A variable wave forming circuit is provided which produces signals of various waveforms (e.g., sine, triangular or trapezoidal waves) and various frequencies. A random access memory (memory means) 121 to store wave formation information on waveform is provided. According to the wave formation information stored in the memory means, the updating or keeping of a digital value in an increment/decrement circuit 123 is controlled and the digital value is digital/analog-converted by a digital/analog (D/A) conversion circuit 124, which is controlled by a digital value control means that includes the increment/decrement circuit 123. By writing appropriate wave formation information into the memory means, it is possible to produce signals of desired waveforms.
An audible tone is generated and attenuated over a wide frequency range, such as throughout the human audible range, the tone selectively being of short duration. During a tone period a digital representation of the sine of a requested tone frequency and amplitude is generated. During an attenuation period a digital representation of a moderately disturbed but continuous sine of decreasing amplitude is generated. During a decay period a digital representation of a continuous function which decays to zero from the zero approach point of the sine half wave is generated. During the attenuation period, at zero crossings, the amplitude value is multiplied by a fractional constant; within zero passing zones, the amplitude between subsequent samples is incremented by temporally reduced values to further attenuate the tone and accumulate a bank of accumulated reductions in increments; and while approaching zero crossings, a sine wave of maximum amplitude equal to the amplitude at the beginning of the prior quadrant minus the bank of accumulated reductions in increments during said prior quadrant is generated; and during a decay period, a digital representation of a continuous function which decays to zero amplitude is generated.
