DTMF

DTMF (Dual-Tone Multi-Frequency) is a signalling system used by telephone networks to represent keypad input as pairs of simultaneous audio tones. It was developed by Bell Labs in the United States and introduced commercially in November 1963 under the registered trademark Touch-Tone. DTMF replaced the older pulse dialling system and became the worldwide standard for telephone keypad signalling.

Background

Before DTMF, telephone networks used pulse dialling, where pressing a digit caused a series of electrical interruptions corresponding to the number dialled. This system was slow, prone to errors, and unsuitable for automated systems. DTMF was developed at Bell Labs in the late 1950s as a faster and more reliable alternative. The system was first shown publicly at the 1962 World's Fair and became commercially available in November 1963.

The layout of the keypad, including the placement of digits with 1-2-3 on the top row rather than the bottom as on a calculator, was determined through empirical human-factors research led by industrial psychologist John E. Karlin at Bell Labs.

How it works

Each key on a telephone keypad is associated with two simultaneous audio tones: one from a set of four "row" frequencies and one from a set of four "column" frequencies. Pressing a key causes the telephone to transmit both tones at once over the voice channel. The receiving equipment decodes the combination of frequencies to identify which key was pressed.

The frequencies were chosen carefully to avoid harmonics and intermodulation products that could cause false decoding. No frequency is a multiple of another, and neither the sum nor difference of any two frequencies equals any other frequency in the set. This design makes DTMF highly resistant to accidental triggering by human speech on the same audio channel.

DTMF frequency matrix

	1209 Hz	1336 Hz	1477 Hz
697 Hz	1	2	3
770 Hz	4	5	6
852 Hz	7	8	9
941 Hz	*	0	#

Use in Payphone Tag

DTMF tones are used in two ways in Payphone Tag:

Normal use

When an agent dials the game number or enters their PIN on a payphone keypad, the payphone generates DTMF tones that travel over the phone line and are decoded by the Operator to register the input. This is how all standard Payphone Tag calls work.

Damaged keypad workaround

If a payphone's keypad is damaged or non-functional, an agent can generate the necessary DTMF tones externally using a tone generator app on a mobile phone. The mobile phone speaker is held close to the payphone handset's mouthpiece, and the tones are played into the microphone. The payphone then transmits them over the line to the Operator as if they had been entered on the keypad directly.

Volume calibration is important: if the mobile phone is too loud, the tones distort and may fail to decode; if it is too quiet, the payphone microphone may not detect them reliably. Adjusting volume while listening through the payphone earpiece is the recommended approach to finding the right level.

See also

• PIN
• Operator
• Phonecard
• Gameplay