INTERPHONE DEVICE FOR SIMULTANEOUS COMMUNICATION

摘要

PURPOSE:To realize suppressing of howling using simple constitution by measuring frequency of generated howling and using a variable frequency filter that can remove specific frequency and making frequency characteristic of the filter follow measured frequency. CONSTITUTION:A part of output signal of an amplifier 2 is inputted to a frequency measuring circuit 3 and a period in which audio signal becomes a fixed level is measured. As the period is indefinite in the case of audio signal, it is not regarded as howling. As sine wave is generated when howling occurs, the period in which the signal becomes larger than a fixed level becomes constant. From this fact it is detected as howling and the frequency is measured. Cut-off frequency of a filter 5 is controlled by a frequency controlling circuit 4 to make the cut-off frequency coincide with measured frequency of howling using measured output.