Tondetektor

摘要

In a tone detector, a periodic signal is received and sampled in a level crossing detector (11) which gates (16) clock-pulse counts (15) to a processor which measures periods between crossings of the level by the signal. Pairs of maximum and minimum thresholds for signal periods corresponding to different desired tones are stored in memory (18) and consecutive periods of the signal are compared with the predefined thresholds. A primary tone detector, responsive to the comparison, counts instances when measured periods fall between a selected pair of minimum and maximum thresholds. Receipt of a first tone is confirmed when the primary tone detector counts to a first predetermined value. A secondary tone detector counts instances when measured periods fall between a new pair of minimum and maximum thresholds. The primary tone detector changes the selected pair of thresholds to the new pair of thresholds when the secondary tone detector counts to a second predetermined value. <IMAGE>