What is a Multiplexer?
Put simply, a multiplexer allows several camera signals to be recorded onto
one videotape. To do this it synchronizes the camera signals (lines them up in
time) and marks each one with a code, allowing every camera to be replayed
independently from tape, regardless of how many cameras are recorded on that
tape. In addition, each image is stamped with a time and date caption.
Many multiplexers also provide the ability to view several cameras
simultaneously on one or more monitors. These groups of pictures, when displayed
on one monitor, are usually called multi-scene pictures. This is particularly
useful when there are a large number of cameras across a site.
As with so many things today, there is a vocabulary which one must learn to
be able to assess the functionality of any particular unit.
Jargon Buster:
| Simplex |
This term is used to describe a multiplexer that will record pictures
to tape or display multiple pictures on a single monitor. A
simplex multiplexer will not perform both functions
simultaneously. When a simplex multiplexer is used to replay tapes, it
will stop recording. |
| Duplex |
This term is used to describe a multiplexer that will record pictures
to tape and display multiple pictures on a single monitor. A
duplex multiplexer will continue to record even when a tape is
being replayed. |
| Multi-scene |
When several images are displayed on a single monitor screen
simultaneously, this is called a multi-scene view. |
| Framestore |
This is an area of memory within a multiplexer that is used to store
pictures when they are digitised. Generally speaking, a simplex machine
can be considered as having one framestore, a duplex machine has two. |
| Digitisation |
The process of turning an analogue signal (most camera signals) into a
digital signal (a collection of binary 1's and 0's used to represent an
analogue value.) |
| Asynchronous camera inputs |
Most multiplexers use asynchronous camera inputs. This means that
cameras within a CCTV system do not have to be referenced to a single
timing source (gen-lock). This usually makes the system both cheaper and
easier to install. |
| Time Division Multiplex |
All video multiplexers perform time-division multiplexing. This refers
to the process of recording all connected cameras by taking a snapshot
of each camera in turn. In other words, a slot of time is allocated to
each camera. |
| Update rates |
This is probably the most often talked about subject when referring to
multiplexers - and the least understood! The update rate is the period
of time between the first and subsequent recordings of any particular
camera. Since the cameras within a multiplexer based system are recorded
in a time-division multiplex, and each camera is allotted a slot of
time, the update rate will be dependent upon the number of cameras
recorded.
For example, if 4 cameras are recorded - for this example we'll call
them 1,2,3 & 4, the sequence of images on tape will be
1,2,3,4,1,2,3,4,1,2,3,4 etc. When camera 1 is replayed from tape, images
2,3 and 4 will not be displayed, but the time allotted to them on the
tape will elapse before the image of camera 1 is updated. When using
time lapse VCR's that extend the length of time a single tape will
record, the update rate will be a function of the VCR recording mode
(i.e. 12, 24, 48-hour modes etc.) and the number of cameras recorded.
THIS IS ALWAYS INDEPENDENT OF THE MULTIPLEXER USED. |
Hints
and tips on choosing and using a multiplexer based CCTV system
- Always go for the best quality multiplexer your budget will permit.
Cut-price models may appear attractive initially, but a poor multiplexer
will ruin the performance of all other elements in the systems.
- If possible, allow room for future expansion in your system design.
Regardless of how many cameras are initially required, once the usefulness
of such a system has been proven, more cameras are very often required.
- When using time-lapse VCR's, use the lowest time-lapse mode that will fit
with the operational requirements of the site. For example, if someone is on
site to change videotapes every 12 hours, use this in preference to a 24hour
regime. This will use two tapes per day as opposed to one, but will double
the update rate and the number of pictures recorded - hence giving a better
chance of capturing that all important image in the event of a incident.
- Ensure that all cameras are set up correctly and adequately lighted. A
multiplexer, no matter how good, will not improve a poor camera signal.
- Plan camera views carefully. If faces or numberplates need to be easily
recognised, avoid the use of wide-angle lenses to keep the subject as large
as possible in the field of view. Remember that telemetry controlled cameras
can be facing the wrong way when an incident occurs, it's often safer,
therefore, to use more static cameras rather than one fully functional
model.
- Use a good quality VCR & tapes. If the budget can stretch to it, an
SVHS video recorder will offer a substantial improvement, in terms of
picture quality, over a standard VHS machine.
- Do not re-record over videotapes more than 12 times. Each time a tape is
run through the VCR, some of the metal oxide is rubbed off of the tape. Many
recordings have been ruined simply because a tape has been used too often.
Once the oxide has been rubbed off, you will be attempting to record on what
is basically sellotape!
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