DTMF stands for ‘Dual Tone Multi Frequency’. This is a system used for dialing numbers in the telecommunications industry. Numbers consist of a pair of tones transmitted at the same time. The duration and relative amplitude of these tones has been selected to minimise the probability of these occuring within normal human speech. A measure of the performance of a DTMF decoder is the rate of false detections over a period of time, this parameter is called “talk off”.
The frequencies of the DTMF digits are in accordance with the table below.
| Digit | Low Tone | High Tone |
|---|---|---|
| 1 | 697 | 1209 |
| 2 | 697 | 1336 |
| 3 | 697 | 1477 |
| 4 | 770 | 1209 |
| 5 | 770 | 1336 |
| 6 | 770 | 1477 |
| 7 | 852 | 1209 |
| 8 | 852 | 1336 |
| 9 | 852 | 1477 |
| * | 852 | 1209 |
| 0 | 852 | 1336 |
| # | 852 | 1477 |
| A | 697 | 1633 |
| B | 770 | 1633 |
| C | 852 | 1633 |
| D | 941 | 1633 |
The minimum tone duration is 65mS, there is no maximum duration. A tone is defined as present when it is greater than 90% of its nominal amplitude.
The minimum interdigit gap where no tones are generated is 65mS. Again there is no maximum period. A gap is defined as when the tones are less than 10% of its nominal amplitude.
The high tone shall be between -7.0dBV and -11.0dBV in amplitude. The low tone shall be between -8.5dBV and -13.0dBV in amplitude. The high tone shall be higher in amplitude than the lower tone by between 1.0dB and 4.0dB. This is called the “twist”. Any interfering (unwanted) signals (example speech) shall be at least 20dB lower in amplitude than that of the low tone.
These parameters are what are required for telecommunications equipment to operate within Europe and are in accordance with EC Directive 91/263/EEC and defined in TBR 21:1998