Introduction

Dual-Tone Multiple-Frequency (DTMF) signaling is used in telephone keypad tone generation. It is accomplished by mixing two different sine waves of well-defined (see Fig. 1) frequencies and amplitudes to generate a composite waveformfor each of the keypad positions (note, the keys in the far right column are not normally shown on the standard telephone keypad.)

Specifications:

The following characteristics are defined for DTMF waveforms:

• Frequencies must be within 1.5% of the specified values
• Column level is +2 dBm on Row level (i.e. Row = 0.794 Column)
• Pulse: 50 ms signal / 50 ms dead time (minimum)
• Total Level: 0 dBm into 600 W (24.5 mV into 600 W )
• Individual spurs/harmonics: -33 dBm from 300 Hz to 3.400 kHz, falling 12 dB/octave through 50 kHz, -80 dBm at 50 kHz
• Total spur/harmonic power: -20 dBm referenced to Row power level

Arbitrary Waveform Construction:

While it is possible to construct a DTMF tone by generating two sine waves and then combining them through a summing node, an arbitrary waveform generator makes the task easier and removes the need for two sources and a summing network. The following example will use the commercially available HP 33120A function/arbitrary waveform generator (see Fig. 2.)

There are several important points to consider when using an arbitrary waveform generator (ARB) to generate DTMF tones. First, the "end-point discontinuity" must be minimized. Such a discontinuity exists when the start point and the end point of the arbitrary waveform do not match, as when a non-integer number of cycles is used. An end-point discontinuity will add high-frequency spurs and harmonics, which can be heard as a repetitive popping on the signal. Second, in the case of the HP 33120A, it is best to use all 16,000 available arbitrary waveform points to minimize time-quantization error.

To construct the waveforms, first round off the row and column frequencies to the nearest "10" position (see Fig. 3) and as the DTMF specification allows up to 1.5 percent deviation the largest rounding-off error in this case will be 0.4 percent.

The next step is to design eight waveforms -- one for each frequency. Each waveform will be constructed of 16,000 points, and will contain multiple cycles of the tone. For example, the 1630 Hz tone will contain 163 cycles in the 16,000 points that define the waveform (approximately 100 points per cycle.) Setting the output frequency to 10 Hz will then reproduce the 1630 Hz signal.

This type of waveform can be generated using the HP BenchLink ARB software. Simply generate 163 cycles of a sine wave, and then expand the 163 cycles to fill up all 16,000 waveform points using the "Expand to Fit" command. Alternatively, these values can be generated programmatically. The following command sequence will generate the 163 cycles of sine wave with 16,000 points:

FOR X=1 TO 16000
WF1(X) = SIN (2*PI*X*163/16000)
NEXT X

Keep in mind that the specification that the Column frequencies must have an amplitude of 1.000 to Row frequencies’ amplitude of 0.794.

After creating all eight pure tones, they can be mathematically added together to create the DTMF signal required. The ARB software has a MATH pull-down menu for adding any two waveforms. Alternatively, this can be done programmatically by adding the two arrays in a "For-Next" loop, and then downloading the resulting signal to the HP 33120A and outputting the waveform.

Up to four 16,000-point arbitrary waveforms can be stored in the HP 33120A’s non-volatile memory at a time. It takes approximately 100 ms to select and output a new arbitrary waveform in this manner. If more than four waveforms are required for a given application, they can be downloaded from a PC as required. If a tone burst is required, the user can take advantage of the instrument’s burst modulation capability. For example, setting the burst count to 1 cycle and the burst rate to 1 Hz can generate a 100 ms burst of tones every second.

While this example has dealt specifically with the generation of DTMF signals for the frequencies defined for a touch-tone phone, the technique can be applied for any multi-frequency waveform. Once downloaded to the HP 33120A, the frequency and amplitude of the output can be adjusted without altering the basic shape of the waveform. The output can even be pulsed, if desired, at this point.

Biographical Sketch:

Mark D. Bailey is the voltmeter product manager at Hewlett-Packard Company’s Electronic Measurements Division. Bailey joined HP in 1983, when he worked in service support and then in on-line field sales support. He has now been involved in customer research, product definition, and several product launches, including the HP 34401A Digital Multimeter, the HP 33120A Function Generator, and, most recently, the HP LogicDart Advanced Logic Probe.

Prior to joining Hewlett-Packard, Bailey worked for nine years as an R&D design engineer for Tektronix, Inc., where he was project leader on several different instruments; he holds a BS in Electrical Engineering, which he earned at Brigham Young University. He also owns MDB Sound, a business specializing in sound reinforcement.

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