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Dithering and Frame Rate Control (FRC)
Interpolation / Scaling of Resolution
Aspect Ratio Control and 1:1 Pixel MappingBurn-In of Images / Image Persistence
Dead Pixels
On Screen Display (OSD)
DVI Cables
HDCP Support
Pivot / Rotate / TiltCableComp
Vista Compatibility (DDC/CI Compliance)
Dithering and Frame Rate Control (FRC)
TN Film screens are traditionally more economical than other technologies. In fact, they only display 64 red, 64 blue and 64 green shades. The maximum amount of colours achievable from liquid crystal rotation alone is 262 144. In order to reach 16 million colours and above, panel manufacturers commonly use two technologies: Dithering and Frame Rate Control (FRC). These terms are often interchanged, but strictly can mean different things.
Spaital Dithering - The dithering method involves assigning appropriate color values from the available color palette to close-by pixels in such a way that it gives the impression of a new color tone which otherwise could not have been created at all. In doing so, there complex mappings according to which the ground colors are mutually assigned, otherwise it could result in color noise / dithering noise. Dithering can be used to allow 6-Bit panels, like TN Film, to show 16.2 million perceived colours. This can however sometimes be detectable to the user, and can result in chessboard like patterns being visible in some cases.
Frame Rate Control / Temporal Dithering - The other method is Frame-Rate-Control (FRC), also referred to sometimes as temporal dithering. This works by combining four colour frames as a sequence in time, resulting in perceived mixture. In basic terms, it involves flashing between two colour tones rapidly to give the impression of a third tone, not normally available in the palette. This allows a total of 16.2 reproducible million colors. Thanks to Frame-Rate-Control, TN panel monitors have come pretty close to matching the colors and image quality of VA or IPS panel technology, but there are a number of FRC algorithms which vary in their effectiveness. Sometimes, a twinkling artefact can be seen, particularly in darker shades, which is a side affect of such technologies. Some TN film panels are now quoted as being 16.7 million colours, and this is down to new processes allowing these panels to offer a better colour depth compared with older TN panels. See this article for more information.
Interpolation / Scaling of Resolution
While TFT screens are best run at their native resolutions, it is possible to run them at lower resolutions if need be. This interpolation of the resolution, from below the native, leads to some loss in image clarity and sharpness as the image is stretched across pixels. In office use this can be a problem and can look quite poor, but in gaming, it is generally not so much of a problem. The ability of a TFT to interpolate the image depends on the particular panel used, and some manufacturers have been able to improve the ability of their panels to run outside the native resolution. Generally though it is not recommended to run outside the native resolution on a TFT if you can help it.
Where screens are having to handle lower resolutions, the image will normally be interpolated and stretched to fill the screen completely. This can cause particular problems with widescreen format monitors since a 4:3 resolution would look very strange stretched to fill a 16:10 screen. One way manufacturers can get round this is with the use of aspect ratio retention methods including what is commonly referred to as 1:1 pixel mapping.
Aspect Ratio Control and 1:1 Pixel Mapping
This feature is particularly important on widescreen format monitors and refers to the hardware ability to maintain an aspect ratio of a source image. For instance, if you tried to play a 4:3 aspect game on a 16:10 format display, the image would normally be stretched to fill the screen, stretching the aspect ratio horizontally. However, if the hardware is capable of maintaining the aspect ratio, the screen can display the source in it's normal 4:3 ratio, and will add black borders along each side.
This aspect ratio retention can be achieved in two ways. The most reliable and easy to use is through the hardware (monitor) itself and normally involves the availability of preset modes in the OSD. There are normally options to:
"fill" the screen ignoring any aspect ratio differences between source and display. a 4:3 source would be stretched to fill the screen regardless.
"aspect", to maintain whatever aspect ratio is used in the source, but interpolate the image to fill as much of the screen as possible. This would normally result in black borders along the right and left hand sides on a WS format screen displaying a 4:3 source.
"1:1" used to literally only use the exact number of pixels specified in the source resolution. For instance a 1024 x 768 source resolution would be displayed on a 1920 x 1200 resolution monitor, only using the pixels required and would not be interpolated or stretched. This would result in black borders on all sides of the image. This would be like using a smaller screen within the larger screen and is handy for those wanting to run games at lower resolutions but without losing image quality through interpolation.
The other option for aspect ratio retention is through software. NVIDIA's display drivers for instance can achieve similar settings to above, when using the digital interface. However, results can be a little more variable and difficult to achieve.
See the Widescreen Monitor Guide for more information
Backlight
leakage refers to the problem some screens exhibit where in a darkly lit room,
and with a dark image on the screen, you can clearly see areas, particularly
around the edges, where the backlight shines through. This is a problem with the
manufacturing stage and quality of the monitor build. A lot of panels will show slightly uneven backlighting,
with perhaps a little light noticeable at the edges of the screen. This is
nothing really to worry about, but panel uniformity can vary from one screen to
the next. Of particular interest here is the Dell 2005FPW which has been sadly
plagued by excessive backlight leakage since it’s release.
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If backlight bleeding is excessive, a return to the manufacturer is normally easy enough.
Banding of
colours across gradients, click for larger version
Banding is an issue which you can sometimes spot on a monitor, and involves blocking and gradation of colours to a considerable level. This is most evident when running some colour gradient programs, to show a gradual change in shade in a certain tone. A certain degree of gradation in this gradient can be expected from many monitors, despite the fact that in an ideal world, the gradient would be smooth and all transitions would be transparent. However, in some cases the gradation is more noticeable and results in what is popularly referred to as "banding".
Some users think that striped gradients are due to the use of 18-bit matrixes instead of 24-bit ones, but this is not exactly true. The lower color depth of the matrix may indeed lead to stripes in gradients if the Frame Rate Control is poorly implemented (this is the technology that emulates 16 million colors while the matrix itself is only capable of displaying 262,000), but the real reason is usually different. Before outputting the image on the screen, the monitor performs a series of calculations and transformations: color temperature correction, gamma compensation, contrast correction, etc. If the accuracy of those calculations is low, you see striped gradients. The matrix’s color depth has nothing to do with it. Even an “honest” 24-bit matrix cannot guarantee that the monitor will correctly process the data before sending them to the matrix.
Some monitors have made banding rather infamous and so many potential buyers now cite this as an important test of a screen, and something which can really seperate the good from the bad. A lot of this is quite exagerated however, with far too much concern about even slight gradation across colour gradients. The early releases of the Dell 2xx7 series were a classic example of where colour banding became a concern. The early releases did show some pretty bad banding, which was promptly fixed by Dell with firmware upgrades. However, it has resulted in many users critisising displays for even slight gradation, and not really considering whether it is really an issue in real use. For the majority of users, it would probably not be an issue in practice, and you'd probably be hard pressed to see any adverse affects of this issue in anything other than colour gradient tests. I would advise caution about the talk of banding on displays, and consider whether there is really as much of an issue as some people make out.
The Screen Door effect is so called because sometimes it is possible to clearly see the individual pixels in a panel and the gaps between them. This is quite rare, but can be distracting if you are using a TFT up close.
This phenomenon is not widely acknowledged across review sites, but is often the subject of discussion in enthusiast forums. The premise is that an LCD display shows a degree of lag between the image being sent to the screen from the graphics card, and what is actually shown on the panel. This is most easily noticeable when comparing a TFT and CRT side by side in clone mode, and you can see that the image on the TFT is lagging a little behind the CRT in some tests. The degree of this input lag varies from one screen to another. It doesn't seem to be linked to anything in particular, but it's presence is thought to be linked most likely to screen electronics and components. Other factors such as panel technology and interface used don't seem to exhibit a particular pattern of input lag.
The best way to record this accurately is using a stopwatch application running in clone mode on both the TFT and CRT. Using a high shutter speed on a digital camera allows you to capture images of this running. You can then record the delay between what is shown on the CRT and what is shown on the TFT. This can vary from generally low < 10ms lag on average, to sometimes much higher average lag of > 50ms. Below shows a graph of average input lag across several screens tested at TFT Central (top section), and also average input lag as recorded from other sources (bottom section) on some popular and well established models:
As you can see, input lag
really can vary from one screen to another. Some screens such as the Viewsonic
VX922 show minimal lag of 2ms, whereas others such as the Viewsonic VX2435WM
show up to 36ms on average.
In practice, input lag is unlikely to affect too many users. There is quite a
lot of fuss made about it on forums, but in reality I would doubt many people
will see any real issues on the majority of displays. Some professional gamers
who rely on being able to match their key presses and mouse movements with what
is shown on the screen might suffer in some cases, so it is something to be wary
of. Generally though, I would avoid worrying too much about this issue for most
average users.
The
following article at BeHardware is also an interesting read on the subject.
Cleartype was
introduced by Microsoft for use with LCD displays to make fonts more rounded and
less jagged. This is effectively a filter used to blur the fonts a little which
some people prefer the look of. This can vary from one TFT to another, and it is
easy enough to turn on and off to allow you to decide which you prefer.
Microsoft’s article about the Cleartype filter can be found here:
http://research.microsoft.com/~jplatt/cleartype/
Image burn in was traditionally a problem with CRT displays, where prolonged images on the screen could leave a ghost image behind after it has changed. This was a problem with older CRT displays and was the reason for the introduction of screen savers. With TFT’s this is not really a problem as the image cannot be burnt into the screen by the cathode ray gun, as the pixels all operate individually. Some older screens can very occasionally show some lasting imprint of an image if the same picture is left on the screen for long periods of time, but it is nothing permanent. This can often be easily solved by looking at some fast moving scenes or gaming. For the sake of electricity more than anything else though, it is probably easiest to use the power settings on your PC to turn the screen off when not in use.
One of the main concerns people have when buying a TFT relates to the problem of dead pixels. Pixels can sometimes be ‘dead’ (stuck on black or white). Sometimes the sub pixels which make up the pixel can be dead which leaves the pixel looking red, green or blue. Sometimes the sub pixels can be ‘lazy’ and with a bit of luck can come back to life.
Dead pixels / sub pixels defects are normally caused during the manufacturing stage, and it is very rare for a panel to generate a pixel fault at a later stage unless you have a tendency to prod the screen. Nowadays, manufacturing levels are very good and it is quite rare for a pixel to be ‘dead’, and you will see some manufacturers like Samsung and Viewsonic for instance, offering zero dead pixel policies. Dead sub pixels are still a problem, and the policy will not cover these in most cases. Dead pixels are not really considered a fault with a TFT monitor and you will need to consider this before purchasing. Refer to the manufacturer to find out what their dead pixel policy is.
The OSD refers to the On Screen Display available on nearly all TFT monitors. This allows the user to change settings ranging from brightness, contrast and colour levels (typically RGB) to more advanced features like aspect ratio and monitor preset features like Senseye and MagicTune for instance. One thing to note is that some features like contrast, phase and pixel clock are only available when using the VGA (analogue) interface and become greyed out when using the DVI (digital interface) as they are no longer required. Proper configuration of a monitor requires RGB levels to be altered and brightness and contrast to be set correctly. More advanced features are often accessible and modern OSD often offer a wealth of selections. Some OSD also offer factory menus and information about the screen or panel being used which can be particularly useful for the enthusiast.
DVI is the
first digital standard and supports a dual link mode, which allows resolutions
up to 2048 x 1536 and beyond. The DVI specification supports hot plug and play
display devices.
There are 3 main different configurations when it comes to DVI:
1. DVI-A is designed for analogue only connections
2. DVI-D is designed for digital signals only and is 24 pin
3. DVI-I (Integrated) is a single connector which is designed for both
digital and analog use, and is backward compatible with analog displays. DVI-I
is 29 pin
A DVI
> VGA Adapter
DVI-D Cable Connection
Most LCD monitors that support digital signal have DVD-D connectors. A DVI-D
cable connection is shown.
The cables: DVI-I single link configuration provides bandwidth sufficient for
res up to 1600 x 1200 and high speed transmission up to 4.95Gbps. DVI-I dual
link config can do 2048 x 1536 @ 9.9Gbps, this is the same for a DVI-D dual link
configuration.
Cable suppliers recommend: "Most customers should use a DVI-D to DVI-D cable.
We suggest that you do not order a DVI-I to DVI-I cable unless you are certain
that it will work for your application."
It is possible to get DVI – VGA converters. These will not offer any improvements over a standard analogue connection, as you are still going through a conversion from digital to analogue somewhere along the line.
HDCP's main target is to prevent transmission of non-encrypted high definition content. In basic terms source material (HD-DVD, BluRay, Next Gen Video Games etc) will be encrypted with HDCP protection. In order to view / use these sources you will need each step in the 'chain' to have support for this protection technique. This includes graphics cards in PC's, DVD players and ultimately (and where we are interested in), the monitor / display device. If all these steps do not have support for HDCP, any encrypted material will have problems playing, typically being reduced to a much lower resolution, showing "not supported" type messages, or not showing any image at all.
HDCP functions over digital interfaces only; and so DVI and HDMI ports are those most affected at present. For a monitor or TV to be truly HD compliant, it must offer HDCP compatibility. Many modern displays do offer this support, but bare in mind they would need digital interfaces to offer this. You may want to consider whether a monitor has HDCP support or not when making your decision, since it may well have an impact on your use in the future. While it was thought that HDCP would not be implemented for many years, it seems it may well be sooner than we had expected. See the below links for further information.
Many screens today have the added functionality of pivot, rotate or tilt functions, or sometimes more than one of these. Rotate can be handy for switching to portrait or landscape mode, and normally the screen comes coupled with software which automatically rotates your desktop on demand. This can also easily be achieved with most graphics card software. This can be handy for office and photo work in particular. Be careful of screens with limited tilt and pivot functions, as they might be restrictive when it comes to aligning them to your line of sight. The use of some features, particularly rotate, becomes a little questionable on the larger screens such as 24" models, but some may still find them useful and an attractive buying point.
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It is possible to detach the stand from some TFT models and instead mount them to a swinging arm on a wall or desk. If this is something you might wish to do, look for TFT’s which specify compatibility with VESA wall mounts. These can provide improved alignment of a monitor and easier use depending on your needs.
The main area to consider really here is the backlight. TFT’s are backlit using CCFL lamps, and these typically have a life expectancy of >40,000hrs. To give you an idea that would be 4.5yrs of the panel being on 24/7 or 9yrs with a more realistic 12/7. Basically by the time the TFT has had it’s better days, you will probably be wanting to buy a new one anyway.
NEC have introduced their "CableComp" technology which automatically compensates for signal quality differences caused by long cables and weak signals. The technology allows you to use cables of up to 20m and still support the 1600 x 1200 resolution over DVI. This is as compared with normal operation of about 7m. For 1280 x 1024 resolution on DVI you can use cables up to 30m in length, and for VGA / Analogue connections, up to 100m! This is achieved through the use of a booster and intermediate replicater, and does not lead to any delay either.
Vista Compatibility (DDC/CI Compliance)
One of the requirements for a Vista compatible monitor is DDC/CI (Display Data Channel Command Interface) compliance. This provides full bidirectional communication between host (PC) and display. Through DDC/CI support, user can use appropriate software to adjust the monitor instead of using the limited monitor control keys.
Colour reproduction is related to the ability of a panel to produce the colours desired. Typically, the colour reproduction is measured by reviewers and recorded on graphs similar to this:
This graph shows the difference between the desired color shade and the one actually displayed. Basically you are looking at how low the lines are down the Y-axis. As you move from point 0 to 100 on the X-axis, you are moving from black to white. The lower the lines are down the Y-axis, the better and the more reliably produced the colours are.
If DeltaE >3, the color displayed is significantly different from the theoretical one, meaning that the difference will be perceptible to the viewer.
If DeltaE <2, LaCie considers the calibration a success; there remains a slight difference, but it is barely undetectable.
If DeltaE < 1, the color fidelity is excellent.
The colour reproduction of monitors is generally very good nowadays. Black levels (on the left) have traditionally been a problematic area, especially for TN film panels, but advances have been made more recently. All in all, colour reproduction is very good nowadays for TFT monitors, even with TN film panels. If this were a panel being measured DeltaE is mostly < 2 so colours are quite well produced, but black production is quite poor.
The graph of Gamut represents the spectrum of colours that a panel can produce. It also represents the richness of colours displayed. The points of the triangle indicate the richness of colours producible by the panel and the nearer to the corners of the true green, blue and red, the better for that colour range. The overall range of producible colours is indicated by the size of the triangle, and the more area covered, the better.
This curve indicates the contrast value measured at a given brightness adjustment on the OSD. In theory, brightness and contrast are two independent parameters, and good contrast is a requirement regardless of the brightness adjustment. Unfortunately, such is not the case in practice.
The brightness adjustment is shown on the X-axis, contrast on the Y-axis. The contrast is expressed here as a percentage of the maximum value measured using the ANSI test protocol. As you can see from this example, the optimum brightness setting for this particular TFT would be around 50. This measurement of contrast stability is conducted by some reviewers.
Again, some reviewers like Tom’s Hardware Guide. It is useful for considering the uniformity of the panels lighting, and relates to backlight bleeding sometimes when uniformity is particularly bad. The screen is set at 50% brightness and 50% contrast and the uniformity of the lighting on a white image separated into 64 areas of equal size is measured. The brightest point is considered to be 100%, and the previously measured black value is considered 0%, with the other values obtained distributed between them. The diagrams like the one above help to show how uniformly lit the panels are.