Modulators – Pulse or interrupted continuous wave modulator – Pulse width modulator
Reexamination Certificate
2001-06-05
2002-10-29
Cunningham, Terry D. (Department: 2816)
Modulators
Pulse or interrupted continuous wave modulator
Pulse width modulator
C345S082000, C345S690000, C345S589000
Reexamination Certificate
active
06472946
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a modulation circuit for outputting pulse signals modulated in accordance with the values of input data at a predetermined period and an image display and a modulation method using the modulation circuit and, more particularly, relates to a modulation circuit of a drive signal for a light-emitting diode (LED) and an image display comprising LEDs.
2. Description of the Related Art
Since the invention of the blue LED, LED color displays that use LEDs to form pictures by pixels emitting the three primary colors have been widely and generally fabricated. LEDs are highly durable and can be used semi-permanently, so they are optimal for long-term use outdoors. Therefore, LEDs have been extensively used for large-scale displays in stadiums and event sites and for information display panels and advertisements on sides of buildings and inside railway stations. In recent years, along with the increasing luminance and lower prices of blue LEDs, such LED color displays have been spreading rapidly.
FIG. 1
is a view of a drive circuit of an LED forming a pixel of an LED display.
In
FIG. 1
, reference numeral
100
indicates a drive circuit and
200
an LED. In addition, Spx represents a video signal supplied to each individual pixel; and, Id represents a current flowing through the LED
200
.
The drive circuit
100
outputs a current corresponding to the video signal Spx to the LED
200
, while the LED
200
emits light according to the supplied current. An LED display is comprised of exactly the same number of circuits consisting of the drive circuits
100
and LEDs shown in
FIG. 1
as that of the pixels. By making the LEDs of the pixels emit light with luminances according to the video signals Spx supplied to the pixels, a person viewing the screen can recognize a picture. The video signal Spx supplied to each pixel is generally input to the drive circuit
100
as a digital value of a certain number of bits.
FIG. 2
is a view of the waveform of the current flowing through the LED
200
in FIG.
1
.
In
FIG. 2
, the ordinate and abscissa indicate the current flowing through the LED
200
and time by relative values, respectively. In addition, Ipulse indicates the peak value of the pulse-shape d current flowing through the LED, tw indicates the time width of the current pulse, and T indicates the period of the current pulse.
As shown in
FIG. 2
, the current flowing through the LED forming a pixel of an LED display has a periodic pulse-like waveform. The luminance is controlled by pulse width modulation to make the pulse width tw variable.
In principle, it is possible to use a direct current as that flowing through the LED and make the current value variable in accordance with the video signal Spx to adjust the luminance, but in this case, it is necessary to finely control the current value by the drive circuit, and there is a disadvantage that the circuit for this control ends up increasing the number of parts. Because it is easier to increase the resolution of the time than the resolution of the current value, in general, the pulse width modulation system generating a current of a waveform as shown in
FIG. 2
is adopted.
Due to the nature of human senses, light blinking in a manner staying lit for less than {fraction (1/60)} of a second is perceived to have a constant luminance. Therefore, even a LED is driven by a current of the waveform shown in
FIG. 2
, if the period T of the current is shorter than the aforesaid time, the blinking light from the LED will be perceived by people as light of a constant luminance. Further, the luminance of an LED perceived by the human senses is proportional to the current flowing through the LED averaged over time. Therefore, the luminance changes in proportion with the duty of the pulse current.
The level of a video signal input to an LED display, however, is normalized in advance to match the luminance characteristics of a cathode ray tube (CRT). If such a video signal is input as it is to an LED, which has different luminance characteristics from a CRT pixel, the following problem arises.
FIG. 3
is a view of the relation of the luminances of a LED and CRT pixel with the level of an input signal.
In
FIG. 3
, the ordinate represents the luminance of a LED or CRT pixel, while the abscissa represents the level of the signal input to a LED or a CRT pixel all by relative values. The curves indicated by A and B show the luminance characteristics of a CRT pixel and a LED, respectively.
Note that for the luminance characteristic A of a CRT pixel, the signal level is expressed by the voltage value of the video signal, while for the luminance characteristic B of an LED pixel, the signal level is expressed by the value of the current flowing through the LED.
As shown in
FIG. 3
, the luminance of an LED has a linear relation with the signal level, while the luminance of a CRT pixel has a nonlinear relation with the signal level. In general, the luminance of a CRT pixel is proportional to the 2.2th power of the voltage level of the video signal (&ggr; characteristic). If a current proportional to a video signal normalized to match such a &ggr; characteristic is directly supplied to an LED, the LED appears brighter than a CRT pixel in the region of low output of light, but it appears darker than a CRT pixel in the region of high output of light. Consequently, a picture formed by such pixels has a ratio of luminance of the bright portions and dark portions different from the original picture, so it looks unnatural to viewers.
In order to solve this problem, in an LED display of the related art, a signal corrected to eliminate the influence due to the above luminance characteristic of the video signal is input to the drive circuit
100
as the above video signal Spx. Specifically, for example, when driving an LED of a linear luminance characteristic by a video signal produced to match a CRT pixel emitting light of a luminance proportional to the 2.2th power of the signal level, a signal proportional to the 2.2th power of the video signal is generated to drive the LED.
However, if the bit length of the input video signal is not sufficiently large, the binary data obtained by raising this digitalized image data to the 2.2th power is incapable of expressing fine changes of value in the region where the value of the input video signal is small. In other words, if the bit length of the digitalized video signal is small, the grey scale ends up rough in the low luminance region, resulting in an unnatural picture. In order to avoid such a problem, it is necessary to increase the bit length of the video signal. Specifically, in a LED display of the related art, it is necessary to generate a video signal of a length of 12 to 16 bits to reproduce a picture which can be expressed by a video signal of a length of 8 bits in the case of a CRT. If the bit length of the video signal is increased in this way, the bit length of the pulse width modulation circuits for driving the LEDs also has to be increased, so the overall circuit scale becomes large and the cost and power consumption rise.
Further, a pulse of the waveform shown in
FIG. 2
is generally generated by counting clock signals serving as a time reference. Increasing the bit length of the video signal means increasing the number of times to count the clock signals by that extent, so when using clock signals of the same frequency, the period T of pulse width modulation ends up longer. For example, when generating a 12-bit video signal, which is 4 bits larger than an 8-bit video signal, performing pulse width modulation for it and comparing it with the same frequency of the clock signal, the period T of pulse width modulation becomes 16 times that of an 8-bit video signal. Since the period T of pulse width modulation is set according to the characteristic of the human senses described above, if this period is too long, flickering where the blinking of the light will be perceived by the human eyes will be caused and the picture will
Cunningham Terry D.
Kananen, Esq. Ronald P.
Tra Quan
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