Abstract: Time-correlation apparatus for collecting correlation data when the noise level is low and inhibiting the collection of correlation data when the noise level is high without losing valid correlation data which has already been obtained. An incoming signal is checked for data pulses during each correlation measurement pulse of a synchronizing signal. The correlation measurement pulses are counted by a first counter, and the coincidences of data pulses and correlation measurement pulses are counted by a second counter. A noise monitor samples the incoming signals 100 times during each of a continuous series of noise analysis periods and counts the number of times the signal (assumed to be noise) exceeds a predetermined threshold. A high noise signal is generated if the threshold is exceeded 30 or more times, and a low noise signal if it is exceeded less than 30 times.