Frequently Asked Questions
Q. Why are my 15" CRT and my 15" LCD different in size?
A. A 15” CRT has a shadow mask and as a result, the active area is compromised because you cannot write to the edge of the tube. Alternatively, the specifications for LCDs are given as the pixel viewing area, which is the true corner-to-corner diagonal. As a result a larger area is displayed by the equivalent size LCD although it takes up less space!
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Q.
Why can I see the picture elements in my LCD more clearly than on my CRT?
A.
Each pixel area in an LCD has a well-defined etched boundary where the electron beam of a CRT has a Gaussian shape that has much more gradual edges.
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Q. Is there such a thing as a flexible LCD?
A. Yes there is a flexible Passive Matrix LCD which Sharp has offered since 1998. For Active Matrix or highly multiplexed displays, these are not yet available because the processing temperatures for TFT arrays are too high for currently available plastic substrates.
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Q. What is the difference between an A/V TFT and a Digital TFT?
A. The difference is in the signal interface. A/V panels expect RGB, NTSC, or PAL format signals. The Digital TFT expects digital graphic signals designed for TFTs.
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Q. How can I control the backlight intensity?
A. In the case of CCFTs, there are techniques for using frame rate modulation, driving them for a smaller percentage of the time at full voltage. They do create a small buzz when you do this which is sometimes objectionable, and the range of control is limited. For more dynamic range, LED backlights are easily controlled from full-on to full-off.
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Q. How can I drive 6 bits of color from a controller that outputs 8 bits of color?
A. Converting to six bits from eight bits selection is straightforward. It simply entails using the most significant bits and discarding the least significant as shown in the figure below.
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In most LCDs, there is a digital-to-analog converter built-in that takes the digital data and translates it to the analog signal that actually gets sent to the display. The data comes in on a 8-bit or 6-bit data path (all zeros equate to black and all 1s equate to white). If you have a controller that can output 8 data bits, but a display that can only accept 6, you simply drop the low-order bits and adjust the least significant bit to give a smooth progression.
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Q. My LCD does not look as good with a touch panel. Why not?
A. A resistive touch panel is typically made of glass or plastic. Between the touch panel and the LCD it is common practice to have an air gap. Each air-glass or air-plastic interface has about 4% reflection. The effect is not only loss of light, but a loss of contrast.
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Q. How does Sharp Advanced Super View compare to SuperIPS?
A. Both techniques are used to achieve wide viewing cones. Advanced Super View has the best color uniformity on the diagonals as well as superior response times and contrast ratios. The off-axis gamma curves for SuperIPS is slightly better than Advanced Super View. The patents for SuperIPS are held by Hitachi in Japan.
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Q. I see noise on my signal. Can this be reduced?
A. An LCD is basically like an integrated circuit. For a VGA display, you are sending a 60 MHz signal on the connection cable. This connection is a transmission line which can cause problems if it is not properly terminated at each end. To minimize noise, you should design in a termination resistor and capacitor that can be tuned to the cable characteristics. This will allow you to soften the edges and reduce the noise. On an LCD, you can actually see the effects of noise. If the screen tears sideways, there is generally a problem with noise on the horizontal sync signal causing irregular resets to the left-hand side of the screen. If you see a vertical rolling effect, it is generally noise on the vertical sync line or a missing sync signal.
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Q. What happens if I power up the display but do not clock in data for several seconds or longer?
A. There are two things about power sequencing that are very important. You must be careful about how you apply power. Typically, you must turn on the Vcc supply voltage, then within 10-20 milliseconds apply all of the data signals including the data bits, sync signals and clocks. If you apply the data signals before you apply power, you get into a latch-up condition. The LCD looks like a CMOS device that can draw high current if a signal is applied to the gate before Vcc is turned on. Most Sharp LCDs have ESD protection diodes. This makes the potential of a high current path to ground from the signal lines if Vcc is not applied before the data signals.
A second problem can occur if you apply power and wait too long before you apply data. This can be a more subtle error. The display needs an AC signal to function properly. If only a DC signal is applied, damage can occur. Generally the damage occurs slowly and is a function of duration, temperature and other conditions, but the effect is cumulative and failures will eventually occur. If you follow the timing guidelines in the specifications for applying the signals, you should have no problems.
You can also have a latch-up problem if you wait too long to apply the Sync signals. Some displays have a built-in voltage doubler. If you don't apply the clocks quickly enough, you can create another latch-up condition in the display. A power sequencing latch-up problem almost always shows up as an all white or all black display. Then, if you turn it off and back on it may start working. Latch-up conditions are almost always recoverable. The only time when permanent damage occurs is when excessive power flows through the ESD diodes on the column drivers.
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Q. How do I control or reduce EMI noise on my display and interface to ensure passing UL Certification requirements?
A. Most noise radiation from the display comes from the dot clock - a synchronization clock that runs at very high speeds. On a VGA display it runs at 25 MHz. For a UXGA display it can approach 140 MHz. The use of small R-C circuits or inductance on the clock lines can soften the edges and lower the radiation. Shielding of the connection cable is another method to reduce noise, but it tends to be more expensive. Another technique for larger displays is to use LVDS or TMDS transmission systems where the TTL signals are transmitted as low voltage differential signals. This is the technique used currently in most laptops.
It is important to ground the display frame to a good AC signal ground. This helps contain the noise generated by the display as well as to become a shield for any electronics located behind the display. Sometimes a metal foil is placed behind a display to help shield other noise sources. SHARP displays, by themselves, will generally pass FCC class B tests.
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Q. What techniques are available to increase viewing angle?
A. The traditional approach by display manufacturers is to add a compensation film to the front of a display. Compensation film works fairly well, but only gets to about 140 x 120 degrees. The simplest alternative solution is IPS from Hitachi which uses lateral electric fields applied within the cell structure. IPS requires higher driving voltages and LCD materials that limit the temperature range of operation. A second alternative is PVA technology from Samsung. This is also a technique for applying a lateral field. It produces fine looking displays, but still is difficult to manufacture in volume. The MVA process from Fujitsu is a third alternative.
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Q. What is the Vcom adjustment?
A. For displays with a low level interface where the customer must supply many of the interface components, there is an adjustment called Vcom. This adjusts for small differences in the electrical characteristics of the glass. The drive circuitry must be tuned to match these characteristics for each individual display. The customer design must allow for this manual adjustment which is done by observing the display and minimizing optical patterns or distortions.
For Sharp displays with a higher level interface built-in, the Vcom adjustment is made at the factory and rarely requires any field adjustment.
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