DTMF (Dual-Tone Multi-Frequency signaling)

May 20, 2023

DTMF, which stands for Dual-Tone Multi-Frequency signaling, is a telecommunication signaling technology used to send numerical codes or commands over a phone line. It is a type of in-band signaling, which means that the signal is sent over the same channel as the voice data.

DTMF was developed in the 1960s by Bell System engineers, and it quickly became the standard way to send commands and data over telephone lines. Today, DTMF is still in widespread use, and it is used for a wide variety of applications, including automated customer service phone systems, voicemail systems, and remote control of devices.

How DTMF Works

DTMF works by sending two simultaneous frequencies over the phone line. Each frequency is chosen from a set of eight frequencies, which are organized into four pairs. The pairs are chosen so that they are easily distinguishable from one another by both humans and machines.

The frequencies used in DTMF signaling are:

  • 697 Hz
  • 770 Hz
  • 852 Hz
  • 941 Hz
  • 1209 Hz
  • 1336 Hz
  • 1477 Hz
  • 1633 Hz

When a button is pressed on a DTMF keypad, the corresponding pair of frequencies is sent over the phone line. For example, if the “1” button is pressed, the pair of frequencies 697 Hz and 1209 Hz is sent.

DTMF signaling is designed to be robust and reliable, even in noisy and low-quality phone lines. The two frequencies are chosen to be far enough apart that they can be easily distinguished, but close enough together that they can be reliably transmitted over phone lines.

Applications of DTMF

DTMF is used in a wide variety of applications, including:

1. Automated Phone Systems

DTMF is used extensively in automated phone systems, where callers can interact with a computerized voice to access information or perform tasks. For example, a caller might be able to check the balance of their bank account, pay a bill, or schedule an appointment using DTMF commands.

2. Voicemail Systems

DTMF is also used in voicemail systems, where callers can listen to and manage their messages. DTMF commands are used to navigate through the menu system and perform tasks such as deleting or forwarding messages.

3. Remote Control

DTMF can also be used to remotely control devices. For example, a security system might be controlled using DTMF commands sent over a phone line, allowing the user to arm or disarm the system from a remote location.

4. Emergency Services

DTMF is also used by emergency services, such as police and fire departments, to communicate with dispatchers and other personnel. DTMF codes can be used to indicate the type of emergency, the location of the emergency, and other important information.

DTMF Keypads

DTMF keypads are designed to make it easy to send DTMF signals over a phone line. The keypad consists of twelve buttons arranged in a grid of four rows and three columns.

Each button corresponds to a particular pair of frequencies, and pressing the button causes those frequencies to be sent over the phone line. In addition to the ten numeric buttons (0-9), there are also two special buttons: the “*” button and the “#” button.

The “” button is used to indicate the start of a sequence of DTMF codes. For example, if you want to access an automated phone system, you might need to dial a phone number and then press the “” button followed by a sequence of DTMF codes.

The “#” button is used to indicate the end of a sequence of DTMF codes. For example, if you are leaving a voicemail message, you might need to press the “#” button when you are finished speaking to indicate that you are done.