Technology and Special Features of LCD Monitors in the Security and Broadcast Sectors
The rapid development of LCD monitors in recent years in conjunction with the use of LEDs for background lighting more and more offsets the disadvantages over the good old cathode ray tubes. And the organic light emitting diode – OLED – is a technology now preparing the monitor market for another revolution. We take a look at the current state of technology and explain the special features of security and broadcast monitors.
It was only four years ago when we discussed the advantages and disadvantages of TFT and CRT monitors in VIEW – an article that was subsequently also taken up and published by trade journals, and which is still ranked very highly by Google & Co. when the corresponding search terms are entered. In the meantime, CRT monitors have fully disappeared from our website and you will find only LCD monitors in our used equipment shop. The cathode ray tube has had its day, even though right up to the very end it still had its fans who found it very difficult to finally turn their back on the bulky boxes. Not only has the technology of LCD monitors experienced rapid development – the trend from analogue to digital or IP-based technology also puts the general importance of professional monitors in a new light. Which technological trends can be identified? What is the significance of the inclusion of the word “professional” in the security and broadcast sectors? Is a LCD monitor from electrical discount stores not “professional” enough?
Operation of liquid crystal displays
LCD stands for “Liquid Crystal Display”. The characteristics of liquid crystals have been researched since the start of the 20th century, and as long ago as in 1968 the first functional LCD was rolled out in the USA by the Radio Corporation of America, although it had little in common with the liquid crystal displays we know today. In simple terms, operation of a LCD can be compared to a venetian blind: light comes through when the slats are positioned horizontally, when the slats are vertical the light remains outside. In the LCD this function is carried out by the crystals floating in the liquid that can change their alignment to either allow light through or block it. The light comes from the background lighting emitted by fluorescent tubes or LEDs (more about this later). Every pixel consists of three chambers are filled by liquid crystals, and a colour filter (red, green, blue) is specified for each chamber. Alignment filters, that enclose the layer of liquid crystals with voltage applied to the crystals, build up an electric field that aligns the crystals in the same way as the slats of a venetian blind. Depending on the crystal alignment, light from the background lighting is able or unable to pass through the polarisation filter before its moves through colour filter. The additive mixture of the light creates the required colour on the glass plate of the LCD.
Background lighting with CCFLs
Normal cold-cathode fluorescent lamps (CCFLs) used for background lighting are arranged in a tube shape next to each other or one below the other. Their light strikes a light-conducting material that has the purpose of distributing the light as uniformly as possible before it passes through the polarisation, alignment and colour filters mentioned above. In practice though, it is frequently extremely difficult to achieve uniform light distribution and areas with different brightness occur, normally where the CCFL tubes are positioned. Another disadvantage is the low contrast values: due to the construction design it is only possible to dim all the light tubes at the same time. For scenes containing dark as well as light regions all CCFLs must therefore be stepped up – black becomes grey, and details are consequently lost in the dark regions. Every science fiction fan has already noticed this when the space ship ventures into black outer space that has a somewhat grey hue. Advantages of LCDs with CCFLs are the light intensity of the lamps that can be used in brightly lit rooms without problems, as well as the now inexpensive production that results in low market prices.
Background lighting with LEDs
A fundamentally new technology is not described with LED monitors – they are liquid crystal displays with the technology described above, but the background lighting is provided by LEDs instead of CCFLs. With these LED backlight monitors distinction is drawn between edge-lit and direct-lit as well as full LED. For the edge principle, also referred to as edge background lighting, the LEDs are arranged only at the edges of the monitor and light is distributed over the screen by a conductor system. The advantage of this arrangement is the extremely flat monitor construction and low power consumption; though the disadvantages of LCD monitors with CCFLs remain – non-uniform illumination as well as low contrast values. With the full LED principle, i.e. direct LED background illumination, the LEDs are placed over the entire surface at the rear of the screen with up to 3,000 LED lamps according to the monitor size. These LEDs are grouped into individual clusters whose brightness is controlled individually by local dimming. Even though this control does not have pixel precision, panel illumination is very much more uniform and contrast values are significantly increased. Further advantages over illumination with CCFLs are the lower power consumption and the nearly double service life of approximately 100,000 hours – a factor that can be crucial particularly in the professional sector.
OLED – Organic Light Emitting Diodes
The latest technology in flat screens is the organic light emitting diode, abbreviated as OLED. As compared to traditional electronics based on inorganic substances such as copper or silicon, OLEDs are based on carbon. OLED screens get by without background lighting and emit coloured light themselves, so that a colour filter is not required. The result is a much higher contrast (ten times higher than for current LCDs) and a very much better colour display. Further advantages are the extremely small screen depth, which is even less than that of LCDs with the edge principle, and response times exceeding the values of LCDs by 1,000 times: they are even as low as 0.001 milliseconds on some devices. Problematic at the moment is the low service life of OLEDs that lies at only a few thousand hours, which is quickly reached especially in continuous operation.
Features of professional broadcast monitors
The most important feature of professional broadcast monitors is – in addition to the very high picture quality and outstanding contrast values – the capability of colour calibration as stipulated by the European Broadcasting Union (EBU). But also functions, such as integrated waveform display and a vector stop which allows checking of colour and picture quality without external measuring instruments for instance, are now indispensable for outdoor shooting when things sometimes have to go fast. And which consumer TFT monitor already features SDI inputs with up to 3 Gb/s support? Of course this is in addition to Composite, S-Video, Component, RGB and HD/SD/SDI inputs. Furthermore, Class 1 broadcast monitors also feature binding, true-colour playback of different colour spaces such as SMPTE-C, REC 709, DCI-P3 as well as EBU and User Defined that allow absolutely realistic display of colours ensuring that the delivered end product has a standardised colouration which is a true reflection of reality.
Security monitors: developed for 24/7
Whilst the benefits of professional broadcast monitors are therefore very apparent, they are more difficult to recognise in the video surveillance sector. Composite video inputs can be identified on the rear of the monitor, but what else? And whoever at all needs extra video inputs with IP-based monitoring when they are travelling – because connection to the PC is made over DVI or HDMI. So off to the electrical discount store to buy a cheap TFT monitor from a consumer brand? In principle this is of course possible, and no difference will be discerned in the display of video pictures – not at the beginning anyway. Though it should be remembered that these monitors were produced to meet requirements other than those in video surveillance and, for instance, even a gamer who loses himself in various game worlds has to go to sleep at some time and switch off the monitor therefore giving it a break. These panels were not designed for round-the-clock use, and they will therefore fail sooner rather than later – a risk that should be avoided in the sensitive field of security applications. TFT monitors developed for CCTV applications have been designed for 24/7 monitoring, their panels are more robust with a longer service life, and they don’t need a break. Additionally, they frequently come with important additional functions that, for instance, prevent image burn-in – an important feature particularly for scenes with little movement. As opposed to CRT monitors, on which the screen image really could be burnt into the phosphor layer of the cathode ray tube, this phenomenon occurs on LCD monitors from a static charge on parts of the screen when the picture does not change over a long time, which is frequently the case in surveillance situations. Professional monitors for video surveillance therefore frequently feature suitable anti burn-in functions. Furthermore, the picture quality of professional CCTV monitors is often additionally improved by a digital 3D comb filter decoder that prevents colour streaks and distorted colour borders by adding forward and back images to the input and therefore giving better separation to the interleaved colour and black/white signals.
Development in the monitor segment is moving ahead at a tremendous pace, and in particular monitors with full direct LED background lighting now achieve a picture quality that should appease even the last fans of CRT monitors. And the outlook for OLED technology, which is already used in some monitors from our supplier TVlogic, shows that picture quality of flat screens will be even higher than that of cathode ray tube monitors in the future. Manufacturers forecast that this technology will have asserted itself by 2016 when OLED monitors will be priced at the same level of LCDs today. In the broadcast sector the need for professional monitors is indisputable. In the security sector, on the other hand, there is a great temptation to use cheap consumer monitors. Though it should be borne in mind that the higher price of a monitor developed especially for video surveillance is fully justified. The load that panels have to withstand in continuous operation is enormous – and consumer LCDs should not be relied upon for this.