Video Switching and Multiplexing
Multiplexers
Multiplexers are a more functionally useful way of handling a multi-camera system. Like a switcher, several cameras (usually up to 16) can be connected to the multiplexer. A single field or frame from each camera is successively output via the multiplexer onto a video recording. Playback would take place again via the multiplexer which would decode the recording to the monitor.
So, for example, if four cameras were connected to the multiplexer, on playback, the particular camera to be reviewed would be selected and an updated image produced every 0.87 seconds (Sony YS-DX316P and SVT-5050P). The more cameras that are recorded onto one tape, the fewer images per camera are captured – if eight cameras were connected, that refresh rate would extend to one image every 1.73 seconds.
In the majority of cases, these types of refresh rates provide sufficient information to enable incidents to be reviewed effectively. Problems may arise if the video recorder is a time-lapse machine recording many cameras over long periods. For example, if 16 cameras were recorded over 72 hours you would have to wait
9.87seconds for the image from each camera to be updated. It is therefore important to ensure that an acceptable ratio of cameras to multiplexers/VCRs is utilised.
Duplex multiplexers can display in multi-picture mode at the same time as recording, as above. Simplex multiplexers can either record or display, but cannot do both simultaneously.
The Sony Guide to CCTV . Issue 3