21-08-2013, 04:46 PM
Applications of The MT8870 Integrated DTMF Receiver
Introduction
The purpose of this Application Note is to provide information on the operation and application of DTMF Receivers. The MT8870 Integrated DTMF Receiver will be discussed in detail and its use illustrated in the application examples which follow.
More than 25 years ago the need for an improved method for transferring dialling information through the telephone network was recognized. The traditional method, Dial pulse signalling, was not only slow, suffering severe distortion over long wire loops, but required a DC path through the communications channel. A signalling scheme was developed utilizing voice frequency tones and implemented as a very reliable alternative to pulse dialling. This scheme is known as DTMF (Dual Tone Multi- Frequency), Touch-Tone™ or simply, tone dialling. As its acronym suggests, a valid DTMF signal is the sum of two tones, one from a low group (697-941Hz) and one from a high group (1209-1633Hz) with each group containing four individual tones.
Development
Early DTMF decoders (receivers) utilized banks of bandpass filters making them somewhat cumbersome and expensive to implement. This generally restricted their application to central offices (telephone exchanges).