Sample Chapter: 5 Monitors
Another important and often overlooked part of a CCTV system is the monitor. Ultimately the picture taken by the camera and the lens is displayed on the monitor. The monitor’s performance and adjustment will have an affect on the picture seen by the system operator.
In the same way that cameras, being analogue devices, have adjustments that enable the best picture quality to be obtained so monitors, also being analogue devices, have settings and adjustments that enable the best picture to be displayed. If the controls on the monitor are not correctly set then, similarly, the money spent on expensive high performance cameras, lenses and control equipment will be a waste because the picture displayed on the monitor will not do justice to the rest of the system. Consequently, it is vital to understand the principles of the normal monitor controls, their effect on picture quality and the correct way to set the controls properly.
Monitors are available in different screen sizes. The reason for this is that the size of the monitor depends on the viewing distance. If the incorrect size or position of a monitor is used then at best the monitor will be awkward and unpleasant to use; at worst the picture will be too small to differentiate detail or so large that the picture appears grainy and low quality.
In this chapter the principles of operation of monochrome and colour monitors will be explained in a simplified way, leading to the principles and effects of their controls. The correct procedures used to set the controls to obtain the best picture quality will be described. Finally, the principles of choosing the correct number, size and positioning of monitors will be discussed so as to get the maximum from this normally undervalued part of CCTV systems.
The Principles of Monochrome Monitor Operation
Apart from the use of transistors, integrated circuits and other solid state devices in the circuits of monitors the major part of the monitor, the television or Cathode Ray Tube (CRT), has remained essentially unchanged since the first TV monitors were developed.
As shown in Diagram 5.1 the CRT consists of a glass tube with all the air removed. An electron gun at the back of the CRT (a special material that when heated “boils off” electrons) generates a stream of electrons. These are attracted to the front screen at very high speed by a high voltage of several thousand volts. The inside of the screen is coated with a special phosphor that glows when struck by the electron beam, the stronger the beam the brighter the spot generated.
Scanning coils around the neck of the tube generate a magnetic field. The magnetic field affects the position of the striking point of the beam on the screen. By changing the voltage on the scanning coils the striking point of the beam can be scanned across the screen of the CRT to create a series of lines; when the beam moves back across the screen, during the retrace, the beam is turned off so that only the line and not the retrace is visible. By selecting the correct wave shape and frequency the same 625 line frame and 50 fields per second patterns as produced by the camera can be re-created For descriptions of fields, frames and the way that the camera produces these see Chapters’ two and three
The video signal is used to control the strength of the beam. The brightness of the beam at any point along a given line will be proportional to the level of the video signal. This is consequently proportional to the light intensity at that point on the image sensor of the camera. In this way the picture captured by the camera can be recreated on the screen of the monitor and observed by the system operator.
In a basic monitor the video signal input enters the monitor and is terminated in a seventy-five ohm load. This matches the output impedance of the camera and the coaxial cable (see Chapter three). A sync separator separates the video signal and sync pulses. The sync pulses are used to synchronise the line oscillator of the monitor to the line oscillator of the camera being viewed. The line oscillator and field oscillator respectively control the scanning coils that scan the electron beam into 625 lines. Field sync pulses control the scanning coils to produce 50 fields. The horizontal and vertical hold controls adjust the frequency of the line oscillator. Consequently, these can be used to compensate for differences in the sync pulse frequencies coming from the camera.
A high voltage generator is used to accelerate the electron beam. The strength of the beam is controlled by the output of an amplifier. The input of the amplifier is the video signal. In this way, the level of the video signal controls the brightness at any point on the screen. The brightness control sets the basic level of the beam and therefore the general brightness of the picture. The contrast control controls the amplification or gain of the amplifier. The greater the contrast the greater is the effect of the video signal on the brightness. At low contrast, the picture will appear grey and uninteresting. At excessive contrast, the blacks and whites in the picture are very harsh and the picture is unpleasant to view. At the correct brightness and contrast levels, the picture will appear natural with many shades of grey. The DC Restoration affects the overall voltage level of the video signal. Sometimes this is needed because the voltage is modified as it passes through capacitors in the circuits of cameras and control equipment. With the DC restoration turned off there will be a grey “raster” when no video is input to the monitor. With the DC restoration turned on the screen will be completely black when no video is input.
Principles of Colour Monitor Operation
A colour monitor works in basically the same way as a monochrome monitor except that there are three electron guns. These three guns are for the three primary colours, red, green, and blue. The guns are aligned to the mask on the phosphor screen. If a TV screen is examined closely, it can be seen that it is a matrix of very fine red, green and blue dots. This is why the resolution of colour monitors is typically lower than monochrome monitors.
A combination of all three dots is needed to generate white compared with a single dot for a monochrome monitor. This means that for the same number of pixels the ability to resolve black and white lines may be up to three times less on a colour monitor. When the beam from the correct gun strikes a spot or pixel on the corresponding mask then the pixel glows red, green, or blue. As previously explained in Chapter two, combinations of these three basic colours can be used to form any colour in the spectrum. The firing of the guns in combination by the colour composite video signal recreates the colour picture viewed by the camera.
After sync separation the combined chrominance and luminance signals are processed by decoder and amplifier circuits. These are divided into separate signals to control the strength of the red, blue and green electron guns. Besides the normal brightness and contrast controls there is also a colour control that affects the general chrominance of the picture. With the control wound to minimum, the image will be monochrome. When the control is turned to maximum the colours will be very saturated and will normally be too unpleasant to view.
Usually a composite colour video input is provided but on some monitors a Y-C or Super VHS input will be provided. Alternatively, an input is provided where all three colour signals are brought in separately. This is known as an RGB (red, green, blue) input. The advantage of either Y-C or RGB inputs is that there is no filtering as associated with colour composite video. The bandwidth available is higher, and consequently higher resolution is available if the Y-C or RGB inputs are used. That is, provided of course that Y-C or RGB has been used throughout the system.
Understanding monitor performance specifications
As with cameras, the vertical resolution of a monitor is the number of black to white transitions or lines that can be distinguished from the top to the bottom of the picture. In addition, as with cameras the limiting factor is the 575 lines that make up the picture. The figure for resolution that is normally given in monitor data sheets is, as for cameras, the horizontal resolution. That is to say, the number of black to white transitions or lines that can be resolved along one horizontal line of the picture.
The major difference between resolution performance figures for monitors and resolution for CCD cameras is that the figure for monitors is given for the centre of the picture. This is where the resolution is highest.
The reason for this is that the picture is made by 625 horizontal lines produced by the scanning coils using a magnetic field to drive the beam of electrons across the phosphor screen. However, it is very difficult to get a magnetic field to have an even or linear effect across the entire surface of the screen. At the edges of the screen, the magnetic field tends to be non-linear and both the horizontal and vertical lines seen on the screen will appear bent. The electron beam also tends to defocus towards the edges. This reduces the ability to distinguish fine lines at the corners and sides of the screen and reduces resolution at these areas. For example, a monitor with a resolution at the centre of 600 lines might only have a resolution of 400 lines at the corners. This is a very important point to remember in choosing a monitor and in positioning a camera on the screen to see the most detail. The object to be viewed must be placed in the centre of the screen to get the sharpest picture.
The problems of non-linearity became worse with the advent of flatter and squarer tubes, because the scanning beam, which is linear had to travel further to the edges of the screen than it did to the centre. This problem was is overcome with a compensation circuit called ‘S’ correction. This causes the beam, now non-linear to move slower towards the edge and faster in the centre.
Monochrome monitor horizontal resolution is normally quite high, between 750 and 800 lines for a nine-inch monitor. The reason is because the coating of phosphor on the inside of the screen is continuous and the spot size is determined by the electron beam focus. Consequently, in monochrome systems the monitor is not the limiting factor for the resolution of the system. The resolution tends to decrease slightly as the monitor size increases because it is more difficult to manufacture large TV tubes with a fine phosphor coating.
In colour monitors, however, because there are three spots to make each point, red, green and blue, the resolution is very much lower typically 330 to 350 lines. The highest resolution that is being achieved at this time is about 450 lines. This is assuming that the Y-C input of the monitor is used. That, of course, has the proviso that all the other parts of the system are Y-C and have the same or higher resolution figures.
Bandwidth is also linked to resolution (see Chapter 3 and the section on camera resolution) The greater the bandwidth the higher the possible resolution of the monitor and the sharper the pictures will be. For a 750-line monitor the bandwidth might typically be about 10MHz.
This article is an extract from chapter 5 of 'The Principles & Practice of CCTV' which is recognised as the benchmark for CCTV installation in the UK.