[0001] The present invention relates to a method for driving a plasma display panel by providing
video data, adjusting the gain and/or the offset of the video data, processing the
video data on the basis of power level information and controlling the plasma display
panel respectively. Furthermore, the present invention relates to a corresponding
apparatus for driving a display panel.
Background
[0002] A PDP (plasma display panel) uses a matrix array of discharge cells, which can only
be "ON", or "OFF". Also unlike a CRT (cathode ray tube) or LCD (liquid crystal display)
in which gray levels are expressed by analog control of the light emission, a PDP
controls the gray level by modulating the number of light pulses per frame (sustain
pulses). This time-modulation will be integrated by the eye over a period corresponding
to the eye time response. Since the video amplitude is portrayed by the number of
light pulses, occurring at a given frequency, more amplitude means more light pulses
and thus more "ON" time. For this reason, this kind of modulation is also known as
PWM, pulse width modulation.
[0003] For all displays using pulse width modulation, the number of real gray levels is
limited. For PDP, in case of standard coding the number of gray levels is more or
less equal to 256.
[0004] These various gray levels can only be used when the dynamic of the input picture
is at its maximum (in case of 8 bit signal, video values between 0 and 255). In other
cases, when the dynamic is reduced (in particular because of contrast or brightness
parameters), the number of displayed levels will further decrease.
[0005] The problem is that the picture quality is affected when the number of displayed
levels is reduced.
[0006] Unfortunately, when reducing the contrast (by dividing by a certain factor) and/or
the brightness (subtracting a certain coefficient from the picture), the maximum value
of the picture decreases and so the picture quality is reduced.
[0007] Contrast and brightness controls are usually part of the so called "front-end", while
PDP specific functions (gamma function, Sub-field encoding, etc) are part of the so
called "back-end" of the display (see Fig. 3).
[0008] In the back-end of a PDP an APL function is used to control the power. The computation
of this Average Power Level (APL) is made through the following function:

where I(x,y) represents the picture to display, C the number of columns and L the
number of lines of the PDP.
[0009] The aim of power management is to keep the power consumption constant (see Fig. 1)
and to have a peak luminance as high as possible. So for every APL value, the maximal
number of sustain pulses to be used is fixed. This number of sustain pulses decreases
when the APL increases, and vice versa as shown in Fig. 2.
[0010] In peak-white pictures (low APL at the left side of Fig. 2), the number of sustain
pulses is not limited by the power consumption, but by the available time for sustaining.
For this reason, the power consumption of peak-white pictures will be lower than for
the other pictures. Consequently, also the power consumption decreases for low APL
levels (compare Fig. 1).
[0011] The following table shows an allocation of the values of the number of sustain pulses
to the average power levels according to Fig. 2.
APL |
Total Number of sustain pulses |
0 |
1000 |
1 |
1000 |
2 |
1000 |
3 |
1000 |
4 |
1000 |
5 |
1000 |
. |
. |
. |
50 |
1000 |
51 |
1000 |
52 |
1000 |
53 |
1000 |
54 |
1000 |
55 |
1000 |
56 |
999 |
57 |
998 |
58 |
996 |
59 |
994 |
60 |
991 |
61 |
988 |
62 |
984 |
63 |
979 |
64 |
975 |
65 |
971 |
66 |
966 |
67 |
962 |
68 |
958 |
69 |
954 |
70 |
950 |
71 |
946 |
72 |
942 |
73 |
938 |
74 |
933 |
75 |
929 |
. |
. |
. |
295 |
449 |
296 |
448 |
297 |
447 |
298 |
446 |
299 |
445 |
300 |
444 |
301 |
442 |
302 |
441 |
303 |
440 |
304 |
439 |
305 |
438 |
. |
. |
. |
. |
. |
. |
600 |
225 |
601 |
224 |
602 |
223 |
603 |
223 |
604 |
222 |
605 |
222 |
606 |
221 |
607 |
221 |
608 |
221 |
609 |
220 |
610 |
220 |
611 |
219 |
612 |
219 |
613 |
219 |
614 |
218 |
615 |
218 |
. |
. |
. |
1005 |
102 |
1006 |
102 |
1007 |
102 |
1008 |
102 |
1009 |
102 |
1010 |
102 |
1011 |
101 |
1012 |
101 |
1013 |
101 |
1014 |
101 |
1015 |
101 |
1016 |
101 |
1017 |
100 |
1018 |
100 |
1019 |
100 |
1020 |
100 |
1021 |
100 |
1022 |
100 |
1023 |
100 |
[0012] As indicated above, the problem of the standard implementation of power management
is that when the energy of the input picture of the back-end decreases, the number
of sustain pulses increases. So the energy of the displayed picture decreases hardly.
[0013] Fig. 3 shows a principle block diagram of the driving unit of a plasma panel 1. The
video input signal is first processed in the front-end 2. The front-end includes a
scaling unit 4 for adapting the size of the picture to that of the panel. The scaled
input signal is supplied to a brightness/contrast control block 5. This control block
5 receives external signals for tuning the brightness and/or the contrast of the picture
(= adjusting the gain and/or the offset of the video data). The video signal is processed
accordingly and supplied to the back-end 3. Within the back-end 3 the signal is processed
in a usual path including a gamma block 6, a dithering block 7 and an encoding block
8. The gamma block 6 performs a data transformation with a look up table in accordance
to a nearly quadratic gamma function. The output signal of the gamma block 6 is transmitted
to the dithering unit 7 which will add for example 4 bit dithering in order to render
more discrete video levels at the output. Afterwards, the sub field encoding 8 generates
sub field data for the video signal. The resulting sub field data are sent to the
plasma panel 1.
[0014] In a parallel path within the back-end 3 the output signal of the front-end 2 is
input into an APL measurement block 10. This block supplies an APL level of the brightness/contrast
tuned video signal to the power management 9. The power management 9 controls the
gamma unit 6 and the encoding unit 8. Furthermore, the power management 9 delivers
sustain information to the plasma panel 1.
[0015] With this arrangement, it is for example interesting to see what happens when the
user is decreasing the contrast and/or the brightness.
[0016] When decreasing the contrast and/or the brightness, the APL (measured in the back-end
3) is decreasing; this means that the number of sustain pulses is increasing. This
increases partly the contrast.
[0017] For example, the user wants to reduce the contrast by 2 for a picture, which has
an APL of 300 (10 bit value). So originally this picture has in average approximately
444*300/1024=130 sustain pulses/cell, and can have a peak luminance of 444 sustain
pulses (compare table shown above).
[0018] To obtain in average 65 sustain pulses/cell, the user in fact has to reduce the contrast
of the picture by around 4. (for an APL value of 70, according to the table, the average
number of sustain is equal to 950*70/1024=65). The peak luminance in this case is
also reduced since all brightness levels of the whole picture are divided by more
than 4, the maximum value of the picture will not be higher than 255/4.3=60 (this
represents 950/4.3=222 sustain pulses). But since, the picture is divided by more
than 4, the number of gray levels really used is also divided by around 4. The picture
quality is rather low in this case.
Invention
[0019] In view of that, it is the object of the present invention to provide a driving apparatus
for a plasma display panel which improves the picture quality, when the brightness
and contrast of the picture are reduced. Furthermore, a respective method shall be
provided.
[0020] According to the present invention this object is solved by a method for driving
a plasma display panel by providing video input data, providing attenuation information,
adjusting the gain and/or the offset of the video input data in order to obtain adjusted
video data, measuring a power level of said adjusted video data and providing a respective
first power level information, generating a second power level information on the
basis of said first power level information and said attenuation information, changing
the level of said adjusted input data on the basis of the attenuation information
and obtaining respective changed video data, adjusting the number of sustain pulses
per sub-field on the basis of said second power level information and, driving said
plasma display panel with said adjusted number of sustain pulses and said changed
video data.
[0021] Furthermore, according to the present invention there is provided an apparatus for
driving a plasma display panel including brightness/contrast control means for receiving
video input data, for adjusting the gain and/or the offset of the video input data
and for outputting adjusted video data, attenuation data means for providing attenuation
information, power measurement means for measuring a power level of said adjusted
video data and providing a respective first power level information, power level generating
means for generating a second power level information on the basis of said first power
level information and said attenuation information, compensating means for changing
the level of said adjusted input data of said brightness/contrast control means on
the basis of the attenuation information and for outputting respective changed video
data, power management means for adjusting the number of sustain pulses per sub-field
on the basis of said second power level information and driving said plasma display
panel with said adjusted number of sustain pulses and said changed video data.
[0022] According to a specific embodiment of the present invention the attenuation information
directly depends on the adjustment of the gain and/or the offset of the video input
data. Thus, attenuation evaluation means are not necessary in the back-end, since
the attenuation information is directly produced in the front-end by the brightness/contrast
control means for example.
[0023] Alternatively, the attenuation information may be evaluated during the step of adjusting
the gain and/or the offset of the video input data, whereby the video input data are
pre-given maximum input data. In this case, the implementation of the system would
be simplified.
[0024] Preferably, the attenuation information may be evaluated during a blanking time between
video frames. Thus, the blanking time can be used for adjusting the power management
in the back-end.
[0025] Furthermore, component attenuation information may be evaluated for the red, green
and blue components of the adjusted video data and the component attenuation information
corresponding to the less attenuated component is used as the attenuation information.
Such differentiation of the components leads to an improved colour rendition.
[0026] Preferably, the attenuation information is evaluated for each frame of the video
input data. This guarantees a dynamic improvement for each picture.
Drawings
[0027] The present invention will now be explained in more detail along with the attached
Figures, showing in:
- Fig. 1
- a diagram of the power consumption over the average power level;
- Fig. 2
- a diagram of the number of sustain pulses over the average power level;
- Fig. 3
- a block diagram of a driving unit of a plasma display panel according to the prior
art, and
- Fig. 4
- a diagram of a driving unit of a plasma panel according to the present invention.
Exemplary embodiments
[0028] The purpose of the invention presented here is to improve the behavior of the power
management 9 (cf. Fig. 3) regarding contrast and brightness control.
[0029] The idea is that when contrast and/or brightness decrease, the power management should
not increase the number of sustain pulses. Otherwise, the user needs to further decrease
the contrast and/or the brightness, thereby further reducing the picture quality.
The total number of sustain pulses should preferably decrease.
[0030] So the solution is to estimate the reduction of the dynamic occurring in the front-end
2, and to compensate it in the back-end 3 as shown in Fig. 4. The global structure
of the block diagram of Figure 4 is the same as the one shown in the block diagram
of Figure 3. However, some gray blocks are additionally included into the front-end
portion 2 and the back-end portion 3. Block 11 only comes to effect if an attenuation
value or attenuation information has to be evaluated, preferably during the blanking
time explained below.
[0031] For evaluating an attenuation value, an attenuation evaluation generator 11 is connected
in front of the brightness/contrast control block 5 as shown in Figure 4. The attenuation
evaluation generator 11 inputs into block 5 maximum video input data as reference
data, on the basis of which the attenuation shall be evaluated. In the back-end 3
an attenuation evaluation unit 12 calculates the attenuation of the maximum input
data tuned by the brightness/contrast control means 5. A respective attenuation factor
or attenuation information is provided for a power level generating block 13 which
provides an adapted power level information APL
Att for the power management unit 9. This adapted power level information APL
Att is generated on the basis of the attenuation information of block 12 and the power
level measured by the power level measurement block 10 of the back-end 3. Furthermore,
a modified attenuation information Att' is input to a compensation unit 14, which
processes the video data of the brightness/contrast control means 5 of the front-end
2 and outputs compensated video data having increased dynamic to the gamma block 6.
In the following, the function of the circuit of Figure 4 is explained in more detail.
[0032] To evaluate the reduction of the dynamic, the front-end 2 includes the attenuation
evaluation generator 11 in order to be able to deliver an output value for an input
value corresponding to the maximum input value (1023 for R, G and B in case of 10
bit signal). These values (R
Att, G
Att, B
Att) should be evaluated during the blanking time.
[0033] Since it is not intended to modify the color changes made in the front-end, the same
compensation 13 will be applied to the three components R, G, B. So in order to have
no saturation, the coefficient, which will be used for the compensation will correspond
to the less attenuated component. This means that only the less attenuated component
will be fully compensated.
[0034] So the back-end 3 will only use the maximum of the values (R
Att, G
Att, B
Att) to determine the value of the compensation to apply. This maximum value has to be
evaluated for each frame since the contrast and/or the brightness can change at any
time.
[0035] Then regarding this maximum value (Att) and the value of the APL measured in the
back-end 3 (APL
B), a new APL value APL
Att is chosen with the help of a Look-Up Table: comp_APL. The total number of sustain
pulses of the new APL value (APL
Att) corresponds to the total number of sustain pulses of APL
B compensated for the value Att. If Att is encoded with 10 bits:

[0036] MinSustainNumber corresponds to the total number of sustain pulses when the APL is
maximum, i.e. MinSustainNumber = SustainNumber(APL_max) and γ corresponds to the gamma
used in the Gamma LUT 6. This is done to reduce the total number of sustain pulses.
[0037] The compensation has to take into account the fact that the new APL value, APL
Att, is limited by the maximum value of APL. This can be done by another LUT: Att2Att'.

[0038] The inputs of this LUT will be the same as the inputs of comp_APL: APL
B and Att since APL
Att is a function of APL
B and Att.
[0039] Then the video level has to be increased to compensate the reduction of the total
number of sustain pulses. This can be done:
- either by a multiplication by a coefficient, mult, located in a LUT (mult_video)

wherein the mult_video LUT has only Att' for input
and

to be evaluated in the IC of the front-end 2 and/or the back-end 3,
- or by a LUT (comp_video)

wherein this LUT uses Att' and Videoln as inputs.
This LUT only needs to be defined for Videoln comprised between 0 and Att', since
the maximum value at the input of the front-end gives at maximum Att at the output
of the front-end (and so at the input of the back-end), and Att ≤ Att'.
[0040] As seen previously, this solution is based on LUTs. Usually, they need to be on an
external memory (EPROM or Flash) and only in the case of the multiplication made by
LUT, one (a subpart of comp_video) needs to be loaded in the on-chip memory of the
IC.
[0041] The content of the front-end 2 and the back-end 3 of the example of Figure 4 are
only given as examples. It is only mandatory in the front-end 2 to have the "Attenuation
Evaluation Generator" function 11 before the brightness/contrast control 5. Of course
also another function that could increase or decrease the video level can be used.
[0042] At a given time of the blanking time, the "Attenuation Evaluation Generator" 11 should
give as input to the front-end 2 the maximum value for R, G and B. Then, the "Attenuation
Evaluation" 12 should read the output values of the front-end 2 during this time.
During the remaining time of the frame, the "Attenuation Evaluation Generator" 11
should have a bypass function.
[0043] After having read the output values of the front-end 2 (R
Att, G
Att, B
Att), the "Attenuation Evaluation" 12 should evaluate the less attenuated value (Att)
by computing the maximum of these three values (R
Att, G
Att, B
Att).
[0044] Then, using this value, Att, and the APL value, APL
B, the "Att' / APL
Att" block 13 will pick-up in the Look-Up Tables (comp_APL and Att2Att') located in the
external memory the value of the attenuated APL, APL
Att, and the new value of the attenuated value, Att' which only differs from Att when
APL
Att=1023. These two LUTs have the same inputs (Att and APL
B) and the same size: 1024*1024*10bit, if each value uses 10 bits. These values are
only read once per frame, and so the LUTs can typically be stored in the external
memory.
[0045] The value APL
Att will be used by the Power Management 9 to choose the correct sustain information
sent to the PDP, and to load the corresponding LUTs in the Gamma 6 and Decoding blocks.
[0046] The value Att' is used by the Compensation Video Block 14. This block 14 can be defined:
- either to use a multiplication by a coefficient, mult. In this case, this coefficient
is picked-up in the LUT (mult_video) located in the external memory.

[0047] This mult_video LUT has only one input (Att'), and its size is 1024*16bit (in case
of 10 bit signals). Then the following expression has to be computed (this is only
a 10bit x 16bit multiplication and a bit shift)
- or to load at the beginning of the frame a subpart of the comp_video LUT located in
the external memory. This comp_video LUT has a size of 1024*512*10bit (in case of
10 bit signals). The first input is Att' and the second one is the video coming from
the front-end. This LUT is only defined for a video input value comprised between
0 and Att', that's why its size is 1024*512*10 bits and not 1024*1024*10 bits.
[0048] This sub-part of the comp_video LUT, which has a maximum size of 1024*10bit, has
to be loaded in the IC for each frame.
[0049] This solution does not change the brightness on the PDP. In fact it decreases the
total number of sustain pulses but it increases in the same ratio the video values
R, G, B. On the PDP there will be used as many sustain pulses as before, only the
number of unused sustain pulses will be smaller. This means that, advantageously,
the power consumption will be reduced in this case.
[0050] Coming back to the example of the introductory part of the description. The user
wants to reduce the contrast by 2. As previously seen, it is necessary to divide the
video level by around 4.3 (after gamma correction).
[0051] During the blanking, the "Attenuation Evaluation Generator" 11 and the "Attenuation
Evaluation" functions 12 will evaluate the attenuation: the "Attenuation Evaluation
Generator" block 11 sends 1023 on the three components, and the "Attenuation Evaluation"
block 12 reads

for the three components (R
Att, G
Att, B
Att) since in this example (with γ=2.2) the front-end 2 applies the same process on the
three components . So the maximum of these three values leads to: Att = 528.
[0052] The measurement of the APL 10 in the back-end 3 leads to APL
B = 70.
[0053] The Look-up Table comp_APL located in the external memory gives for the given values
of Att (528) and APL
B (70), a new value of APL, APL
Att, in block 13. This new APL has 4.3 times less sustain than APL
B. So since SustainNumber(70) = 950, SustainNumber(APL
Att) = 222 and APL
Att = 605.
[0054] Then an updated value of Att is looked up in the LUT Att2Att'. Here the value is
the same: Att' = 528.
[0055] Depending on the choice which has been made for the implementation, a LUT or just
a coefficient has to be loaded in the IC in order to proceed to the multiplication.
[0056] So in the first case, the Compensation Video Block 14 will pick-up the LUT corresponding
to Att' = 528 in the comp_video LUT located in the external memory. This LUT has only
528 inputs since the front-end delivers values between 0 and

even if the place on this IC should be equal to its maximum size, i.e. 1024*10bit.
This LUT is loaded only once per frame, and is used for every pixel of the picture.
The LUT will multiply the video data by

so that the video will be able to use the full dynamic of the PDP.
[0057] The other solution is to make this multiplication in the IC. In this case only the
coefficient to be used for the multiplication (mult) has to be loaded in the IC. This
coefficient is in the mult_video LUT located in the external memory. This value is
loaded only once per frame, and is used for every cell of the picture. The entry to
this LUT is Att' = 528, and the output is

Then, for every cell of the panel 1, the following multiplication has to be computed:

[0058] The power management unit 9 uses the value 605 as input. The average number of sustain
pulses is equal to 222*(70*4.3)/1024=65, but the maximum value of the picture will
be

So the picture is able to use the same number of levels than before the reduction
of contrast.
[0059] This means that the number of gray levels really used is four times as big as in
the standard implementation. So finally the picture quality is significantly improved.
1. Method for driving a plasma display panel (1)
characterized in that it comprises the following steps:
- providing video input data,
- providing attenuation information,
- adjusting the gain and/or the offset of the video input data in order to obtain
adjusted video data,
- measuring a power level of said adjusted video data and providing a respective first
power level information,
- generating a second power level information on the basis of said first power level
information and said attenuation information,
- changing the level of said adjusted input data on the basis of the attenuation information
and obtaining respective changed video data,
- adjusting the number of sustain pulses per sub-field on the basis of said second
power level information and,
- driving said plasma display panel (1) with said adjusted number of sustain pulses
and said changed video data.
2. Method according to claim 1, wherein the attenuation information is generated when
adjusting the gain and/or the offset of the video input data.
3. Method according to claim 1, wherein said attenuation information is evaluated after
said step of adjusting the gain and/or the offset of the video input data, whereby
pre-given maximum input data are used as video input data.
4. Method according to claim 3, wherein said attenuation information is evaluated during
a blanking time between two video frames.
5. Method according to claim 3 or 4, wherein component attenuation information is evaluated
for each of the red, green and blue components of the adjusted video data, and the
component attenuation information having the lowest level is used as said attenuation
information.
6. Method according to one of the preceding claims, wherein said attenuation information
is provided or evaluated for each frame of said video input data.
7. Apparatus for driving a plasma display panel (1)
characterized in that it includes:
- brightness/contrast control means (5) for receiving video input data, for adjusting
the gain and/or the offset of the video input data and for outputting adjusted video
data,
- attenuation data means (11, 12) for providing attenuation information,
- power measurement means (10) for measuring a power level of said adjusted video
data and providing a respective first power level information,
- power level generating means (13) for generating a second power level information
on the basis of said first power level information and said attenuation information,
- compensating means (14) for changing the level of said adjusted input data of said
brightness/contrast control means (5) on the basis of the attenuation information
and for outputting respective changed video data,
- power management means (9) for adjusting the number of sustain pulses per sub-field
on the basis of said second power level information and driving said plasma display
panel (1) with said adjusted number of sustain pulses and said changed video data.
8. Apparatus according to claim 7, wherein said attenuation data means (11, 12) include
a data generator (11) for inputting pre-given maximum input data into said brightness/contrast
control means (5) and further including attenuation evaluation means (12) for evaluating
an attenuation applied on said pre-given maximum input data when being adjusted in
the brightness/contrast control means (5).
9. Apparatus according to claim 8, wherein said data generator (11) is capable of providing
pre-given maximum input data for each of the video components red, green and blue
and said attenuation evaluation means (12) is capable of evaluating for each of said
components component attenuation information, wherein that component attenuation information
having the lowest level is used as said attenuation information.