Martin Kane of SITO continues his commentary on the CCTV module of the 1851 qualification. In a previous session he considered switchers. In this session he considers multiplexing (putting two or more signals on to a single channel) and signal transmission.
There are some system applications where continuous recording of all cameras is required. This can be achieved in two ways: either by linking one video recorder to each camera or using video multiplexing.

Video multiplexing is a high-speed switching system that enables 'snapshots' or frames of a number of camera signals to be recorded on to a video tape at very high speed. Most modern multiplexers will do this in either colour or mono.

If, for example, we had eight cameras linked to the system the unit would start at camera 1, taking a sample of the video signal and sending it to the video recorder (Fig 2). It would then do the same for cameras 2, 3, 4, 5, 6, 7 and 8. Having completed the cycle, it would return to camera 1 and start again, producing a continuous stream of camera shots on to the video tape.

The system is capable of producing 25 of these single shots every second. Because the switching is done at such high speeds we are able to record a number of cameras on to a video tape with very little time loss between each individual camera frame.

In normal direct viewing the camera will complete 25 complete pictures or frames every second which are recorded on to the video tape. This gives the effect of live and simultaneous imaging.

Robotic motion

In multiplexing, the same number of frames can be recorded on to a real time video tape but the number of frames per second for each camera will be less as we are recording a number of different cameras in the same time period.

The longer between frames from individual camera scenes, the more picture information is lost. It appears less like continuous movement and more like robotic motion.

As each sample of signal is processed and sent to the recorder it is given a 'tag', which the system uses as an identification. The process of recording and tagging samples of each camera very quickly is referred to as encoding. When the tape is replayed the tags from each frame are 'read' by the system. The system can then identify frames from each camera and select only the ones required for viewing.

The frames selected can then be processed and displayed on the monitor. Those from other cameras will be ignored. This process is called 'decoding'.

Most multiplex systems also offer multi-camera displays within the system. This means that all or a number of cameras are displayed together, by splitting the screen into sections or quadrants.

Monitor outputs

Multiplexing systems normally have one control monitor for control and review. Some systems also offer a second monitor output known as the spot monitor. This operates independently of the multiplexer encoding and decoding system and does not display any of the data that appears on the main review monitor.

The spot monitor is linked to a separate video switcher within the unit which operates as a normal desk top switcher. Because multiplex systems sample the inputs from all the cameras, it is necessary to feed all the camera inputs directly into the unit.

Unlike normal video switchers, though, the output to the monitor is separate from the output to the video recorder.

This is because of the way the system works, encoding and decoding the signal on to, and from, the video tape. The video recorder becomes an integral part of the multiplex system, the video output from the unit is fed directly to the review monitor.

Multiplexing types

There are two types of multiplex systems in use: the single duplex unit and the full duplex unit.

The single duplex enables the operator to encode information to tape and review by decoding. These two functions cannot be carried out at the same time – it is necessary to stop recording before playing back.

The full duplex unit has a second channel built into it which allows the operator to record and encode on one channel while reviewing and decoding through the second channel. This means that continuous recording can be maintained. This requires a second video recorder and review monitor (Fig 1).

Modern multiplex systems are referred to as 'software driven' systems. This means that the system operates on pre-programmed commands held in the system's memory.

Some of these are already present in the system. Others have to be programmed in when the unit is installed. These will include setting site information such as number of cameras, types of video recorder, time/date etc. It is important to follow the manufacturer's set-up procedure carefully as many of the problems that occur are due to incorrect setting up or user error.

Another important element in these systems is user training. User error accounts for a large number of problems, so it is important to ensure operators are properly trained and competent.

Signal transmission

So far we have looked at the camera and how it converts the light from the scene into an electronic signal. We have also looked at how the monitor converts the signal back into a picture and how multiplexing is achieved.

Now we shall consider signal transmission. This is the method used to transport the video signal from the camera to the control point or monitor. The most obvious method of signal transmission is cable, and we shall look at this in some detail. There are also other methods employed in CCTV applications and we shall look at those in general use.

The most common form of cable used is coaxial cable, often shortened to 'coax' (see factfile).

We shall continue with signal transmission in our next session.

Coaxial cable

Coaxial cable (Fig 3) consists of two conductors: a centre core, which is insulated by a non-conductive or dielectric material, and an outer screening, which surrounds the inner core. A final covering of PVC or similar material protects the conductors from damage. While the basic principles appear simple, there are a number of factors that must be considered to achieve effective and efficient signal transmission. There are many different qualities and types of coax and it is important to ensure the most suitable one is selected. Inner conductor This should normally be a solid core of copper wire for general applications. Because the wire is solid, damage through flexing and bending is possible. Cables with stranded wire conductors, which are more flexible, are also available. For the majority of cable runs it is advisable to use cable with a solid core as this has low internal resistance resulting in lower signal losses. The insulator The insulating material that surrounds the inner core is also important and there are several types available. They vary in their ability to handle moisture and this must be considered if the cable is to be exposed. Some types of low loss cable use an air spaced insulator. Outer shield The outer shield protects the inner core from unwanted external signals. It is essential that the screening capability of the cable is high and therefore the construction of the screen is important. There are two basic types: copper braid and aluminium foil. The latter is designed for use in modulated signal transmissions and is not suitable for standard CCTV use as it may distort the composite video signal. For CCTV applications, pure copper braid cable is the most suitable. There are different qualities of braid depending on the number of wire strands used. A tightly woven braid will have a larger number of wire strands than a loose woven braid and will have greater shield qualities. Screening helps protect the signal from corruption by electromagnetic interference (EMI), which is created by heavy duty power cables, motors, trains and communication equipment etc. Outer cover The majority of cable is covered in a PVC casing which protects the cable against damage and water ingress. Care should be taken when handling, as a broken casing can lead to water seeping in to the cable which will reduce effectiveness and lead to long term damage.

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