BNC 630 - Manual (Page 35)

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5.17 DTMF Detection Mode
Introduction
The DTMF (Dual Tone Multi Frequency) Detection mode decodes and displays the Touch-tone dialing tones for the American
telephone network. Signals to be decoded are applied to the External Mod In jack. Once a DTMF digit has been detected, it will
appear on the LCD display and will also be sent to the RS232 terminal port.
The input signal must be 10 Vp-p or less. If the input signal exceeds 10 Vp-p, the waveform will distort and adversely affect the
DTMF detector. Input levels above 50 Vp-p may damage the 630. The DTMF detector in the 630 features an AGC algorithm which
boosts low level signals (20 mVp-p or greater) to detect low level DTMF tones.
For reliable detection, each DTMF digit must be present for a minimum of 100 mS. There must also be a minimum silent period
of 100 mS between each DTMF digit.
The Ext Mod In input is high impedance (about 30K ohms). If your system expects this signal to be terminated, place a resistor (of
the appropriate terminating resistance) across this input.
*** Caution ***
Do NOT connect the Ext. Mod In input directly to a telephone line. High voltages exist on a telephone line which will cause
permanent damage to the 630. See appendix A for a suitable telephone line interface.
Background
Each DTMF dialing tone consists of two sinusoids of different frequencies which are added together to form the output waveform.
These frequencies were chosen so that they are not harmonically related. The table below lists each dialing digit and its associated
dualtone frequencies:
DTMF 0: 941 Hz and 1336 Hz
DTMF 1: 697 Hz and 1209 Hz
DTMF 2: 697 Hz and 1336 Hz
DTMF 3: 697 Hz and 1477 Hz
DTMF 4: 770 Hz and 1209 Hz
DTMF 5: 770 Hz and 1336 Hz
DTMF 6: 770 Hz and 1477 Hz
DTMF 7: 852 Hz and 1209 Hz
DTMF 8: 852 Hz and 1336 Hz
DTMF 9: 852 Hz and 1477 Hz
DTMF
∗
: 941 Hz and 1209 Hz
DTMF #: 941 Hz and 1477 Hz
DTMF A: 697 Hz and 1633 Hz
DTMF B: 770 Hz and 1633 Hz
DTMF C: 852 Hz and 1633 Hz
DTMF D: 941 Hz and 1633 Hz