The recognition of signals having specific periods is described particularly in relation to recognising vowels. Characteristic frequencies in vowels are recognised by circuits which delay the input signals for an interval related to a frequency to be recognised, correlate the delayed signals with undelayed input signals and integrate the resultant over a short period comparable with the duration of the vowel sound. The magnitude and sign of the integrated signal indicates whether the required signal is present. In order to deal with speech an incoming signal is first passed to a special form of AGC circuit and then divided into low and high frequency components. The presence of characteristic frequencies in these components is determined by the technique described above, and logic circuits indicate what combinations of frequencies and thus what vowels are present.