Pulse width modulation signal generating device, image display apparatus including pulse width modulation signal generating device, and pulse width modulation signal generating method
A pulse width modulation signal generating device which generates a pulse width modulation signal for modulating laser beam according to image data includes: a cycle signal output circuit which outputs a cycle signal having a cycle corresponding to a drawing rate for drawing the image data by the laser beam; and a pulse width modulation circuit which receives supply of the image data and the cycle signal and modulates a pulse wave having a cycle determined by the cycle signal by changing duty ratio of the pulse wave based on the image data to produce the pulse width modulation signal.
1. A pulse width modulation signal generating device which generates a pulse width modulation signal for modulating a laser beam according to image data, comprising:
a cycle signal output circuit adopted to output a cycle signal having a cycle corresponding to a predetermined drawing rate for drawing an image by the laser beam in accordance with the image data; and
a pulse width modulation circuit adopted to receive the image data and the cycle signal and modulate a pulse wave having a cycle determined by the cycle signal by changing duty ratio of the pulse wave based on the image data to produce the pulse width modulation signal, wherein the predetermined drawing rate increases as the drawing position rises in a vertical direction of the image drawn by the laser beam.
2. The pulse width modulation signal generating device according to claim 1 , wherein the cycle signal output circuit includes a phase synchronization circuit adopted to receive a drawing clock having a cycle corresponding to the predetermined drawing rate and output a signal having frequency equal to and phase synchronized with the drawing clock as the cycle signal.
3. The pulse width modulation signal generating device according to claim 2 , wherein the cycle signal is a signal having ramp wave, triangle wave, or saw tooth wave.
4. The pulse width modulation signal generating device according to claim 1 , wherein the predetermined drawing rate becomes a maximum value at a center position in a horizontal direction for a same position in the vertical direction, and decreases as the position shifts from the center position toward an outside position in the horizontal direction for the same position in the vertical direction.
5. The pulse width modulation signal generating device according to claim 1 , further comprising:
an image data memory unit adopted to temporarily store the image data, and adopted to cyclically output respective pixel data constituting the image data based on a data read clock received by the image data memory unit,
wherein the cycle signal output circuit includes
a parameter calculating unit adopted to calculate the position of pixel data to be read from the image data memory unit on the image data in the vertical direction and calculate a parameter corresponding to the predetermined drawing rate determined by the position in the vertical direction,
a phase synchronization circuit adopted to receive a system clock having a predetermined cycle and the parameter and output a signal having a cycle calculated by dividing the predetermined cycle by a number corresponding to the parameter as the cycle signal, and
a waveform shaper adopted to shape an oscillation signal from the phase synchronization circuit and output the shaped signal as the data read clock.
6. The pulse width modulation signal generating device according to claim 5 , wherein the cycle signal is a signal having ramp wave, triangle wave, or saw tooth wave.
7. An image display apparatus comprising:
the pulse width modulation signal generating device according to claim 1 ; and
a controller adopted to receive an image signal containing the image data from the outside, and adopted to transmit a drawing clock having a cycle corresponding to the predetermined drawing rate based on the image signal to the pulse width modulation signal generating device.
8. An image display apparatus comprising:
the pulse width modulation signal generating device according to claim 5 ; and
a controller adopted to receive an image signal from the outside, and adopted to transmit image data contained in the image signal to the image data memory unit, and a vertical synchronous signal and a horizontal synchronous signal contained in the image signal to the parameter calculating unit.
9. A pulse width modulation signal generating method which generates a pulse width modulation signal for modulating laser beam according to image data, comprising:
outputting a cycle signal having a cycle corresponding to a predetermined drawing rate for drawing an image by the laser beam in accordance with the image data; and
receiving the image data and the cycle signal and modulating a pulse wave having a cycle determined by the cycle signal by changing duty ratio of the pulse wave based on the image data to produce the pulse width modulation signal, wherein the predetermined drawing rate increases as the drawing position rises in a vertical direction of the image drawn by the laser beam.
10. The pulse width modulation signal generating device according to claim 1 , wherein a frequency of the cycle signal increases as the predetermined drawing rate increases, and the frequency of the cycle signal decreases as the predetermined drawing rate decreases.