Dual-tone multi frequency (DTMF) is the signal used on telephone systems for tone dialing. DTMF decoders are integrated circuits (ICs) that detect the tone frequencies used in DTMF and translate them to digits or commands that can interface with microcontrollers or computer systems. DTMF decoders allow devices to accept numerical input through a phone line or process keypad/dial pad input. This article provides an overview of how DTMF decoders work and the common ICs used.
How DTMF Encoders and Decoders Work
A DTMF tone is composed of two superimposed sinusoidal waves – one tone from a high frequency group and the other from a low frequency group. The high frequency group consists of 1209, 1336, 1477 and 1633 Hz tones while the low frequency group uses 697, 770, 852, and 941 Hz tones. By combining one frequency from each group, 16 unique f1/f2 tone pairs can be generated as shown below:
| High \ Low | 697 Hz | 770 Hz | 852 Hz | 941 Hz |
|---|---|---|---|---|
| 1209 Hz | 1 | 2 | 3 | A |
| 1336 Hz | 4 | 5 | 6 | B |
| 1477 Hz | 7 | 8 | 9 | C |
| 1633 Hz | * | 0 | # | D |
A DTMF encoder generates these tone pairs for dialing on a keypad. A DTMF decoder receives the tone input and detects the frequencies to determine the corresponding keypress digit or symbol.
The decoder measures the incoming signal and uses digital signal processing techniques like the Goertzel algorithm to isolate the high and low frequency components. By identifying which specific f1/f2 pair is present, the decoder can output the associated digit or command. Some DTMF decoders may have additional logic to handle timing between tones, recognize special key sequences, etc.
Common DTMF Decoder ICs
Here are some commonly used DTMF decoder ICs:
MT8870
One of the most widely used DTMF decoder chips, the MT8870 uses a digital counting technique to determine the DTMF tone frequencies. Key features:
- Decodes DTMF signals into 4-bit binary output
- Operates with low voltage 2.5V – 5.5V power supply
- Typical detect time: 35ms
- Integrated bandsplit filter separates high and low tones
- 28-DIP package
The MT8870 has two digital output buses – one for the high frequency and another for the low frequency component. By decoding which signals are active on the buses, the digit can be identified.
CM8870
The CM8870 is a clone of the MT8870 chip manufactured by various companies. It has nearly identical functionality and pinout:
- Decodes DTMF signals into 4-bit output
- Detects tones from keypad/dialpad input
- Built-in filter separates high and low frequency groups
- Similar 35ms detect time
- 2.5V – 5.5V operation
The CM8870 is offered in 28-DIP format like the original as well as smaller 20-SSOP surface mount packages for compact PCBs.
HT9170B
The Holtek HT9170B DTMF decoder uses digital counting methods and also incorporates some programmable options:
- Detect and decode all 16 DTMF signals
- Adjustable detect threshold and timing validation
- Mcros can read status via 2 control pins
- Valid signal output flag
- Early steering output mode before full decode
This IC allows adjustments like setting the minimum duration of a valid tone pair and early output signaling to the microcontroller when a valid tone is initially detected. The HT9170B comes in 18-DIP and 20-SOIC packaging.
MT88L70
MT88L70 is an upgraded version of the ubiquitous MT8870DTMF receiver. The new features are:
- Lower voltage operation with 1.8V – 5.5V supply
- Reduced power consumption in the microamp range
- Built-in dial tone rejection circuitry
- Improved decoding accuracy with fuzzy logic
The dial tone reject avoids false triggering and improved accuracy helps in noisy phone line conditions. It is available in space saving 16-SSOP package.
M-88D
The M-88D from PulseLarsen is an advanced DTMF decoder delivering high performance with accuracy exceeding standard telephone requirements.
Key features:
- Exceptional decoding accuracy in noise/distortion
- Detect time under 15ms
- Single-ended or differential input
- Built-in programmable dial tone rejection
- Early low, early high, steering output flags
- 20-QFN packaging
This DTMF receiver achieves fast and precise decoding even under challenging real-world phone line impairments. Its small QFN-20 surface mount package enables compact PCB implementation.
Interfacing DTMF Decoders to Controllers
DTMF decoders like the MT8870 and HT9170B provide digital outputs that can interface with microcontrollers and other logic devices to implement DTMF-based control or dialing interfaces.
Here is a typical connection schematic:
The audio signal from the phone line is fed into the DTMF receiver IC which processes the tone frequencies. When a valid tone pair is detected, the corresponding digit line (D0 to D3) is activated with a high logic output. The microcontroller periodically scans these digit output pins to determine which button was pressed on the keypad or dialpad.
Some additional control line connections may include:
- Valid Signal (VS) – indicates a valid tone is currently present
- Leading Edge (LE) – signals early detection of valid tone
- Trailing Edge (TE) – marks end of valid tone
These extra outputs allow the microcontroller to precisely time and validate the decoded tones. Some software development is required to handle interfacing with the outputs and performing actions based on dialed codes.
DTMF Decoder Applications
DTMF decoders are used in many systems that need to interpret touch-tone dialing or interface with DTMF-based control panels. Some examples include:
Telephone Switching – Decode DTMF tones from users to look up and dial numbers.
Voicemail Systems – Recognize numbers pressed when users access voicemail boxes to retrieve messages.
Ham Radios – Allow making calls over the air using touch-tone dialing.
Home Automation – Remotely control appliances, lighting, HVAC based on DTMF sequences sent over phone line or radio.
Security Alarm Panels – Press phone keypad to arm/disarm security system or enter passcodes.
PBX/IVR Systems – Users select menu options on automated phone response systems.
Amateur Robotics – Use a DTMF decoder to remotely steer and control robots by sending tone commands.
Control Toys – Add a dialing interface to toys by incorporating a DTMF decoder.
Conclusion
DTMF decoders continue to see wide use for interfacing telephone and remote control systems to microcontrollers. With their ability to convert the standard touch-tone signals to digital outputs, DTMF decoder ICs enable direct integration with logic circuits and programmable devices. Modern chips feature improved decoding accuracy and noise tolerance – making them suitable for even more applications.
Frequently Asked Questions
What is DTMF?
DTMF stands for Dual-tone multi frequency. It is the audible tone signaling used by touch-tone phones and dialpads to represent digits, symbols or commands. DTMF uses two distinct sinusoidal tone frequencies, one from a high group and the other from a low group, which are combined to transmit keypresses.
How does a DTMF decoder IC work?
A DTMF decoder IC uses digital signal processing to analyze the input tones and distinguish the lower and higher frequency components. It measures their spectral content to determine which specific f1/f2 pair is present and matches it to the corresponding dialpad digit, letter or symbol. This information is provided on digital output pins.
What are the main applications of DTMF decoder chips?
DTMF decoders are widely used for telephone switching and dialing interfaces. Other common applications include home automation (controlling devices remotely by phone), amateur robotics, security systems, toys, ham radios, and IVR/voicemail systems that require tone dialing input. They allow microcontrollers to interface with DTMF keypads, dialpads, touch-tone phones etc.
What are the key specifications of DTMF decoder ICs?
Important specifications include the detect time (under 50ms is common), acceptable frequency deviation, digital output format (number of bits), signal to noise tolerance, dial tone rejection capability, operating voltage range, and packaging type like DIP, SOIC, QFN etc. that determines PCB footprint.
What logic/interface is required to connect DTMF decoder to microcontroller?
The digital outputs of the DTMF decoder that represent the detected number/symbol are connected to input pins on the microcontroller. Additional interface logic may include filtering on the analog input and pull up/pull down resistors on the digital out lines. Software reads the digital inputs to determine pressed keys and perform programmed actions. Timers may be needed to validate minimum tone duration.
