Report
on
Wireless Data Transmission Using DTMF
Designed By:
MOHD KASHIF
Department of Electrical & Electronics Engineering
Krishna Institute of Engineering & Technology
DTMF (Dual Tone Multiple Frequency)
In telecommunication, a caller needs to dial the number of the callee. The earlier versions of
telephones used to have rotary type dials which are now obsolete. Almost all the landline and
mobile phone handsets now use pushbutton keypads.
What is DTMF?
DTMF is a signalling system for identifying the keys or better say the number dialled on
a pushbutton or DTMF keypad. The early telephone systems used pulse dialling or loop
disconnect signalling. This was replaced by multi frequency (MF) dialling. DTMF is a multi
frequency tone dialling system used by the push button keypads in telephone and mobile sets
to convey the number or key dialled by the caller. DTMF has enabled the long distance
signalling of dialled numbers in voice frequency range over telephone lines. This has
eliminated the need of telecom operator between the caller and the callee and evolved
automated dialling in the telephone switching centres.
DTMF (Dual tone multi frequency) as the name suggests uses a combination of two sine
wave tones to represent a key. These tones are called row and column frequencies as they
correspond to the layout of a telephone keypad.
A DTMF keypad (generator or encoder) generates a sinusoidal tone which is mixture of the
row and column frequencies. The row frequencies are low group frequencies. The column
frequencies belong to high group frequencies. This prevents misinterpretation of the
harmonics. Also the frequencies for DTMFare so chosen that none have a harmonic
relationship with the others and that mixing the frequencies would not produce sum or
product frequencies that could mimic another valid tone. The high-group frequencies (the
column tones) are slightly louder than the low-group to compensate for the high-frequency
roll off of voice audio systems.
The row and column frequencies corresponding to a DTMF keypad have been indicated in
the above figure.
DTMF tones are able to represent one of the 16 different states or symbols on the keypad.
This is equivalent to 4 bits of data, also known as nibble.
DTMF (Dual Tone Multiple Frequency)
Most DTMF decoders can process at least 10 tones per second under the worst of conditions,
so DTMF can easily convey 40 (10 x 4) bits or 5 bytes of data per second which is nowhere
near to the performance of a good communication modem, which can operate nearly 600
times faster (28,800 bits per second). But DTMF signalling is lot more robust under noisy
line conditions.
It should be noted that the numbers and symbols on the keypad do not always match the
binary values of DTMF decoders. Most notably, the ‘0’ on the keypad is represented in
DTMF by a decimal value of 10 or binary value of 1010.
The binary codes corresponding to symbols in a DTMF keypad are listed below.
DECIMAL VALUE
BINARY VALUE
KEYBOARD SYMBOL
0
0000
D
1
0001
1
2
0010
2
3
0011
3
4
0100
4
5
0101
5
6
0110
6
7
0111
7
8
1000
8
9
1001
9
10
1010
0
11
1011
*
12
1100
#
13
1101
A
14
1110
B
15
1111
C
Applications
DTMF tones are thus mainly used at the telephone switching centres for detection of
dialled/called number. They are also used by certain radio and cable TV networks. These
networks use DTMF tones to signal a network station or local cable operator when a local
advertisement is to be inserted or for station identification. In broadcasting, this is known as
local insertion. DTMF tones were also used by terrestrial stations for turning on and shutting
off remote transmitters.
DTMF Decoder
Pin out of DTMF decoder IC M-8870
DTMF (Dual tone multiple frequency) is the most popular and nowadays ubiquitously used
telecommunication signalling method. A DTMF decoder detects the DTMF tones and
generates the binary sequence corresponding to key pressed in a DTMF keypad. The circuit
of this project presented here is a DTMF decoder. DTMF keypads are used in almost all
landline and mobile handsets. The DTMF decoders, therefore, are used at the telephone
switching centres to detect the number dialled by the caller. The DTMF version used in
pushbutton telephones is called touch tone and is a registered trademark of AT&T.
The circuit of this project uses a DTMF decoder IC (HT9170). The DTMF tones are
generated by the keypad of a cell phone or by the computer software available on www.polarelectric.com.
The tones generated from the speaker (audio signals) of keypad are given to microphone
which converts audio tones into electrical signals. The signals from the microphone are
processed by the DTMF decoder IC which generates the equivalent binary sequence as a
parallel output.
The electrical signals from microphone (mic) are fed to inverting output (pin2; VN) of Opamp, present in IC, through a series of resistance and capacitance of value 100 k and 0.1 µF
respectively. The non-inverting input pin (pin1; VP) of Op-amp is connected to pin4 (VREF).
The voltage at VREF pin is VDD/2. A feedback signal is provided, by connecting the output of
Op-amp (pin3; GS) to inverting input pin (pin2; VN) through a resistor R2 (100 k ).
The output of Op-amp is passed through a pre filter. The output from this pre filter is then
supplied to low group and high group filters. These filters consist of switched capacitors and
divide DTMF tones into low and high group signals. High group filters pass the high
frequencies while low group filter pass low frequencies. These frequencies are then passed
through frequency detector and code detector circuits. Finally the four-digit code is latched
on the output pins of HT9170. The output on these pins has been used to drive a set of
four LEDs.
The whole process, from frequency detection to latching of the data, is controlled by steering
control circuit. So it is very important part of the whole process and is mainly controlled by
RT/GT and DV pins of DTMF decoder IC. RT/GT pin is connected to VDD through a
capacitor of 0.1 µF. The EST pin is connected to RT/GT pin through a resistor of 300k .
Pin6, PWDN pin (active high) inhibits the working of oscillator thereby stopping the working
of circuit; and Pin5, INH pin (active high) inhibits detection of the tones of character
A,B,C,D. The pin 10; OE (output enable; active high) enables the latching of the data on the
data pins. Pin15; DV (Data valid pin) becomes high on detection of valid DTMF tone
otherwise it remains low. An oscillator of frequency 3.579545 MHz is connected between
pins 7 and 8.
Basic principle and use of DTMF in communication
What is DTMF (Dual Tone Multi Frequency)? DTMF is a common communication term for
touch tone phones. The tones formed when dialing on the keypad (DTMF digits or DTMF
number) on the phone can be used to characterize the digits, and some different tone is used
for each digit. DTMF signaling is mainly used in DTMF phones, telephone switching system.
Yet, there is always a possibility that a random sound will be on the similar frequency which
will trip up the DTMF sounds system. It was recommended that if two tones were used to
represent a digit, the probability of a false signal happening is ruled out, thus the name ‘Dual
Tone’. This is the basis of using dual tone in DTMF communication. DTMF dialing uses a
keypad with 12 or 16 buttons. Each key pressed on the keypad generates two tones of
particular frequencies, so a voice or a random signal cannot mimic DTMF signaling tones.
One tone is generated from aHigh DTMF frequency group of tones and the other from Low
DTMF frequencygroup.

When a button is pressed, both the row and column tones are generated by the
telephone or touch tone instrument.

These two tones will be distinctive and different from tones of other keys. So there is
a low and high frequency associated with a button, it is essentially the sum of two
waves is transmitted.

This elementary principle can be extended to a range of applications.
Probe further: How to decode DTMF tones?
Row and Column frequency correspondence
The frequencies generated on pressing different phone keys are shown in the Table
DTNF
Frequency
1209
1336
1447
697
1
2
3
770
4
5
6
852
7
8
9
941
*
0
#
Each row and column of the keypad corresponds to a certain tone and creates a specific
frequency. Each button lies at the intersection of the two tones.
DTMF Frequencies generated on Key press
The frequencies generated on pressing different phone keys are shown in the Table.
Button
Low DTMF
frequency
(Hz)
High DTMF
frequency
(Hz)
1
697
1209
2
697
1336
3
697
1477
4
770
1209
5
770
1336
6
770
1477
7
852
1209
8
852
1336
9
852
1477
0
941
1336
*
941
1209
#
941
1477
DTMF signals can be transmitted over a radio to switch on or switch off home appliances,
flash lights, motors, cameras, warning systems, irrigation systems and so on. These encoded
data can be stored and processed in a microcontroller to perform different tasks.
Components required
1. DTMF decoder IC (M-8870)
2. Resistors (100kΩ; 70kΩ; 390kΩ)
3. Capacitors (0.1µFx 2)
4. Crystal oscillator (3.579545MHz)
What is the Need of DTMF Decoding?
In the premature days, our telephone systems were operated by human operators in a
telephone exchange room. The caller will pick up the phone, giving instruction to the
operator to connect their line to the destination. It is a kind of manual switching. As more and
more people entered in the telephone technology as useful communication gear, manual
switching becomes a time consuming tedious task.
As technology established, pulse or dial tone technique was invented for telephone
communication switching. It employs electronics and computer to support switching
operations. DTMF (Dual Tone Multi Frequency) is the ultimate technique used in any of the
Mobile, Telephone communication systems.
The operation of DTMF method as follows.

Caller generates a dial tone consisting of two frequencies. It is transmitted via the
telephone line (communication media).

Telephone exchange consists of a DTMF decoder, which decodes the frequencies in
to digital code.

These codes are the address of destination subscriber; it is read and processed by a
computer and connect caller to the destination subscriber.
Working of DTMF decoder circuit

DTMF (Dual Tone Multi Frequency) keypads are employed in almost all landline and
mobile handsets. Thus, DTMF (Dual Tone Multi Frequency) technology used in the
telephone switching centers to identify the number dialed by the caller.

A DTMF (Dual Tone Multi Frequency) decoder distinguishes the DTMF tones and
produces the binary sequence equivalent to key pressed in a DTMF (Dual Tone Multi
Frequency) keypad.

The circuit uses M-8870 DTMF decoder IC which decodes tone generated by the
keypad of cell phone.

DTMF signal can be tapped directly from the microphone pin of cell phone device.
Cut the microphone wire and you will get two wires red and green. The red wire is the
DTMF input to the circuit.

The signals from the microphone wire are processed by the DTMF decoder IC which
generates the equivalent binary sequence as a parallel output as Q1, Q2, Q3, and Q4.
Table showing DTMF Low and High frequency tones and decoded output
Button
Low DTMF
frequency
(Hz)
High DTMF Binary coded
frequency
output
(Hz)
Q1 Q2 Q3 Q4
1
697
1209
0
0
0
1
2
697
1336
0
0
1
0
3
697
1477
0
0
1
1
4
770
1209
0
1
0
0
5
770
1336
0
1
0
1
6
770
1477
0
1
1
0
7
852
1209
0
1
1
1
8
852
1336
1
0
0
0
9
852
1477
1
0
0
1
0
941
1336
1
0
1
0
*
941
1209
1
0
1
1
#
941
1477
1
1
0
0

There is an inbuilt Op Amp present inside the M-8870 DTMF decoder IC. The
electrical signals from microphone pin are fed to inverting input of the Op Amp via a
series of resistance (100kΩ) and capacitance (0.1 µF).

The non inverting input of Op-amp is connected to a reference voltage (pin4 -VREF).
The voltage at VREF pin is Vcc/2.

Pin 3 (GS) is the output of internal Op Amp, the feedback signal is given by
connecting the output pin (pin3- GS) to inverting input pin (pin2- IN-) through a
resistor (270kΩ).

The output of Op Amp is passed through a pre filter and low group and high group
filters (Filter networks). These filters contain switched capacitors to divide DTMF
tones into low and high group signals (High group filters bypass the high frequencies
whereas low group filter pass low frequencies).

Next processing sections inside the IC are frequency detector and code detector
circuits. Filtered frequency passed through these detectors.

At last the four digit binary code is latched at the output of M-8870 DTMF decoder
IC.
Uses of other pins:

The entire process, from frequency detection to latching of the data, is controlled by
steering control circuit consisting of St/GT, Est pins, resistor (390kΩ) and a capacitor
(0.1µF).

5th Pin, INH is an active high pin, inhibits detection of A, B, C, D tones of character.

6th Pin, PWDN is an (active high) inhibits the working of oscillator thus stops the
working of our circuit.

The 10th pin 10; TOE is the output enable pin which is active high logic and enables
the latching of the data on the data pins Q0, Q1, Q2, and Q3.

15th Pin StD is the Data valid pin, turn out to be high on detection of valid DTMF
tone or else it remains low.

Pins 7 (OS1) and 8 (OS2) are used to connect crystal oscillator. An oscillator of
frequency 3.579545 MHz is used here.