Patent ID: 7532543
Filing Date: 2009-05-12
Classification: H03F

Abstract:
1. An acoustic signal generating apparatus comprising: a PWM signal generator, operable to generate a first pulse signal and to generate a second pulse signal at least one of before and after generating the first pulse signal; and a low-pass filter, operable to convert the first pulse signal output from the PWM signal generator into a first analog acoustic signal, and to convert the second pulse signal output from the PWM signal generator into a second analog acoustic signal, wherein a potential level of the first acoustic signal is changeable in a predetermined range between a first potential level and a second potential level higher than the first potential level based on a duty ratio of the first pulse signal; wherein a potential level of the second acoustic signal is changeable in the predetermined range based on a duty ratio of the second pulse signal; wherein the PWM signal generator gradually changes the duty ratio of the second pulse signal such that the potential level of the second analog acoustic signal is smoothly changed from one of the first potential level and the second potential level to a center potential level in the predetermined range, or is smoothly changed from the center potential level to one of the first potential level and the second potential level; and wherein the first analog acoustic signal has an audible frequency and the second analog acoustic signal has an inaudible frequency.