12.3: Black Frame Insertion Technique with Charge ShareTaesung Kim, Jaeho Oh, Byunghyuk Shin, Sunkwang HongLCD BusinessSAMSUNG ELECTRONICS CO., LTD#200, Myeongam-Ri, Tangjeong-Myen, Asan-City, Chungnam-Do, KOREA, 336-841 AbstractBlack Frame Insertion (BFI) has been widely used to improvethe moving image quality of LCDs. The easiest way of doing thisis to control back light brightness, but the slow response ofCCFL lamps makes it difficult to be implemented. Anothermethod for this is to insert black frame regularly, but in thiscase, the refresh rate has to be doubled so that the systemfrequency goes up, the charging time of pixel is shorten, andthus the brightness is decreased accordingly. Therefore in thiswork, a new driving scheme using charge share feature ofcolumn driver has been proposed and it allows us to implementBFI algorithm without increasing refresh rate, and also theinsertion ratio is very flexible so that we don’t need to make itfixed at 50%.1. IntroductionThe moving picture quality of LCD is inferior to CRT due to thecharacteristics of hold-type display. Each pixel of CRT displayemits the light by the collision of electrons from electron guns,and new data on each frame updates the image on the screen.However, the emission time of each pixel is so short comparedto the frame time that the pixel stays at OFF state for the mostframe time. This is so called impulsive emission display. Thus aclear image is formed on our eyes every time even though theobject is moving.However, LCD has fundamental differences with CRT in twoaspects. First, due to the slow response of liquid crystal, theresidual image of moving object persists over two frames andthis is seen as edge blurring. Secondly, because the pixel voltageis held constantly until it is refreshed by next frame image, themoving object is perceived as an integration on the retina of oureyes causing motion blur which makes the moving edgesobscure [1]. As the response time of LCD gets faster, the firstphenomenon is supposed to be improved, but the second motionblur is not improved theoretically.The difference between CRT and LCD like this is represented inFigure 1. One method to improve this kind of motion blur is toput black data between frames for a certain period of timedividing them into separate images. The most straightforwardway to do this is to double the frame frequency and make everyodd or even frames black. But this can’t be a good solutionbecause the system clock frequency is doubled and the chargingtime becomes half. Therefore in this paper, we proposed a newmethod that can insert black frame without increasing framerate.2. Black Data InsertionWe can think of two ways of inserting black frame betweennormal data frames according to duty ratio. The first one is tomake the time ratio between black frame and data frame as 1:1,and in second method, the time ratio of inserted frame to normalframe could be arbitrary (1:x). Let’s find out pros and cons ofthese two methods.2.1 Fixed 50% Duty RatioThe easiest and simplest way is to make both of black frametime and data frame time 1:1 as in Figure 2. That is, the original60Hz frame is sampled at double speed becoming 120Hz, andevery odd or even frame is displayed as black so that 50% dutyratio can be achieved easily. As well known in eye perceptiontheory, if periodic black data is inserted between normal frames,the residual image of previous frame on our eyes can beeliminated achieving clear edge of moving object [3].However, the data processing frequency becomes twiceresulting higher power consumption, higher EMI emission, andsystem level cost increase. Besides, due to no light emissionduring the black frame time, the brightness level is decreasedconsiderably, and flickering artifact due to dramatic luminancechange is perceived becoming problematic in real application.2.2 Variable Duty RatioSince the brightness loss during black frame proportional to theinsertion time is quite inherent and fundamental, in order tominimize the brightness loss black insertion time also needs tobe minimized. But in case of fixed 50% duty ratio, the ratiochange is not possible at all. Therefore we need more flexibleway of duty ratio modification with great deal of flexibility, asshown in Figure 2(b), so called variable duty ratio method.In this figure, 25% of black insertion time is described, and asthe time of data frame gets longer compared to black frame, itcomes close to the original 60Hz hold type situation and thusthe black insertion effect gets smaller.Figure 1. Emission characteristics: CRT vs. LCD [2]152 • EuroDisplay 2005(a) 50% duty ratio(b) Variable duty ratioFigure 2. Black frame insertionIn contrast, the longer the black frame time becomes, thedimmer the screen will be, but moving picture quality becomesbetter since liquid crystal behaves more likely impulsive.Therefore it is the most important to find out the optimum trade-off relationship between brightness and black insertion effect.2.3 Double Scan DrivingThe only way that we can freely change the black frame ratio isto scan the same frame twice. That is, two scan pulses arescanning down the screen, and the first pulse asserts black datainto the pixel and, after a while, the following pulse updates thesame pixel with next frame image. The black insertion ratio willbe decided according to the distance between these two pulsesso that the duty ratio can easily be changed. Figure 3 representsthis concept.Two scan pulses, Set and Reset, exist on the same frame, andthe reset pulse RESETS the pixel voltage previously chargedwith previous frame data, and the set pulse UPDATES the pixelwith next frame data. Looking at from the pixel point of view,the time that pixel stays in the black state (25% in this case)shall decide the insertion ratio in connection with the full frametime.3. Time Multiplexing Pixel ChargingDouble scan driving can be operated at 60Hz frame rate andhence the frame frequency is not increased at all, which is ahuge benefit in terms of pixel charging time. But instead, onehorizontal line time needs to be time multiplexed between blackdata and normal image data. That is, normal CD output data hasto charge the pixels whose gate electrodes are connected to theSET pulse, and the black data voltage needs to be stored in thepixels being driven by reset scan pulse.By the way, the black data that is to be charged during someportion of horizontal line time can be created by about 3different ways. Let’s find out what they are.(a) 120Hz frame rate(b) Double scan drivingFigure 3. Double scan driving3.1 Digital Black Data TransmissionThe simplest way of black data generation is to make it indigital manner within timing controller. The timing controller,TCON in short, transmits black data just like normal image datain specific period of time within horizontal line time, as shownin Figure 4. But in this case, the data needs to be transmitted indouble speed and also column driver IC also has to run at thesame speed, which creates a lot of unfavorable effects from thesystem point of view. Row scan pulse with 1/2H pulse widthwill be asserted twice with certain period of time gap, one forblack and one for data. No doubt that the charging time isdecreased to 1/2H, but since black data is created and suppliedfrom TCON, arbitrary gray can be inserted, which allows us tobe flexible in dealing with insertion gray level.Figure 4. Digital black data transmission3.2 Analog Black Data TransmissionOne possible way to send the black data without increasingbandwidth requirement is to send direct analog voltage tocolumn driver. As in Figure 5, data input frequency has nochange, but instead the analog voltage that corresponds to blackEuroDisplay 2005 • 153data needs to be supplied to CD side, and one signal (Bout) is needed to specify when the black voltage is driven to the panel within 1H time period. So to speak, CD outputs the analog black voltage level to the panel only when Bout pulse is high, and normal image data will be driven for the rest of the line time. And it is possible to input a different gray voltage by changing the analog voltage level being supplied to CD.Figure 5. Analog black data3.3 Black Data by Charge ShareFigure 6. Charge share circuitIn most column driver ICs being recently used, there is a feature called “charge share” and what that means will be presented from now. Figure 6 shows four consecutive column driver output stages, and when LCD panel is driven, SW1s are closed and SW2s are turned off charging the data line capacitance, C DL , with target voltage.Due to the AC driving requirement of liquid crystal, 1st and 3rd outputs are driven to positive polarity, and 2nd and 4th voltages become negative. After one horizontal line time, each capacitors will be charged as +V1, -V2, +V3, -V4 respectively. At this time, the amount of charge stored at each capacitor is as follows:Q 1=C DL V 1, Q 2=C DL (-V 2), Q 3=C DL V 3, Q 4=C DL (-V 4)On the other hand, the opposite polarity voltage needs to be applied in order to charge pixels with next line data. By connecting all output nodes together by closing SW2 for a while, the existing charge can be re-used to form the commonnode voltage. At this time, finally induced voltage, V final , can be calculated as follows:Q TOT = Q 1+ Q 2+ Q 3+ Q 4=C DL (V 1-V 2+V 3-V 4) = C DL V finalBy the way in the actual application, if thousands of data lines are tied together, the final voltage approaches to zero volt on average since positive and negative voltages are distributed half and half. In this case, some amount of power can be saved because local charge redistribution made the common node voltage as zero without any current supply from outside. In addition, output transition time can also be improved since the voltage swing starts from common node voltage, rather than opposite polarity voltage. Although the final voltage became zero due to the assumption of positive/negative polarity, the common node voltage settles down around the common electrode voltage(Vcom) in the real application. In this paper, a new method has been proposed that can use charge share voltage level as black data making use of the fact that the black data voltage is around the Vcom voltage level in VA mode LCD. The beauty of this method is that the implementation is very simple because most of column driver ICs commercially available recent days have the charge share feature for better power consumption performance so that any modification is not needed at all. As represented in Figure 7, since CD output voltage comes to close to Vcom level during charge share operation, black data can be applied to pixels by asserting row scan pulse for reset operation. In this case, the allowed charging time for normal data is shorten accordingly, but inadequate charging time is the problem that is commonly applied to any black insertion methods.When we think of the fact that the charging time for black data can relatively be shorter than the one for normal image because the inserted black frame is less sensitive, it will be somewhat waste of charging time if we use the same frame time for both black and normal data frame. However, the proposed method can maximize the charging time efficiency by optimizing respective frames separately. The overall operation timing diagram of this method is shown in Figure 8. As stated earlier, multiplexing time ratio between black and data within 1H can be compromised so as to increase the charging time for normal image data. According to our evaluation results, 3:7 (black vs. data) ratio shows the best performance in terms of charging time black and data.Figure 7. Use of charge share level as a black data154 • EuroDisplay 2005Figure 8. Overall operation timing diagram4. ResultsThe proposed method has been applied to 40” WXGA (1366x768) TV panel with PVA pixel structure and the moving image quality turned out to be dramatically improved. Since we implemented the black frame insertion by making use of charge share time in order not to increase refresh rate and system frequency, and simply changed the row driver timing sequence, there are lots of benefits in terms of charging time. Besides, we’re able to identify the optimum black insertion ratio since the black ratio can be set by simply changing the location of vertical start pulses of black and data. Figure 9 shows the moving image improvement by this method on the 40” PVA panel, and the image is being moved to right direction with 8 pixels/frame speed. The texture of the fabric is clearly visible when BFI is used as in (b).(a) Without black insertion(b) 30% black insertionFigure 9. Experiment results5. ConclusionsSince the charge share feature has been supported by most column drivers recently available, the proposed algorithm allows us to implement the black frame insertion with existing column drivers. A simple modification of driving timing will be adequate to implement the BFI function and it provides a lot of benefits. Also, if the refresh rate has to be doubled to insert black frames, all digital control signals and clock frequencies need to run a lot faster making actual implementation very hard. Besides, because pixel charging time comes down to half, the image quality deteriorates and the yield loss becomes significant. The black insertion ratio has to be fixed as 50%, and thus the brightness is decreased accordingly so that lots of other means have to be added to compensate the luminance loss. However, the proposed method allows us to insert black data without any frequency increase using a minor change of input timing. In addition, the black insertion ratio can be programmed with great flexibility so that optimum black insertion ratio can be easily identified. Therefore, this method can be the best choice of doing black frame insertion to improve a moving image quality without any cost impact.6. References1. Kazuo Sekiya, Hajime Nakamura, “Eye-Trace Integration Effect on The perception of Moving Pictures and A New Possibility for Reducing Blur on Hold-Type Displays,” SID02, DIGEST2. Taiichiro Kurita, “Moving Picture Quality Improvement for Hold-type AM-LCDs,” SID01, DIGEST3. Taesung Kim, et. Al, “An Optimized Boost Table Measurement Method for Response Time Acceleration in LCD,” SID04, DIGESTEuroDisplay 2005 • 155。
