Important factors to consider when evaluating an LCD monitor include resolution, viewable size, response time (sync rate), matrix type (passive or active), viewing angle, color support, brightness and contrast ratio, aspect ratio, and input ports (e.g. DVI or VGA).

Display resolution

One use of the term "Display resolution" applies to fixed-pixel-array displays such as flat-panel plasmas (PDPs), LCDs, front and rear projectors using LCD, DLP or similar technologies and is simply the physical number of columns and rows of pixels creating the display (eg, 852x480; 1368x768 etc). A consequence of having a fixed grid display is that for multiformat video inputs all displays need a "scaling-engine" (a digital video processor that includes a memory array) to match the incoming picture format to the display.

Response time

Response time is the amount of time a pixel in an LCD monitor takes to go from active (black) to inactive (white) and back to active (black) again. It is measured in milliseconds (ms). Lower numbers mean faster transitions and therefore fewer visible image artifacts.

Older monitors with long response times would create a smear or blur pattern around moving objects, making them unacceptable for moving video. High response times can be annoying to a viewer depending on the type of data being displayed and how rapidly the image is changing or moving. Many current LCDs monitor models have improved to the point that this is rarely seen.

A figure of 8 to 16 ms for rise and fall times is typical. Claimed 2 ms response times for LCD displays are beginning to be advertised, but measurements these low are usually taken using "grey-to-grey" transitions, instead of black to white. The response time given by the LCD manufacturer is often the fastest measured, and not the average or maximum response time (which may be significantly longer).

In comparison, a CRT displaying a picture with an update frequency of 60 to 80 Hz could be said to have a response time of 12.5 ms and upwards. However, as the picture is updated completely (and virtually instantly) each time the electron beam passes over the screen, CRTs do not have the same problems with smearing or ghosting. The same is true for plasma displays (however, both CRTs and plasma displays can have problems with flicker).

Matrix Type

Passive-matrix and active-matrix

LCDs with a small number of segments, such as those used in digital watches and pocket calculators, have a single electrical contact for each segment. An external dedicated circuit supplies an electric charge to control each segment. This display structure is unwieldy for more than a few display elements.

Small monochrome displays such as those found in personal organizers, or older laptop screens have a passive-matrix structure employing supertwist nematic (STN) or double-layer STN (DSTN) technology (DSTN corrects a color-shifting problem with STN). Each row or column of the display has a single electrical circuit. The pixels are addressed one at a time by row and column addresses. This type of display is called a passive matrix because the pixel must retain its state between refreshes without the benefit of a steady electrical charge. As the number of pixels (and, correspondingly, columns and rows) increases, this type of display becomes less feasible. Very slow response times and poor contrast are typical of passive-matrix LCDs.

High-resolution color displays such as modern LCD computer monitors and televisions use an active matrix structure. A matrix of thin-film transistors (TFTs) is added to the polarizing and color filters. Each pixel has its own dedicated transistor, allowing each column line to access one pixel. When a row line is activated, all of the column lines are connected t a row of pixels and the correct voltage is driven onto all of the column lines. The row line is then deactivated and the next row line is activated. All of the row lines are activated in sequence during a refresh operation. Active-matrix displays are much brighter and sharper than passive-matrix displays of the same size, and generally have quicker response times, producing much better images.

Contrast Ratio

The contrast ratio is a metric of a display system, defined as the ratio of the luminosity of the brightest and the darkest color the system is capable of producing. High contrast ratio is a desired aspect of any display, but with the various methods of measurement for a system or its part, remarkably different values can be measured of the same subject.

Manufacturers of display devices have traditionally favoured those methods of measurement that isolate the device from the system, whereas designers of practical display systems have more often taken the effect of the room into account. An ideal room would absorb all the light reflecting from a projection screen or emitted by a CRT, and the only light seen in the room would come from the display device. With such a room, the contrast ratio of the image would be the same as the device. Real rooms reflect some of the light back to the displayed image, lowering the contrast ratio seen in the image.

Moving from a system that displays a static motionless image to a system that displays a dynamic, changing picture slightly complicates the definition of the contrast ratio, because of the need to take into account the extra temporal dimension to the measuring process. Thus the ratio of the luminosity of the brightest and the darkest color the system is capable of producing simultaneously at any instant of time is called static contrast ratio, while the ratio of the luminosity of the brightest and the darkest color the system is capable of producing over time is called dynamic contrast ratio.

Digital Visual Interface (DVI)

The Digital Visual Interface (DVI) is a video interface standard designed to maximize the visual quality of digital display devices such as flat panel LCD computer displays and digital projectors. It was developed by an industry consortium, the Digital Display Working Group (DDWG).

The DVI interface uses a digital protocol in which the desired brightness of pixels is transmitted as binary data. When the display is driven at its native resolution, all it has to do is read each number and apply that brightness to the appropriate pixel. In this way, each pixel in the output buffer of the source device corresponds directly to one pixel in the display device, whereas with an analog signal the appearance of each pixel may be affected by its adjacent pixels as well as by electrical noise and other forms of analog distortion.

Video Graphics Array (VGA)

Video Graphics Array (VGA) is an analog computer display standard first marketed in 1987 by IBM. While it has been obsolete for some time, it was the last graphical standard that the majority of manufacturers decided to follow, making it the lowest common denominator that all PC graphics hardware supports prior to a device-specific driver being loaded. For example, the Microsoft Windows splash screen appears while the machine is still operating in VGA mode, which is the reason that this screen always appears in reduced resolution and color depth.

The term VGA is often used to refer to a resolution of 640×480, regardless of the hardware that produces the picture. It may also refer to the 15-pin D-subminiature VGA connector which is still widely used to carry analog video signals of all resolutions.

Src Wikipedia

Related Articles