JPH0659645A - Driving method for liquid crystal electro-optical device - Google Patents

Abstract

PURPOSE:To suppress fluctuation of effective voltage due to a display signal by adopting a predetermined period of the latter half in one scanning period of each picture element set in a single frame period as a selection period of this picture element. CONSTITUTION:A liquid crystal is interposed between a pair of transparent insulative substrates and in one of the transparent insulative substrates a plurality of switching elements each composed of a non-linear element or an active element and picture element electrodes electrically connected to one end of the switching element are formed, while in the other transparent insulative substrate a counter electrode is formed. In the case where this liquid crystal panel is driven, a predetermined period of the latter half in one scanning period of each picture element set in a single frame period is adopted as a selection period of this picture element. Namely, a scanning period Ts is divided into halves for AC driving, a single scanning period Tse1 adopting each half scanning period selection level is further divided into a plurality of periods (fine scanning periods 10), and only a part of the fine scanning periods 10 is adopted as a selection level.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION Liquid crystal electro-optical devices have become extremely popular in recent years as display devices for optical switches, calculators and electronic watches. Furthermore, there is a demand for use as a display device for small personal computers and the like. However, the current liquid crystal material has a maximum duty ratio of about 1/30 that can be driven by multiple presses, and lacks a large amount of information display capability. As a display method devised from such a background, (1) Address method using a non-linear element as a switching element-Varistor address-Metal-insulator-metal- (MIM) element address-Diode address-Discharge tube address ( 2) Address method using active element as switching element-Thin film transistor address-MOS transistor address-Triac address (3) Light / heat / writing method-Laser thermal writing-Photoconductor writing (4) Dual frequency addressing method, etc. The development of a liquid crystal display device having a large amount of information has been earnestly pursued.

The present invention relates to (1) a liquid crystal electro-optical device using a nonlinear characteristic element and a switching element, and (2) a driving method of a liquid crystal electro-optical apparatus using an active element as a switching element.

More specifically, the present invention relates to a matiplex drive for suppressing the fluctuation of the effective voltage applied to the pixel of the liquid crystal electro-optical device.

[0004]

2. Description of the Related Art FIG. 1 shows a typical M of non-linear elements.
The current-voltage characteristic of the IM element is shown. Parista, P
Also in the case of a reverse series-connected diode utilizing the breakdown voltage of the N junction, etc., it has a non-linear current-voltage characteristic similar to FIG. Further, any element having non-linear characteristics such as high resistance in the low voltage region and low resistance in the high voltage region as shown in FIG. 1 can be adopted as the switching element.

It is known that when a liquid crystal electro-optical device is constructed by using these non-linear elements, multiple-digit multiplex driving can be performed as compared with general multi-press driving. This is understood as follows.

FIG. 2 is an equivalent circuit diagram for one pixel electrode.
It is composed of a liquid crystal capacitance C ^LC (1), a resistance ^RL _C (2), an equivalent capacitance C ^NL (3) of a non-linear element, and an equivalent resistance R ^NL (4). R ^NL (4) has low resistance at high voltage and high resistance at low voltage depending on the voltage applied to the non-linear element. Now, consider adding a liquid crystal drive signal to the terminals of the equivalent circuit.

FIGS. 3A to 3C show an example of the 1/50 duty, 1/5 bias method. FIG. 3A shows a matrix-shaped display pixel including scan electrodes 5-1 to 5-10 and signal electrodes 6-1 to 6-50. Each scanning electrode 5
Scan signals SCAN1 to SCA are included in -1 to 5-50, respectively.
N50 is applied. Moreover, each signal electrode 6-1 to 6-50
The display signals SIG1 to SIG50 are applied to each. Here, the display pixels (M, N) corresponding to the scan electrodes 5-M and the signal electrodes 6-N are turned on, and the other display pixels (plural) on the column of the signal electrodes 6-N are turned off. Consider a case.
The waveform of the scanning signal in this case is shown in FIG. 3 (B), and the waveform of the display signal is shown in FIG. 3 (C). In these figures, ts is a scanning period in which a signal is applied to the entire display pixel, and tsel in FIG. 7B is a selection period of the scanning signal SCANM in which the scanning electrode 5-M is selected. According to FIG. 6C, the signal electrode 6-N has the lighting level VON during the selection period tsel, and has the non-lighting level VOFF during the period of the scanning signals other than SCANM. Therefore, the voltage applied to the display pixel (M, N) is V (M, N) = SCANM-, as shown in FIG.
Given by SIGN. The solid line in FIG. 4A is the waveform V (M, N) of the applied voltage when the display pixel (M, N) is at the lighting level in this way, and the solid line in FIG. It is the voltage waveform VNL of the non-linear element, and the solid line in FIG. 7C shows the voltage waveform VLC applied to the liquid crystal layer. That is, they have the following relationship.

V (M, N) = VNL + VLC Further, the broken lines shown in (a), (b) and (c) of the same figure show the case where the display pixel (M, N) is at the non-lighting level. .

Next, based on FIGS. 5 (a), 5 (b) and 5 (c), the operation concept of the non-linear element and the liquid crystal layer will be schematically described. FIG. 5A shows applied voltage VNL vs. current ± of the non-linear element.
Shows the relationship, and the non-linear element has a low resistance in the area of 7,
In the region of No. 8, the non-linear element has high resistance.

FIG. 1 (b) shows a non-linear element resistor RNL4.
Shows the current flow i when the resistance is low (almost 0), and FIG. 6C shows the current flow when the resistance 4 is high (almost infinite).

First, as shown in FIG. 1B, when the non-linear element enters the low resistance region, most of the driving voltage is applied to the liquid crystal layer and the liquid crystal layer is charged.

The time constant at this time is shown by the following equation from the equivalent circuit of FIG.

[0013]

[Equation 1]

In the above equation, the resistance RNL of the non-linear element
Is almost 0, the current i excessively flows to charge CLe1. At this time, all the voltage is applied to the liquid crystal layer.

Next, when the display pixel V (M, N) enters the non-selection period, the region resistance 7 shifts to the high resistance region (8). Therefore, RLC << RNL holds, and most of the excessive current i flows through RLC as shown in FIG. 5B. The time constant is approximately given by τ = (CLC + CNL) RLC (2). Generally, it is sufficiently possible to be used in a field effect type liquid crystal display panel.

When the display pixel indicated by the broken line in FIGS. 4A, 4B and 4C is at the non-lighting level, the GPF voltage VNL is generated.
Since it does not enter the lighting region even at the peak time, the liquid crystal layer is not charged and VCL remains at a low level.

Therefore, the effective value ratio of the lighting level to the non-lighting level of the liquid crystal layer is larger than that of the conventional driving method based on the voltage averaging method which does not have a non-linear element. Therefore, the liquid crystal driving method using the non-linear element can perform multiplex driving with a higher digit number, and can realize a large capacity of the liquid crystal display device.

[0018]

However, in the multiplex drive as described above, the effective voltage applied to the liquid crystal layer fluctuates depending on the state of the display signal in the non-selection period as described below. There is a drawback that.
This defect will be described with reference to FIGS. 6 (a), 6 (b) and 6 (c).

As described above, in the non-linear element liquid crystal display device capable of increasing the display capacity, in the multiplex drive, the effective voltage applied to the liquid crystal layer varies depending on the display signal in the non-selection period. There are drawbacks. (A) is
In the signal electrode row 6-N, the voltage waveform VLC in the case where only the display signal electrode 5-M is lit, (b) is the voltage waveform VLC among the row 6-N.
The voltage waveform VLe in the case of lighting every row, (c) is the column 6
It is a voltage waveform VLC in the case where all of -n are turned on. The broken line indicates the voltage V (M, N) applied to the display pixels M and N, and the solid line indicates the voltage VLe applied to the liquid crystal layer.
Looking at (a), (b), and (c), the same signal electrode (SIG)
It can be seen that VLc fluctuates significantly depending on whether the other pixels above are on or off. The same applies to display pixels (M, N)
Is also applicable when is not lit.

Therefore, conventionally, the minimum value E ONmin of the effective voltage of the lighting waveform is larger than the saturation voltage Vsat of the liquid crystal, and the maximum value E OFFmax of the non-lighting waveform is E OFFmax.
Is smaller than the threshold value Vth of the liquid crystal, and the binary display is performed. For this reason, the non-linear element liquid crystal display device is applied only for binary display, and grayscale display is impossible. Also, the above E ONmin, E OFFmax
When the margin is used as a margin, the required characteristics of the non-linear element are severe, which is a manufacturing difficulty. Further, when the saturation voltage is not clear like the guest-host effect, there is a problem in display quality that variations in effective value are displayed as variations in contrast.

The present invention has been made in view of the above drawbacks, and is intended for application to gradation display, prevention of contrast variation, and increase of margin by suppressing fluctuations in effective voltage due to display signals. The variation of the effective voltage can be suppressed unless E ONmin and E OFFmax are brought close to an intermediate level. Another object of the present invention is to make the discharge of the liquid crystal constant when the switching element is in the OFF state by subdividing one scanning period into a plurality of selection levels and non-selection levels. Therefore, as the switching element, the driving method of the present invention can be applied not only to the non-linear element but also to the active switching element (TFT, MOS transistor).

[0022]

According to the present invention, when a liquid crystal display device in which an element having a non-linear characteristic is installed on at least one substrate constituting a liquid crystal display panel is driven by a two-frame AC system, one frame is used. It is characterized in that the absolute value of the average value of the voltage applied to the pixels in the non-selected period within the cycle is substantially the same for all pixels or some pixels.

[0023]

EXAMPLES The present invention will be described below based on examples.

FIG. 7 shows a conventional driving method (B) for driving a display panel (A) composed of matrix-shaped display pixels,
It is a comparison with the driving method (C) of the present invention. In FIG. 7A, the display pixel column 6-0 is a case where only the pixel row 5-M is lit, and the display pixel column 6-N is a case where each pixel row is lit every other column, Display pixel column 6-P is a case where all pixel rows are lit.

FIG. 7 considers the 1/50 duty, 1/5 bias method as in the conventional example. In the normal voltage averaging method shown in FIG. 7B, the scanning cycle Ts is divided in half for AC driving, and is further divided by 50 digits to make a total of 10
It is divided into 0 (this 1 unit period is referred to as 1 scanning period 9).
Then, each scanning signal SCAN has a selection level for one scanning period Tsel once every half scanning period, and has a non-selection level for the other scanning periods.

On the other hand, in the driving method of the present invention shown in FIG. 7C, one scanning period Tsel having each half scanning period selection level is further divided into a plurality of periods (this is the fine scanning period (1
(0)), and a scanning signal is generated in which a part of the fine scanning period is set to the selection level and the other is set to the non-selection level. The method of dividing one scanning period into fine scanning periods can be selected with various ratios, unequal intervals, and even intervals, but for simplicity,
The case of equally dividing into two will be described.

FIG. 7C shows. In the case of this 1/2 division, in the driving method of the present invention shown in FIG.
This is the 1st scanning signal, and the duty is apparently half.
It is the same as the scanning signal of / 100 duty. or,
The display signals added to the display pixel columns 6-0, 6-N, and 6-P are shown as SIGO, SIGN, and SIGP, respectively. Similar to the scanning signal SCANM, each display signal is divided into a plurality of one scanning period (9) (half in this case), and a part of the fine scanning period (10) (in this case, the first half scanning). Only the period) is set to the same level as the normal voltage averaging method, and the remaining fine scanning period is set to the opposite level, that is, the non-selection level for the selection level and the selection level for the non-selection level. . As a result, the display pixel row 6
The display signal waveforms applied to each of −0, 6-N, and 6-P are SIGO, SIGN, and SIGP. In the drive system (C) of the present invention, the waveform is 1 / centered around the reference level.
Although the waveform changes apparently as in the case of 100 duty, considering the average value of the non-selection period within the half scanning period,
It can be seen that the average values are almost equal in both cases.

FIGS. 8A, 8B, and 8C show display pixels (M, O) (M, O) in the driving system of the present invention, respectively.
N) shows the voltage VLC (shown by the solid line) applied to the liquid crystal layer with respect to the voltage (shown by the broken line) applied to each of (M, P). Here, comparing with the case of the conventional driving method shown in FIG. 6, it can be seen that the VLC waveform according to the present invention of FIG. 8 draws a discharge waveform which is almost equal except for a slight change due to the display signal. . This is nothing more than the suppression of the fluctuation of the effective voltage of the liquid crystal layer due to the display signal by the present invention.

Thus, the ON-OF on the same signal electrode is
Since the pixel applied effective voltage is determined without being affected by F, the peak level in the selection period, the time for which the selection level is taken,
By modulating the peak level, it is possible to display gray scales, which was not possible with the conventional non-linear element liquid crystal display device, and the maximum of the OFF waveform, which is the voltage margin of the conventional non-linear element liquid crystal display device. According to the present invention, the effective voltage and the minimum effective voltage in the ON waveform are OFF waveform, ON
It is provided between certain levels of the waveform and has the advantage of expanding the margin. However, in the present invention, even when N-digit multiplex drive is performed, 2N-digit multiplex drive is actually used. The increase in the number of digits does not cause a decrease in margin in the conventional matrix panel, but in the case of the non-linear element liquid crystal display device, it is sufficient within the fine scanning period in which the peak voltage of the ON waveform is applied. If the equivalent capacitance CLC of the liquid crystal layer is charged to the level, there will be no problem even if the number of driving digits increases. In fact, this charging period can be considerably shortened, and depending on the characteristics of the non-linear element, a duty of several thousands of sentences is possible.

In the above embodiment, one scanning period is divided into ½, but the present invention is not limited to ½. The point is that the CLC needs time to charge to a sufficient level within the fine scanning period in which the peak voltage value is obtained, and even if there are a plurality of fine scanning periods in which the peak voltage value is obtained within one scanning period. Alternatively, it may be a fine scanning period in which the scanning period is divided into unequal intervals. However, considering the ease of the drive circuit and the small fluctuation of the effective voltage, 1/2
Equal division is considered best.

Further, since the time for which the scanning signal is at the selection level is only the time for which the CLC is sufficiently charged within the selection period of the ON waveform, one scanning period is not limited to one scanning period ts divided by 2N. Does not have to be. That is, 1
The period xts (0 <x ≦ 1) less than the scanning period ts is set to 2
The equally divided period may be set as one scanning period.

FIG. 9 shows the 1/5 bias method, N = 8, x =
The scanning signals SCAN1 and SCAN8 and the display signal SIG1 when 0.8 is shown. In FIG. 9, the display period t
D is a set of eight scanning periods, the pause period tp is a time which does not belong to any scanning period, and all signal electrodes also have this t.
It is at the non-selection level during p. In this case, the display signal is always a non-selected waveform in the present invention during the period tp, but a selected waveform is not sufficient.

The selection level and the non-selection level described in the present invention are not limited to the selection level and the non-selection level of the conventional driving method.

Next, a circuit for producing the drive signal of the present invention will be described.

FIG. 10 is a panel and a panel drive circuit diagram of the liquid crystal display device of the present invention. From the non-linear element dot matrix liquid crystal panel 11, the display electrode driver section 12, the scan electrode driver section 13, and the drive signal generating section 14. It is configured. The liquid crystal panel 11 includes scan electrodes 15 and display electrodes 16. Display electrode driver section 1
2 is a J-stage shift register 17 and J latch circuits 18 connected to the respective outputs of the shift register, a level shifter 19 for converting the logic level of the circuit into a liquid crystal display level, Level shifter 1
It is composed of M demultiplexers 20 that change the lighting level and non-lighting level of the display signal according to the signal from 9. The scanning electrode driver unit 13 is a 2N-stage shift register 21, a level shifter 22, and a k number of demultiplexers that switch between selection and non-selection of a scanning signal by a signal from the level shifter, where k is the number of scanning electrodes. It is composed of 23. The drive signal generator 14 includes demultiplexers 24-29 and drive voltage generating resistors 30-3.
It is composed of 4.

FIG. 10 will be described in detail with reference to the timing charts of FIGS. 11, 12A and 12B.

.Phi.s in FIG. 11 is the transfer clock of the shift register 17, and the display data DATA is changed by .phi.s.
Are transferred from left to right. When J lines of data for one line are transferred, the clock pulse CL of the latch circuit 18
l becomes high level, and the data is latched from the shift register 17 to the latch circuit 18. The data is 1
In the drive signal generator multi-mixer 20 which is level-shifted by 9 and is input to the control terminal of the multi-brexer 20, the display signals DON and DOFF from the drive signal generator 14 are switched by the display data DATA signal. Shift register 2 of scan electrode driver 13
As shown in FIG. 11, the number 1 indicates the transfer of the pulse synchronized with the latch clock pulse CLl and the display data DATA by J / 2 pieces (when J is an odd number, the maximum integer or J which does not exceed J / 2).
(Minimum integer larger than / 2) Data Dscan that becomes high only once in one cycle with the scan clock CLsc of the logical sum of the pulses generated at the end being the clock pulse
Is entered. In the shift register 21, only odd-numbered stages of the 2N-stage outputs are connected to the level shifter 22. Therefore, the outputs SC ₁ , SC ₂ , SC ₃ , etc. of the odd-numbered stages of the shift register 21 are as shown in FIG. 12-a. The signal is supplied to the demultiplexer 23 through the level sister 22. In the demultiplexer 23, the scan signal selection signal S is generated by the signal DSCAN.
The CON and the non-selection signal SCOFF are switched.

The resistors 30 to 34 have a value of -5V to -V-5.
It is made by dividing the V value. Demulti-Brexer 24, 25
Is the frequency signal φ of the liquid crystal AC drive.
Switching according to f, a selective SCON and a non-selective SCOFF signal of the scan electrode are generated. Denalplexer 26,2
Numeral 7 produces the selection signal DSEL and the non-selection signal DNSEL in the drive system in which the display electrodes are fully used, in accordance with φf. 28 and 29 are demultiplexers required in the present invention, and C are used in the multiplexers 28 and 29.
The signal CLsc / 2 obtained by dividing Lsc by ½ divides DS
EL and DNSEL are switched. Thus, the display electrode signals DON and DOFF (FIG. 12b) are formed.

FIG. 13 shows an example of a control circuit for generating the necessary clock pulse of the drive circuit of the present invention. In this embodiment, J = 160 and K = 120 are set, and a 6-bit binary counter 30, a NOR gate 31, an RS flip-flop 32, an inverter 33, D-type flip-flops 34, 35, 39, 41, and a NOR gate 36. , 40,
It is composed of a 6-bit binary counter 37 and an AND gate 38. The clock pulse φs supplied to the shift register 17 of the display electrode driver 12 is counted by the 6-bit binary counter 30, and when J / 2 = 80 is counted, the gate 31 detects this and SR-FF3.
Set 2. The RE-FF 32 is immediately reset in synchronization with the rising of φs. 32 output CL
sc is input to the RESET terminal of the counter 30, and
It is an input to the D-type FF 43. The D flip-flop 34 divides CLsc by 1/2 to generate a signal CLsc / 2,
Type FF35) Supply to D input. Signal CLsc / 2 is D
The signal CLe, which is differentiated by the FF 35 and the NOR gate 36 and has a period of one scanning period, is supplied to the clock pulse input terminal of the latch circuit 18. The counter 37 counts the clock pulse CLsc from the RS flip-flop 32 with CLsc as the clock, and when counting 239 times, the output of the AND gate 38 becomes high,
This FIG signal is delayed by the D-type FF 39 and differentiated by the gate 40. The signal delayed by the D-type FF 39 becomes the delay signal DSCAN and is supplied to the shift register 21 of the scan electrode driver. Also, the gate 40
The pulse differentiated by is a clock pulse of the D-type FF 41, and is supplied to the demultiplexers 24 to 26 that do not generate drive signals as an AC drive signal φf that changes between high and low in each judgment period.

[0040]

As described above, the driving of the present invention can be realized with a relatively simple circuit. Since the variation of the effective value depending on the lighting or non-lighting of the display pixel is reduced, the effect of improving the driving margin by increasing the minimum value of EON and decreasing the maximum value of EOFF together with the above-mentioned effect. Can be expected. Therefore, when the present invention is applied to a non-linear element liquid crystal display device, it is possible to obtain an effect that even gradation display is possible over the entire panel image.

[Brief description of drawings]

FIG. 1 is a diagram showing VI characteristics of a typical nonlinear device.

FIG. 2 is a diagram showing an equivalent circuit of a non-linear element liquid crystal display device.

FIG. 3 is a diagram showing a liquid crystal panel drive waveform example that requires a conventional non-linear element.

FIG. 4 is a diagram showing the operation of a non-linear element liquid crystal display device, in which a is an example of an applied waveform, b is a voltage waveform applied to a nonlinear element, and c is a voltage waveform applied to a liquid crystal layer.

FIG. 5 is a diagram schematically showing an operational concept of a non-linear element liquid crystal display device, in which a is an ON region of a non-linear element, and O
FF region, b is O when non-linear element is ON, c is O
The figure which shows the time of FF.

FIG. 6 is a diagram showing a pixel applied voltage waveform (broken line) and a liquid crystal layer applied waveform (solid line) in a non-linear element liquid crystal display device, where a is pixel ON at the time and other pixel OFF on the same signal electrode.
In the case of, b is an on / off state alternately, and c is a case of all pixels being on.

FIG. 7 is a diagram in which an example of a drive waveform according to the present invention and a conventional drive waveform are compared and drawn, where A is a display example of a liquid crystal panel,
B is a conventional example, C is an example of the present invention (in the parentheses, a voltage waveform applied to a pixel corresponding to an intersection of a scanning electrode and a signal electrode) (SIG
O is the pixel of this pixel is ON, other pixels on the same signal electrode are OFF
Signal waveform in the case of, SIGN is a signal waveform when alternately ON and OFF, and SIGP is all pixels on the same signal electrode
The signal waveform in the case of.

FIG. 8 is a diagram showing a voltage waveform applied to a liquid crystal layer when the present invention is applied, where a is ON for the pixel and OFF is another pixel on the signal electrode, and b is alternately ON and OFF. In the case of, c is a diagram showing when all pixels are ON.

FIG. 9 is a diagram showing an example of drive waveforms when a pause period tp is set according to the present invention.

FIG. 10 is a drive circuit diagram of the liquid crystal display device of the present invention.

11A and 11B are timing charts of FIG.

12A and 12B are timing charts of FIG.

FIG. 13 is a diagram showing an example of a control circuit of a drive circuit of the present invention.

─────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] July 28, 1993

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be amended] Claims

[Correction method] Change

[Correction content]

[Claims]

[Procedure Amendment 2]

[Document name to be amended] Statement

[Name of item to be corrected] 0022

[Correction method] Change

[Correction content]

[0022]

SUMMARY OF THE INVENTION The present invention provides a pair of transparent insulators.
The liquid crystal is sandwiched between the edge substrates, and the transparent insulating substrate on one side does not
Multiple switching with linear or active elements
Element and electrically connected to one end of the switching element
Pixel electrode is formed and faces the other transparent insulating substrate.
Driving a pixel of a liquid crystal display panel with electrodes
In the driving method of the liquid crystal electro-optical device, one frame period
Within the latter half of one scanning period of each pixel to be set within
It is characterized in that a predetermined period is set as a selection period of the pixel .

Claims (5)

[Claims] 1. When a liquid crystal display device in which an element having a non-linear characteristic is installed on at least one substrate constituting a liquid crystal display panel is driven by a 2-frame AC method, 1
A method for driving a liquid crystal electro-optical device, wherein the absolute value of the average value of the voltage applied to the pixels in the non-selected period within the frame period is substantially the same for all pixels or some pixels. 2. A method of driving a liquid crystal display device provided with a non-linear element, wherein one scanning period, which is obtained by equally dividing all or part of each half scanning period by the number of scanning lines, is further divided into a plurality of periods. 2. The driving of the liquid crystal electro-optical device according to claim 1, wherein the scanning signal has a selection level in a part of the fine scanning period of the fine scanning period and a non-selection level in the remaining fine scanning period. Method. 3. A display signal takes on a lighting level and a non-lighting level, respectively, in correspondence with ON and OFF of a pixel in a period synchronized with the fine scanning period in which the scanning signal takes a selection level.
The non-lighting level and the lighting level are respectively set oppositely during the remaining fine scanning period of the scanning period.
Alternatively, the driving method of the liquid crystal electro-optical device according to claim 2. 4. A fine scanning period obtained by dividing one scanning period into a plurality of periods, when a part of the fine scanning periods is set to a selection level and the rest is set to a non-selection level, a time at the selection level and a non-selection level are set. The method for driving a liquid crystal electro-optical device according to claim 1, wherein certain times are equal. 5. The driving method for a liquid crystal electro-optical device according to claim 1, wherein the one scanning period is divided into two equal parts to form a fine scanning period.