Dual-Tone Multi-Frequency Decoder

45 DUAL-TONE MULTI-FREQUENCY DECODER source: Teltone Corporation design: Robert Krijgsman PE ICHY The dual-tone multi-frequency (DTMF) dialling scheme for telephone networks was originally developed by Bell Laboratories and introduced in the United States in the mid-1960s as an alternative to pulse dialling. Offering increased speed, improved reliability, and the convenience of end-to-end signalling, DTMF has since been adopted as standard and recommended for use by telecommunications organizations such as CCITT, CEPT, NTTPC, and others around the world. The versatile DTMF decoder described here is a state-of-the art design based on Teltone"s single-chip DTMF receiver Type M957. The DTMF system has been designed to convey dialling information from a sub- scriber to his local exchange. Since the DTMF keypad is not disabled during the call, the tones may also be sent after the call has been answered. This makes possible a number of interesting applica- tions related to remote signal...