B&K 4071 DTMF Detection Mode, DTMF detection on Ext Mod input 1 2 3 # A, DTMF detected digits

5.17 DTMF Detection Mode

Introduction

Dual Tone Multi Frequency detection mode decodes and displays the Touch-tone dialing tones according to ANSI T1.401-1988 Section 7.2 standard. Input test signal could be applied to the External Mod Input for DTMF decoding. Once a DTMF digit has been detected, it will appear on the LCD display and will also be sent to the RS-232 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. The DTMF detector in the 4071 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 30KO). 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 4071. Input levels above ±25V may damage the 4071. 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 of each dialing digit and its associated Dualtone frequencies could be listed in 5.16

DTMF Detection Mode Parameters

The DTMF Detection mode has the following front panel display:

1

DTMF detection on Ext Mod input:

1 2 3 # A 4

Figure 5.17-1: DTMF Detection mode display

1. DTMF detected digits

All detected digits appear on this line of the display. The display shows up to 40 detected digits.

When the 41st digit is detected, this line of the display is cleared and the new digit is printed on the left hand side. All detected digits are sent to the terminal port. By connecting a computer to the terminal port, the 4071 can be

used as DTMF detection peripheral. See the example program in Appendix A.

2. RS-232 Port

All DTMF detections are printed to the terminal port. When a DTMF digit is detected (the digit 6 is given here as an example), a carriage return and a linefeed plus the following text is sent to the terminal:

DTMF: 6

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BK Precision 4071 User Manual Rev.2.2