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SIGNAL GENERATION CIRCUITS
Application Note 97: Jul 09, 1998
Telephone Tone Generator Requires No Trimming
Many products that connect to phone lines (modems, for instance) incorporate a "callprogress monitoring" function known as CPM. CPM circuits "listen" to the lines as a human would, and respond according to what they "hear." You shouldn't dial a number unless you first hear a dial tone, for example. Neither should your computer. Tone accuracy is not very important when people monitor the call-progress tones, but the use of computers for this purpose has produced a need for tone-accuracy specifications to prevent errors in interpretation.
AN97 Datasheet
