In a portable telephone, a voice filter is connected to an antenna through a receiving circuit. A filter is connected to the voice filter through a switching circuit and filters the voice signal from the voice filter to produce a filtered voice signal. The filter has a first cut-off frequency when the...http://www.google.es/patents/US5933769?utm_source=gb-gplus-sharePatente US5933769 - Portable telephone capable of decreasing noise
