MB87P2020 Datasheet, PDF (173/356 Pages) Fujitsu Component Limited. – Colour LCD/CRT/TV Controller

Graphic Processing Unit
formation. This is done by loading the gamma tables with the appropriate contents. For instance, the inverse
gamma correction would require the contents of address I_mtx to be:
I _act = [I _mtx] = ((I _mtx – I ₀) ⁄ c)^{1 ⁄ γ}
(10)
In a different application the gamma look-up tables might be used to adopt RGB color spaces to special dis-
play characteristics. This is referred to as RGB gamma forcing. In this case, the gamma tables are not avail-
able for YUV gamma correction anymore, although the matrix still can be used for YUV to RGB
conversion.
3.8 Duty Ratio Modulation
3.8.1 Working Principle
This paragraph briefly describes the Duty Ratio Modulator integrated. The purpose of this unit is to provide
additional (pseudo-) levels (shades of hue or gray) for output, i.e. virtually expand the color resolution in
the physical color space. This is achieved by tuning the duty ratio of actually existing physical bits.
Assume there is a frame rate of 100 Hz on a black and white display (1 bit physical color space). Pseudo
gray levels can be obtained when pixels are not set black all 100 frames per second but say 80 frames only.
Thus, the pixel is seen in dark gray instead of black. The less time a pixel is set to black the lighter the gray
becomes. This is equivalent to modulating the duty ratio of the pixel signal.
Vertical sync (i.e. the frame rate) is used to modulate the pixels, since this signal must be common for all
pixels on display because otherwise artefacts may occur. If the decision whether to set a pixel to black or
white to obtain the desired pseudo gray level is made e.g. on a line-by-line basis it depends on the ratio be-
tween the number of lines and the set pseudo gray level where (geometrically on the display) “gray” pixels
are set to black or white. Unsuitable settings could then cause actually “gray” pixels to stay either black or
white or visibly flicker. Therefore, all “gray” pixels of the frame of the same pseudo gray level are set to
the same physical value of either black or white to achieve a unified optical impression.
The number of frames constituting a basic modulation period depends on the number of pseudo gray levels
desired and on the linearity of the display characteristics. For instance, if just black, white and gray is in-
tended the basic modulation period might be limited to two frames. Black pixels are set black every frame
(100% duty ratio), white pixels are not set black at all (0% duty ratio) and “gray” pixels are set black every
other frame (50% duty ratio). However, the “gray” optically perceived might not be 50% black. This is due
to a possibly nonlinear electro-optical characteristics of the display. In order to cope with this adjustments
must be made possible. An optically 50% black might be achieved with a 60% duty ratio, for instance. One
should be able to adjust the duty ratio with an accuracy of some percent (5 or 6 bits precision). Hence, longer
periods i.e. more frames (30…100) are needed.
When using a basic period of e.g. 100 frames a 50% duty ratio is equivalent to an overall 50 frames on and
50 frames off. This implies no statement about when to display the pixel as active and inactive, respectively.
Setting the pixel first for 50 frames active and afterwards the next 50 frames inactive seems to be equivalent
to setting the pixel active every other frame.
Unfortunately, this is not the case in general. Most displays feature a frame rate in the magnitude of 100Hz
which means that the modulation period of 100 frames lasts for one second. Both methods are equivalent
only for displays slow enough to integrate over this rather long period. On faster displays the first method
(simple pulse width modulation with 50 consecutive frames activity) will cause a blinking of 2 Hz clearly
visible for the human eye. Consequently, a simple pulse width modulation is inadequate and a distribution
of on and off values as even as possible is required.
This is achieved using a derivation of Bresenham’s line algorithm. This algorithm was originally designed
to draw a line from point A(x_a, y_a) to point B(x_b, y_b) on a lattice of discrete pixels with minimal deviation
from the ideal graph y = m ⋅ x + n . The basic idea is to scan the distance ∆X = x_b – x_a step by step and draw
pixels at appropriate y-values using a so-called decision variable. This variable tells for a given x(k + 1)
whether to draw the pixel on (x(k + 1),y(k)) or (x(k + 1),y(k) + 1) .
GPU Control Information
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