Abstract: In order to detect the presence of voice frequencies on a telephone line which is periodically sampled for monitoring its activity, with conversion of resulting analog samples into binary words each including a polarity bit, the polarity bits of successive samples of a given line are fed to a zero-crossing detector emitting a pulse on every polarity reversal. The pulses pass through a shift register, which is stepped at the sampling rate, into a counter whose count is periodically read to determine whether a line-signal frequency, as represented by the number of zero crossings per test interval, or the rate of change of that frequency exceeds a certain threshold to indicate a fricative or sibilant sound introducing a spoken syllable.