Pulse Dialing vs. Tone Dialing: Understanding the Differences

This page explains the basics of pulse dialing and tone dialing, highlighting the key differences between the two methods used to communicate telephone numbers to a telephone exchange (telephone company).

As we know, there are two primary ways to inform the telephone exchange of the number you wish to dial: pulse dialing and tone dialing.

Pulse dialing, the older method, is now largely obsolete. Tone dialing is the prevalent and widely used method today.

Pulse Dialing

To use pulse dialing, you need a rotary dial telephone. The dial generates pulses on the local loop by opening and closing an electrical switch when the dial is rotated and released.

For example, dialing the number ‘3’ generates 3 pulses, as shown in the figure above. It takes a total of 0.3 seconds to transmit all 3 pulses (6 x 0.05 seconds = 0.3 seconds). Each pulse is caused by an interruption in the current flow on the local loop for a duration of 0.05 seconds. There’s a 0.05-second pause between each pulse.

Tone Dialing (DTMF)

To use tone dialing, you need a touch-tone telephone. These phones have a DTMF (Dual-Tone Multi-Frequency) keypad, as shown in the figure above.

Most telephones have a DTMF keypad with 12 keys, mapping the numbers 0 to 9 and special symbols like * and #. Some keypads also include 4 additional keys for specific applications. Each key represents two audio frequencies associated with its row and column.

For example, if you press the ‘5’ key, the audio frequency pair (770 Hz, 1336 Hz) is generated and transmitted to the telephone exchange. These tones are recognized at the telephone central office (CO). The tolerance on these frequencies must be within +/- 1.5 percent.

Pulse Dialing vs. Tone Dialing: Key Differences

The primary difference between pulse dialing and tone dialing is speed. Pulse dialing takes considerably longer to dial digits compared to the faster tone dialing method.

For example, dialing the number ‘9’ using pulse dialing takes about 0.9 seconds (calculated as 0.05 seconds x 18 = 0.9 seconds).