Dual-tone Multi-frequency Signaling - Special Tone Frequencies

Special Tone Frequencies

National telephone systems define additional tones to indicate the status of lines, equipment, or the result of calls with special tones. Such tones are standardized in each country and may consist of single or multiple frequencies. Most European countries use a single precise frequency of 425 Hz, where the United States uses a dual frequency system.

The tone frequencies, as defined by the Precise Tone Plan, are selected such that harmonics and intermodulation products will not cause an unreliable signal. No frequency is a multiple of another, the difference between any two frequencies does not equal any of the frequencies, and the sum of any two frequencies does not equal any of the frequencies. The frequencies were initially designed with a ratio of 21/19, which is slightly less than a whole tone. The frequencies may not vary more than ±1.8% from their nominal frequency, or the switching center will ignore the signal. The high frequencies may be the same volume as – or louder than – the low frequencies when sent across the line. The loudness difference between the high and low frequencies can be as large as 3 decibels (dB) and is referred to as "twist." The duration of the tone should be at least 70 ms, although in some countries and applications DTMF receivers must be able to reliably detect DTMF tones as short as 45ms.

European Tones:

As with other multi-frequency receivers, DTMF was originally decoded by tuned filter banks. Late in the 20th century most were replaced with digital signal processors. Although DTMF can be decoded using any frequency domain transform (such as the popular Fast Fourier transform), the Goertzel algorithm is a common algorithm to consider due to its high performance for DTMF.