TN Film, MVA, PVA and IPS ? Which one’s for you?
A Look at Modern LCD Display Specifications
As an introduction to this article, and before we get stuck into the other more complex elements such as panel technologies, it is important to consider how manufacturers record specifications in the modern market, and what to look for when considering the screen in your short list. Obviously, a fair part of what’s listed on a TFT’s spec sheet is often marketing, so always be wary of how figures are quoted and more crucially, how they are measured. The following Buyers FAQ may also be helpful if you are buying a screen for the first time. I’ll keep the following details simple to accomodate the first time buyer, but more information about LCD specifications is available here.
1. Response Times
The lower the response time, the better. This spec determines how quickly liquid crystals can re-orientate and how quickly they can respond to changing images. Modern screens almost always feature a response time quoted in "grey to grey" (G2G) and this figure represents the fastest transition recorded by the manufacturer within all the possible changes in grey shade (note: all LCD transitions are in grey to grey, since the colours are achieved using filters in front of the pixels). Traditionally, response time was measured at the black to white transition, which gave the fastest figures to quote in spec sheets. This was because before overdrive hit the market, the largest change that the liquid crystals would ever need to make was white to black, and so the highest voltage is applied to bring them into their new orientation. More recently, ‘overdrive’ / Response Time Compensation technologies were introduced to boost pixel response times further. This allowed manufacturers to apply this highest voltage to all transitions, and so G2G changes became the fastest, since the crystals did not need to rotate as much as a full black to white change, but still received the higher voltage anyway. I won’t go into too much detail about RTC here, but more information can be found on the following pages .
In theory, the faster the response time spec, the better. However, this still only represents the best case response time, and so might not truly indicate the panels overall performance. In addition to this, the quoted spec might not necessarily reflect true responsiveness in practice, but overall it can be used as a rough guide to the panels speed and how it might perform in fast moving images such as those during gaming.
2. Colour Accuracy
Colour accuracy can vary from one model to the next. It’s important to realise that by default, most screens will at best come with average colour accuracy, so calibrating your screen is a idea. Panels (as discussed later) are usually either 6-Bit with Frame Rate Control (a method used to produce 16.2 million viewable colours) or full 8-bit panels (16.7 million colours). The colour palette can have an impact on the screens ability to show certain shades and 6-bit +FRC panels can also show some artefacts from the FRC technology – a method of switching between like colours to produce a new shade (more info here ). However, colour depth does not necessarily affect colour accuracy, and modern 6-bit TN Film panels are actually very good in this regard with proper calibration of the display.
Out of the box accuracy can really vary, but with proper calibration, accuracy can be greatly improved. Calibration can be done at a basic level using software methods ( see here ), but to achieve decent calibration of colours, a hardware colorimeter would be necessary. There are several different hardware devices available in varying price ranges. These include more budget, but quite popular, models such as the Spyder 2 Express, and more advanced and highly regarded devices like the Gretag One-Eye Display and LaCie Blue Eye Pro for example. With proper hardware calibration, you can achieve some nice results from most modern panels. However, for professional displays, 8-bit colour depth and wide viewing angles are important. See more information later about panel technologies.
3. Brightness, Contrast Ratio and Black Depth
Brightness of a display is recorded in candella per metre squared (cd/m2), and in theory, the higher the value the better. Bear in mind that for comfortable use in normal lighting conditions, the recommended luminance of a display is 120 cd/m2, and so modern displays are often very bright. The higher values do mean an LCD display can show nice bright images and have a good variation in achievable brightness levels. The black depth of a display is also recorded in cd/m2, but almost never listed in manufacturer specifications. However, you can calculate this by considering the contrast ratio of a display. The contrast ratio shows the ratio between the brightest white, and the darkest black. The higher the contrast ratio figure, the better really. If you divide the quoted brightness value by the quoted contrast ratio, the figure given will give you an indication of the black depth of the panel. The lower the black depth the better, as this will ensure good deep blacks and the ability of the display to render dark shades reliably. Be wary of dynamic contrast ratios however, as these do not necessarily reflect true performance in all applications, and can show a difference between black depth / brightness under dynamic control and only in certain circumstances.
4. Viewing Angles
The viewing angle figures are perhaps the most difficult to trust on modern displays, and are often based on varying methods of recording the figures and lapse measurement techniques. The higher the viewing angle figures, the better. However, be wary of overly exagerated figures, for example TN Film panels are often too high on paper to reflect real life performance. Viewing angle figures are handy for indentifying what panel technology is used in many cases, but typically you will see figures of 160 / 160 (TN Film), 176 / 176 (MVA and PVA matrices) or 178/178 (IPS).
5. Movie Noise and Playback
When considering movie playback on an LCD display you need to take into account a few things. Firstly, viewing angles are important especially if you plan on watching the video in a group, or from different positions in your room. Black depth is also important as it ensures darker scenes are rendered well, and detail is distinguishable in dark shades. Finally, noise and artefacts can be evident in some cases. This can really vary depending on the source file (DVD, XVid, HD-DVD etc), graphics card (some have anti-noise technology) and also on the panel used. Some screens are better than others in terms of noise and overall movie playback as detailed later on.