| 摘要 |
A method and apparatus are provided for detecting the transition of digital data signals the two states of which are modulated with two different signal frequencies from one signal frequency to the other signal frequency. Timing elements generate comparison pulses having a first and second duration at each pulse front of the modulated data signal during a second and first signal frequency respectively. The comparison pulses in turn are used to generate control pulses so that the leading edge of the control pulse coincides with the leading edge of the comparison pulse, and likewise, the trailing edge of the control pulse coincides with the trailing edge of the comparison pulse. The control pulses are fed to the up/down counting direction control input of a reversible counter having a clock signal connected to its input. A pair of comparators compare the contents of the reversible counter with a pair of preset separate threshold levels. The comparators generate an output signal upon the attainment of a threshold level in the reversible counter which attainment is interpreted as a transition from one signal frequency to the other signal frequency. The determination of a transition from one signal frequency to the other signal frequency is also utilized to change the duration of the comparison pulse to the duration of the other comparison pulse.
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