Comparing dial-tone signals

Article By : John Dunn

The first dial tone from 1954 is not the same as the sound we are familiar with today.

Telephone dial tones are one aspect of modern life that we take for granted, but there is a history there. Wikipedia offers some interesting reading on the subject.

Back in 1954 when Bell Telephone, semi-affectionately remembered as “Ma Bell,” first introduced dial tones, they made an instructional video:

At one point in the film, at 1:58 in the YouTube video above, there is a five-second sample of what a dial tone sounded like back in 1954. It is the sound that I remember from my childhood, but it is NOT the sound that we are familiar with today. (I really do suggest that you watch the entire film; you will be astonished at the differences that 66 years can make.)

The modern dial tone in North America consists of two simultaneous sinusoidal signals. One tone is at 350 Hz and the other is at 440 Hz. The sketch in Figure 1 illustrates this.

sketch of the simultaneous sinusoidal signals of the modern dial toneFigure 1 The modern dial tone consists of two simultaneous sinusoidal signals.

The 440 Hz sinusoid and the 350 Hz sinusoid are seen in the top two traces, while the sum of those tones is illustrated in the two examples below. An envelope tracing is highlighted in each example.

These two tones at 350 Hz and 440 Hz are the equivalent of a double-sideband (DSB) suppressed-carrier signal centered on the suppressed carrier frequency of 395 Hz, from which frequency we find two sidebands, one at 45 Hz above the 395 Hz suppressed carrier and the other at 45 Hz below the 395 Hz suppressed carrier. The 45 Hz envelope sinusoid of example 1 is the modulating signal, which would be applied to a 395 Hz carrier in DSB generation.

By contrast, the full-wave rectified sinusoid envelope of example 2 yields a hearing perception of 90 Hz and harmonics of that frequency plus the higher pitches of the two original sinusoids.

Here is a sample of a modern dial tone:

Trust me when I say from personal experience that you do not want to sit and listen to that thing for any long period of time, but there it is and it can be compared to the 1954 dial tone as follows.

scope comparison of dial tone signalsFigure 2 This is a composite of two scope photographs to compare the original and modern dial tones.

This picture is a composite of two different photographs and I uncalibrated the scope’s time base in both cases so that the dial tone waveforms could be more easily seen. Please note that the modern one matches example 2 from Figure 1.

John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).

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