An all digital feature based pattern recognition tone detection system distinguishes one or more tones from speech or other background noise and provides extremely reliable, low cost DTMF tone detection. The system includes a digital signal processing system which uses a modified PARCOR signal analysis...http://www.google.com.hk/patents/US4689760?utm_source=gb-gplus-share專利 US4689760 - Digital tone decoder and method of decoding tones using linear prediction coding
