Here’s a side by side comparison of GEdney-9 dial tone versus “Modern” dial tone: Ultimately, as the line is hung up, rich background sounds can be heard in this very high fidelity recording from Evan’s library: Can you hear how different from “Modern” dial tone it sounds? There’s a lot happening in the background. For dramatic effect, after about 50 seconds it transitions to another recording of dial tone, timing out you can hear the panel revertive pulsing in the background as the call is connected to the permanent signal holding trunk. This next clip is a classic example of what is known as “Old City Dial Tone”. Evan Doorbell’s library has high quality recordings of calls dialed from this central office in the 1970’s. GEdney-9 was a wonderful panel exchange in New York City. However, before dial tone was standardized like this, it could sound very different depending on where you were calling from. From the above, you can see and hear how pure and simple it is. When first introduced, it was something new and modern and quite literally signaled the future of telephony. In the bottom trace, the two pure tones have been literally added together mathematically to form the harmonic waveform of dial tone a larger rhythm now encompasses several smaller peaks within. The trace on the top is 350 cycles per second and the one in the middle is 440. If you look at the waveform of these sounds closely with an oscilloscope or audio editor, here’s what they look like: This clip plays each frequency separately, and then both blended together creating the pleasant harmony of dial tone: So-called “Modern” dial tone is a combination of 350Hz and 440 Hz. Fifty years ago, when TouchTone service was introduced, the Phone Company established a set of precise frequencies that would be used to create standard sounds for ringback, busy and dial tones.
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