Waveform shaping apparatus

A waveform-shaping apparatus for a rotational speed sensor which measures a rotational speed of a rotatable member of an automotive vehicle, said waveform-shaping apparatus comprising:

• a waveform-shaping circuit including a comparator which compares an input analog signal with a reference signal, the input analog signal alternating in synchronism with rotation of the rotatable member, the comparator producing a binary-coded signal which is indicative of the rotational speed and which has high and low levels, the high level being provided when the input analog signal is greater than the reference signal, the low level being provided when the input analog signal is smaller than the reference signal;
• a hysteresis providing circuit which provides first and second hysteresis components having first and second hysteresis levels, respectively, the hysteresis providing circuit increasing a difference in level between the input analog signal and the reference signal so as to mask the influence of noise from interfering with an operation of said waveform shaping circuit, the first hysteresis component being determined based on the rotational speed and applied to the input analog signal when the binary-coded signal has the high level, the second hysteresis component being determined based on the rotational speed and applied to the reference signal when the binary-coded signal has the low level;
• and
• a correcting circuit which corrects the first and second hysteresis components in a preselected relation to a given noise factor which interferes with the operation of said waveform shaping circuit.

A waveform shaping apparatus for a rotational speed sensor which detects a rotational speed of a rotatable member of a vehicle, said waveform shaping apparatus comprising:

• waveform shaping means for waveform-shaping an input analog signal from the rotational speed sensor into a binary-coded signal, said binary-coded signal having a high level when the input analog signal is greater than a reference signal and a low level when the input analog signal is smaller than the reference signal;
• hysteresis providing means for providing first and second hysteresis components having first and second hysteresis levels, respectively, said hysteresis providing means increasing a difference in level between the input analog signal and the reference signal so as to mask the influence of noise from interfering with an operation of said waveform shaping means, the first hysteresis component being applied to the input analog signal for a first period of time when the binary-coded signal has the high level, the second hysteresis component being applied to the reference signal for a second period of time when the binary-coded signal has the low level;
• setting means for setting at least either the first and second hysteresis levels of the first and second hysteresis components or the first and second periods of time during which the first and second hysteresis components are applied based on the rotational speed detected by the rotational speed sensor;
• and
• changing means for changing said at least either the first and second hysteresis levels of the first and second hysteresis components or the first and second periods of time during which the first and second hysteresis components are applied according to a given vehicle condition.