame is a consecutive odd and even field of the same camera. In multiplexing and in time-lapse recording, where individual fields of alternating cameras are recorded, there is no opportunity to reassemble consecutive fields of the same video source. Therefore, virtually all multiplexed and time-lapsed applications utilize fields, not frames. In order to fill the screen, the information from the missing field is still needed. This is typically accomplished by duplicating the content of each scan line of the existing field on the following line. This process is called line doubling.
Why can't I "fast search" a tape recorded with a multiplexer?
First, what's the difference between fast forward and fast search? Fast forward is like rewinding a tape, only in the opposite direction. The heads are moved away from the tape and the tape is wound quickly. You can not see images while the tape is in fast forward. Fast search is when the tape is moved past the playback head at four, or, five times the normal playback speed. In this mode, the images appear to be moving very quickly with four, or five lines running through the picture horizontally. Most people erroneously say fast forward when they are talking about fast search.
So why are the lines on the screen in fast search? With a typical home style VCR, you are usually viewing a movie which is a single video source with what appears to be continuous motion. Video is actually a series of still images displayed closely together in time to give the impression of fluid motion. The individual "fields" of video are recorded on the VCR tape in a long diagonal strip. The VCR usually plays back the individual fields in order, and you see the resultant moving picture. When the tape is moved past the read head very rapidly, the head scans across portions of several of these strips of video at the same time. The lines shown on the screen are the dead areas between the fields of video. The fields are separated in time by 1/60th. of a second (NTSC, 1/50th. PAL) so the result is what appears to be a fast moving picture with lines running through it. A time lapse VCR in "real time" two hour mode acts identically to the home VCR.
What's different about a multiplexed tape? Multiplexers record several cameras on one VCR by time-sharing images to tape. With four cameras, you are recording one camera at a time while cycling through all the available cameras (1,2,3,4, 1,2,3,4, etc.) The advantage is that some images of all cameras are recorded on a single tape. The disadvantage is that the recording of each camera is not continuous. When the tape is played back, the system decodes whatever is on the tape. You can view a single camera, or, several cameras in multi-screen format. When you fast search the tape, you still get the horizontal lines which separate the portions of each consecutive field. This time, however, the field portions are from different cameras. You see fast motion, horizontal lines and strips of different camera views. How do you fix this? You can't. It isn't broken. It is a limit of the technology. This is one of the trade offs necessary to allow multi-camera recording on a single VCR.
Why do I need a multiplexer to play back a tape recorded through a multiplexer?
Using the information we learned in the previous section, we know that the multiplexer causes the recorder to record fields of alternating cameras. The multiplexer encodes the time date and camera number in the vertical blanking interval following each field. When played back, the multiplexer decodes this information and displays the camera views which you select with the multiplexer front panel. If you play back the tape without the multiplexer, no decoding takes place and you see exactly what is recorded on the tape, changing images of several cameras. The speed at which the images change depends on the record speed used when the tape was made. A "two hour" tape will be unintelligible, with rapidly changing images (20, or, 30 per second) depending on the multiplexer used to make the recording.
In an "emergency," with no multiplexer available, you can see still images if the playback VCR has a pause/still function. Press the pause/still button to stop the tape. You can then single step to your heart's content. You will quickly agree that using a multiplexer is a more attractive alternative. When the tape is played back through a multiplexer, the individual camera images are decoded and displayed as selected from the front panel. You can view individual cameras, PIP, 2x2, 3x3, or, 4x4. In fact, any screen format you can select in live mode may be selected in tape playback, as well. The update speed is limited by the speed at which they were recorded and the selected playback speed.
