Multiplexers can be used to combine two or more individual video camera signals and send them to a single recorder. This is often referred to as timeshare multiplexing and allows up to 16 video camera signals to be recorded on a single half-inch videocassette simultaneously and played back as individual pictures or combinations of pictures upon command. A multiplexer could be either a simplex multiplexer or duplex multiplexer. The simplex multiplexer can only display a full-screen image of one selected camera or a sequence of selected cameras while recording. A duplex multiplexer can also display multiscreen images while still recording. Essentially, a multiscreen display consists of a split screen that allows for the viewing of all camera images on the system simultaneously (exhibit 2.16).
Timeshare multiplexing can also be used to transmit multiple video camera signals (up to 16) from one point to a second point by a single cable or transmitter (microwave, fiber optic, infrared). Another multiplexer at the second point can be used to separate the multiple video signals back into individual video signal outputs.
A duplex multiplexer is higher in cost than a simplex multiplexer. Generally, a duplex multiplexer is used if someone is watching or operating the system while it is recording; if it is unmanned, as in many school applications, a simplex multiplexer is more cost-effective. A true duplex system allows the user to watch multiple screens while recording without affecting the multiplexed output to the video camera recorder (VCR). A simplex system allows for full-screen or sequenced viewing in the record mode. If multiscreens are activated during the recording, the multiscreen itself might be recorded, thereby not allowing full-screen playback. A duplex system also allows for recording and playback simultaneously if two VCRs are connected. The multiplexer should provide two monitor inputs if this feature is used so live viewing of the facility is not lost. In most applications, a simplex unit is suitable and more economical if recording can be stopped while the video is reviewed. The recorded videotape can then be retrieved in a full-screen or in a multiscreen configuration.
Most multiplexers available from established manufacturers feature camera titling for recording and a permanent time/date stamp on each frame of recorded video.
Another feature is compensation for camera synchronization. Multiplexers are equipped with an alarm input for each camera. When activated, these can be used to generate an output to the VCR to place both the multiplexer and VCR into the 2-hour recording mode (real time) for a predetermined period of time. Some multiplexers allow only images from the alarm camera to be recorded, but others allow a choice of interleaving (every other field). Onscreen programming of the multiplexer allows for simpler programming and review of settings. Programming features should display VCR tables because it is important to synchronize the multiplexer to the particular model and brand of VCR to avoid missing crucial information.
Research Report: The Appropriate and Effective Use of Security Technologies in U.S. Schools