Unless you’re a phone tech (and I know there are more than a few of those who read PCMech), you know what touch-tone dialing is, but don’t know what DTMF is.

DTMF is dual-tone multi-frequency. It is what replaced pulse dialing as the standard for how telephone calls are placed, and is what allows touch-tone dialing to work.

Each time you press a key on your phone, you are not hearing one tone but actually a chord of two. It was developed this way to avoid accidental telecommunication signals through a handset from single tones, such as you whistling. The system only recognizes tones produced by DTMF by design.

Pulse dialing originally operated by mechanical means using what was known as Strowger switches. These did work, but due to their mechanical nature required a ton of maintenance just to keep them operational. DTMF was a phenomenal improvement in how phone calls were made. It not only placed calls much faster but also required far less maintenance for the Telco to keep the system operational because it allowed them to dump all those Strowger switch mechanisms.

For those of you old enough to remember Ma Bell, it’s a good bet that a big chunk of ridiculous prices you were paying for phone service back in the day went towards switch maintenance and repair. Or it could have been the Telco’s just being greedy. Or both. We don’t know for sure.

The video below from 1989 demonstrates Strowger switch pulses, DTMF tones and how each works in a basic sense.

I’m old enough to remember pulse dialing just as it was being phased out, and after seeing this now truly understand why it took so f*#$%ng long for a call to connect across a state line when using pulse.

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