Rate of change detection

Apparatus for monitoring the rate of change of a first signal which comprises:

• means for periodically sampling the amplitude of the first signal to produce a sampled signal,
• means responsive to the sampled signal for generating a reference signal which for each sample period differs from the sampled amplitude of the sampled signal by a predetermined amount,
• means for comparing the amplitudes of the first signal and the reference signal and to generate an output signal when the amplitude of the first signal crosses the amplitude of the reference signal,
• said sampling means comprising clock means which produces pulses at a fixed rate, counter means which counts the pulses produced by the clock means, means to convert the count in the counter means into an analogue signal, said analogue signal being said sampled signal, and means to compare the sampled signal to the first signal and to generate a signal which suspends operation of the counter means when the sampled signal and the first signal are in a predetermined relationship to one another, thereby causing the sampled signal to be held constant for the sample period.