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Renesas Electronics America - Knowledgebase

What is tech_081gl?

Latest Updated:07/01/2006


What is tech_081gl?


Most things which exist in the world have a stable shape or form that always appears as the same ("static"). In reality, the human eye is simply not fast enough to follow things that change at high speeds. We generally cannot recognize the unnaturalness, even if something changes every faster than 70 Hz.

[Dynamic operation]
Dynamic operation (dynamic display), which is used in lighting such as fluorescent and displays such as televisions, exploits this characteristic.
In the area of the 50 Hz frequency of commercial power supplies, fluorescent lights flash repeatedly at a speed of 100 Hz, which is the double. As you can see in the pattern diagram below, this is because the light is illuminated around the + and - peaks, and extinguished in the vicinity of 0 V. The human eye can see it as continuous illumination because it cannot recognize such a fast change.

Coffee Break
People's perception differs. The senses are also affected by age. Thus, some people may be very sensitive to the flickering of fluorescent lights. Manufacturers have tried to solve this problem by lengthening the persistence time of the fluorescent material. There are also products that feature a firefly-like afterglow for a short time after the light is turned off.
A more recent arrival on the market is an inverter light, which doesn't operate with the frequency of the commercial power supply, but rather flashes at a high frequency, performing switching operations at a frequency of 20 to 50 kHz. Since the inverter light flashes at the frequency above the visible frequency range, there is no flickering or delayed illumination as with conventional fluorescent lights.

[Dynamic operation used in displays]
In CRT TVs, the picture is displayed by an electron beam moving across fluorescent material on the screen surface in a horizontal direction (known as a scanning line). In standard definition (SD) TV broadcasts, half a frame (one frame consists of 525 scanning lines) is displayed per 1/60 of a second (half a frame is known as a field; one complete frame therefore consists of one even field and one odd field).

One point is projected for an instant by the electron beam (the projected point is highlighted in red in the figure above), but since the fluorescent material continues to illuminate for a short time after it (shown in pink in the figure above), we cannot observe any flickering. (If the display is photographed by a camera or filmed on video, the flickering (dark areas) is noticeable.)
In LCD televisions, unlike the CRT style of one-by-one point (dot) display, multiple dots in a row are displayed simultaneously. A picture is displayed on the screen by changing the vertical order in which these dot rows are displayed ("dynamic" display). Although it would require about 320,000 signal lines to statically display a screen with 640 horizontal dots and 480 vertical lines, this screen can be displayed dynamically in line units with only 1120 (640 + 480) signal lines.

Another kind of display method is a segment display, used for displaying prefixed images, such as in a calculator. Unlike the dot method used in TVs, the segment display method controls the display through a combination of segment signals and signals that are used for digits. The example on the left below shows the display in a unit of every digit. In this example, 7 signal lines are used to operate the segment and 4 signal lines are used to select the digit, making a total of 11 signal lines used. In the example on the right, two digits are displayed simultaneously, so 14 signal lines are required for segment operation.

[Dynamic display control]
Control of the segment display in the example above is shown in the timing chart below. First, while outputting the segment display data for the first digit to the segment signal, the first digit selection signal is output. This results in the display of one digit. Next, in order to display the second digit, the first digit selection signal is cut off and the data of the segment signal is changed to that of the second digit. The second digit selection signal is then output, resulting in the display of the second digit. When the digit is switched in this way, there is a period in which no selection signal is output. This period exists to prevent blurring of the display, and is known as a blanking period. This blanking period is required in LED or fluorescent displays, but not in LCDs because the display speed in an LCD is too slow to track.

This kind of display, in which the order of digits displayed is switched every fixed period of time, is known as time-division display. Increasing the number of divisions reduces the number of signal lines, but also shortens the display period. This means that the difference between the segments that are being displayed and those that are not being displayed (the contrast) is low. To avoid this, a large current has to be applied to brighten the display as much as possible.

Coffee Break
 The display timing described above works as a concept, but caution is required when dealing with actual waveforms. In LEDs, if the logic of the segment signal is made positive, the digit selection signal (common signal) logic has to be made negative by using a cathode common type signal. In other words, the LED is lit by current flowing from the segment signal to the common signal (figure on the right).
 In fluorescent lights, the segment (the section that lights up) is the anode (positive electrode). The grid electrode is the digit signal, and current flows between the cathode (negative electrode) and the segment. In order for current to flow, the anode and the grid electrode must both be positive against the negative cathode. When both of these are positive, electrons are output from the cathode's heater and flow to the segment electrode, where they light up.
In LCDs (liquid crystal display), if voltage is applied between the electrodes, the liquid crystal array twists, changing the way light passes through. Unlike LEDs, where there is no current flowing in the reverse direction, in LCDs, this change occurs in the same way regardless of whether the polarity of the voltage is changed, so care is required in the non-selection state. Consequently, in addition to full-amplitude voltage, an intermediate voltage is also applied. When displaying something, full-amplitude voltage is applied, and when not displaying anything, intermediate amplitude voltage is applied. Two voltages are therefore insufficient to drive an LCD, and it is difficult to drive an LCD using a normal port.
Voltage must also be applied in both the forward and reverse directions. An LCD driving waveform is shown in the example below. The red sections in the differential voltage chart indicate the period of displaying something.
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