Construction
Small liquid crystal displays as used in calculators and other devices have direct driven image elements—a voltage can be applied across one segment without interfering with other segments of the display. This is impractical for a large display with a large number of picture elements (pixels), since it would require millions of connections—top and bottom connections for each one of the three colors (red, green and blue) of every pixel. To avoid this issue, the pixels are addressed in rows and columns which reduce the connection count from millions to thousands. If all the pixels in one row are driven with a positive voltage and all the pixels in one column are driven with a negative voltage, then the pixel at the intersection has the largest applied voltage and is switched. The problem with this solution is that all the pixels in the same column see a fraction of the applied voltage as do all the pixels in the same row, so although they are not switched completely, they do tend to darken. The solution to the problem is to supply each pixel with its own transistor switch which allows each pixel to be individually controlled. The low leakage current of the transistor prevents the voltage applied to the pixel from leaking away between refreshes to the display image. Each pixel is a small capacitor with a layer of insulating liquid crystal sandwiched between transparent conductive ITO layers.
The circuit layout of a TFT-LCD is very similar to that of a DRAM memory. However, rather than fabricating the transistors from silicon formed into a crystalline wafer, they are made from a thin film of silicon deposited on a glass panel. Transistors take up only a small fraction of the area of each pixel; the rest of the silicon film is etched away to allow light to pass through.
The silicon layer for TFT-LCDs is typically deposited using the PECVD process from a silane gas precursor to produce an amorphous silicon film. Polycrystalline silicon (frequently LTPS, low-temperature poly-Si) is sometimes used in displays requiring higher TFT performance. Examples include high-resolution displays, high-frequency displays or displays where performing some data processing on the display itself is desirable. Amorphous silicon-based TFTs have the lowest performance, polycrystalline silicon TFTs have higher performance (notably mobility), and single-crystal silicon transistors are the best performers.
IPS (in-plane switching) was developed by Hitachi in 1996 to improve on the poor viewing angles and color reproduction of TN panels. Though color reproduction approaches that of CRTs, the dynamic range is lower but this was improved over the years. Fringe Field Switching is a technique used to improve viewing angle and transmittance on IPS displays.[3] IPS technology is widely used in panel sizes of monitor 20"~30" and LCD TV 17"~52".
| Hitachi IPS evolving technology [4] | |||||
|---|---|---|---|---|---|
| Name | Nickname | Year | Advantage | Transmittance / Contrast ratio | Remarks |
| Super TFT | IPS | 1996 | Wide viewing angle | 100 / 100 Base level | Most panels also support true 8-bit per channel color. These improvements came at the cost of a slower response time, initially about 50ms. IPS panels were also extremely expensive. |
| Super-IPS | S-IPS | 1998 | Color shift free | 100 / 137 | IPS has since been superseded by S-IPS (Super-IPS, Hitachi in 1998), which has all the benefits of IPS technology with the addition of improved pixel refresh timing. |
| Advanced Super-IPS | AS-IPS | 2002 | High transmittance | 130 / 250 | AS-IPS, also developed by Hitachi in 2002, improves substantially on the contrast ratio of traditional S-IPS panels to the point where they are second only to some S-PVAs. |
| IPS-Provectus | IPS-Pro | 2004- | High contrast ratio | 137 / 313 | Currently the latest panel from IPS Alpha Technology where contrast ratio are able to match PVA and ASV respectively, no glowing at the angle and wider color gamut. Matsushita will become the major shareholder after acquire Hitachi displays as of Mar 31,09. [5] |
| LG IPS evolving technology | |||||
|---|---|---|---|---|---|
| Name | Nickname | Year | Remarks | ||
| Super-IPS | S-IPS | 2001 | LG.Philips remain as one of the main manufacturers of S-IPS based panels based on Hitachi Super-IPS. | ||
| Advanced Super-IPS | AS-IPS | 2005 | Increasing contrast ratio with better color gamut. | ||
| Horizontal IPS | H-IPS | 2007 | It improves the contrast ratio by twisting the electrode plane layout. The H-IPS panel is used in the NEC LCD2490WUXi and LCD2690WUXi, Mitsubishi RDT261W, HP LP2475w, Planar PX2611W,[6] and Apple's newest Aluminum 24" iMac. H-IPS up close.
| ||
| Enhanced IPS | E-IPS | 2009 | A low-cost AS-IPS display by lowering the aperture ratio to increased transmittance which result some glowing at the angle, lower color gamut and contrast ratio. | ||
ASV
ASV (Advanced Super View), also called Axially Symmetric Vertical Alignment was developed by Sharp, it is a VA mode where LC molecules orient perpendicular to the substrates in the off state. The bottom sub-pixel has continuously covered electrodes, while the upper one has a smaller area electrode in the center of the subpixel.
When the field is on, the LC molecules start to tilt towards the center of the sub-pixels because of the electric field; As a result, a continuous pinwheel alignment (CPA) is formed; the azimuthal angle rotates 360 degrees continuously resulting in a excellent viewing angle. The ASV mode is also called CPA mode.
Electrical interface
External consumer display devices like a TFT LCD mostly use an analog VGA connection, while newer, more expensive models mostly feature a digital interface like DVI, HDMI, or DisplayPort. Inside external display devices there is a controller board that will convert CVBS, VGA, DVI, HDMI etc. Into digital RGB at the native resolution of the display panel. In a laptop the graphics chip will directly produce a signal suitable for connection to the built-in TFT display. A control mechanism for the backlight is usually included on the same controller board.
The low level interface of STN, DSTN, or TFT display panels use either single ended TTL 5V signal for older displays or TTL 3.3V for slightly newer displays that transmits Pixel clock, Horizontal sync, Vertical sync, Digital red, Digital green, Digital blue in parallel. Some models also feature input/display enable, horizontal scan direction and vertical scan direction signals.
New and large (>15") TFT displays often use LVDS or TMDS signaling that transmits the same contents as the parallel interface (Hsync, Vsync, RGB) but will put control and RGB bits into a number of serial transmission lines synchronized to a clock at 1/3 of the data bitrate. Usually with 3 data signals and one clock line. Transmitting 3x7 bits for one clock cycle giving 18-bpp. An optional 4th signal enables 24-bpp.
Backlight intensity is usually controlled by varying a few volts DC, or generating a PWM signal, adjusting a potentiometer or simple fixed. This in turn controls an high-voltage (1,3 kV) DC-AC inverter or an matrix of LEDs.
The bare display panel will only accept a digital video signal at the resolution determined by the panel pixel matrix designed at manufacture. Some screen panels will ignore colour LSB bits to present a consistent interface (8bit->6bit/colour).
The reason why laptop displays can't be reused directly with an ordinary computer graphics card or as a television, is mainly because it lacks a hardware rescaler (often using some discrete cosine transform) that can resize the image to fit the native resolution of the display panel. With analogue signals like VGA the display controller also needs to perform a highspeed analog to digital conversion. With digital input signals like DVI or HDMI some simple bitstuffing is needed before feeding it to the rescaler if input resolution doesn't match the display panel resolution. For CVBS or "TV" usage a tuner and colour decode from a quadrature amplitude modulation (QAM) to Luminance (Y), Blue-Y (U), Red-Y (V) representation which in turn is transformed into Red, Green Blue is needed.
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