Monitoring the situation

When we look at the technology behind a television system we see a lot that is redundant or strictly no longer necessary. This is usually because of advances that have been made over the years. However, there are standards that have been laid down and these must be adhered to, or the system is in danger of failing. This case is well illustrated with relation to display devices.

In the early days of television, a camera scanning system employing interlaced scanning was developed to save on transmission bandwidth. A very clever idea at the time! This meant that the displays used to look at these camera signals also had to operate in an interlaced fashion.

Similarly the display devices were all based on cathode ray tube displays, and however you look at it these devices were just big electronic valves that could show pictures at one end. This simple, if not earth shattering statement, introduces another little wrinkle into the system: thermionic valves do not have a linear response.

That means simply that if you double the amount of signal going in, you do not necessarily get double the amount of light coming out. The relationship between the amount of signal in and the amount of light out is referred to as the Gamma law of the tube. This fundamental property of the display device therefore needs to be accounted for in the processing of the signals from the camera side.

There is no point in having a camera that has one sort of response and displaying it on a device that has a totally different one. In television, we really do want to see a true and correct representation of what the camera sees. Therefore all cameras introduce an intentional distortion of the signal knowing that this will be compensated for when that signal is finally produced on the CRT.

This non-linear transfer characteristic is embedded in television standards and must be observed or else the displayed pictures will not accurately reflect the scene being viewed.

So why is this important? Well, when it comes to good old standard CRT displays all is well, and nothing has really changed over the years.

However, the two things to remember about LCD (liquid crystal display) and Plasma displays are that they are neither interlaced nor non-linear in their operation. This also applies to projection systems. So what we have with the newer displays is a fundamental difference in the principles on which they operate, but yet they still have to work within a system whose origins are over 70 years old.

Implications

Let's just consider the implications for a moment. All of the new panels operate on a progressive system of scan as opposed to the old interlace system. This in itself is no bad thing, and a lot of the new High Definition services now being offered will be produced using progressive scanning throughout.

However, most of the distribution around any CCTV installation is done using PAL composite signals. All composite PAL colour signals are based on an interlaced structure. That means that somewhere within a flat panel display must be the necessary circuitry to convert the interlaced signal into a progressive one, and there will be compromises on how good this process will be, usually based around cost.

To perform interlace to progressive well costs a lot of money, and to do it cheaply, well, doesn't. Keep in mind that every flat panel display needs to perform this function and so in the interests of cost, most manufacturers will do just as much as they think they have to and no more. The other major consideration for flat panel displays is how accurately they can portray the images. This comes back to the issue of gamma mentioned earlier. In general CRT displays are equally good, or bad, depending on your viewpoint, at displaying both dark and light areas of the picture. The result is a reasonable degree of consistency of both grey scale and resolution across all levels of brightness.

The gamma law of LCDs is, at best, variable. Each manufacturer includes some form of circuitry to try to compensate for this, but they again are faced with the constraints of cost. A quick fix is easier and cheaper than a highly complex attempt to compensate electronically for the limitations of the panel.

Limitations

These limitations will probably be noticed most in the dark areas of a scene. When dealing with bright low contrast images flat panel displays look great, but when used to display night time scenes with high contrast lighting they will struggle to show detail in the dark areas. For critical monitoring they do still leave something to be desired. It is important to understand a little of the underlying principles to appreciate just what they can and can't do.

In any security system it is important to understand the parameters and priorities of the operator and choose the best fit of equipment and technology to meet those needs. Unfortunately there are always compromises and other considerations to be taken into account, which are not always under our own control. One of the most recent of these is a new European Union directive placing a ban on the provision of new electrical and electronic equipment containing more than agreed levels of lead, cadmium, mercury, and other chemicals.

While at first glance this may seem a good thing, this RoHS directive has many unforeseen side effects. The one most important to the television industry is that a number of manufacturers are now saying that they cannot produce CRT monitors because of the amount of lead in the glass. So although for some applications you may want to specify CRT monitors, you may find your choice of vendor restricted.

To illustrate, G1 recently completed a total refit of West Quay Shopping Centre's control room in Southampton. The control room was previously dominated by a large console taking up more than half of what is a big control area. After much consultation with the client, and technical furniture designers Gresham Wood Technical Furniture & Design, G1 came up with a design that would include both CRT and TFT monitors. The total available space was carefully mapped and planned. The brief was to provide a monitor wall of CRTs. We had demonstrated to the client the benefits and there was agreement that there had to be 'Glass' monitors in the main monitor wall. Twenty 21-inch monitors from JVC were chosen as they are one of the companies who have committed to continuing the supply of tube based monitors.

Flat panel displays will continue to become better and ever more popular. This fact can be confirmed by a stroll around any of the large electrical retailers. However, the ability of a tube monitor to display superb images in terms of resolution and grey scale should not be underestimated, and on occasions should be actively encouraged and embraced.