Dtmf Signals And Speech In Telephony Computer Science Essay

Dual Tone Multi Frequency ( DTMF ) is an electronic signalling criterion presently used in push button telephone dialling. This method is normally used in synergistic telephone systems such as Telephone banking, Tele selling and Interactive control services offered by service suppliers. DTMF that is used in modern telephone for tone dialling is known as Touch-Tone, was foremost used by AT & A ; T as a registered hallmark which was standardized by the Bell Laboratories. DTMF criterions were so redefined by the International Telecommunication Union ( ITU ) Q.23, which introduced the DTMF signalling as an efficient option to pulsate dialling system in telephone.

The DTMF signal is composed utilizing two overlying sinusoidal wave forms. Every cardinal imperativeness can be represented as a map below, where N is defined as figure of samples taken.

dt [ n ] = wickedness [ I‰Ln ] + wickedness [ I‰Ln ] , 0 a‰¤ n a‰¤ N-1

























High Frequencies

Low FrequenciesThese two moving ridge signifiers are chosen from a set of eight frequences. The eight frequences are from two sets of high and low frequence groups. The frequence allotment is as follows.

Figure 1.1 Standard keys and matching frequences

The frequences used were selected in such a manner that no two frequences are built-in multiples of each other. This characteristic leads to accurate sensing of cardinal imperativenesss by the shift web, even when noise is high in the communicating medium. User friendliness, truth and efficiency of the DTMF method established its use in communicating systems worldwide.

The tabular array 1.1 states the parametric quantities in the AT & A ; T standard These DTMF criterions are important to formalize and decently decrypt a DTMF signal even in extremely noisy having condiA­tions. If the parametric quantities are satisfied by the input signal so the Detected DTMF signal is stated as a valid DTMF cardinal imperativeness.



Signal frequence

Low group

697,770,852,941 Hz

High group

1209,1336,1477,1633 Hz

Frequency tolerance


= & lt ; 1.5 %


= & gt ; 3.5 %

Power degrees per frequence


0 to -25 dBm


Max -55 dBm

Power degree difference between frequences

+4dB to – 8 dubnium

Signal continuance


Min. 40 MS


Max. 23 MS

Table 1.1 DTMF parametric quantities by AT & A ; T standard [ 4 ]

2. Purposes and Aims

As the undertaking subject illustrates, favoritism procedure between DTMF signals and address signals in telephone is the chief intent of this undertaking. The concluding result of the undertaking should acquire a information watercourse consisting of DTMF signals, added address and noise as the input. The system should make the noise and speech favoritism procedure from the information watercourse.

A graphical user interface should expose the keys pressed in relevancy with the DTMF tone as the end product. The system should see about the truth of the DTMF sensing procedure and efficiency of the overall system.

The aims of this undertaking are as follows

Discover the features of the DTMF pulsations and comparison, contrast them with the features of the address signal in telephone.

Investigate the efficient filtering map that can be used to filtrate out noise and speech signal accurately

Select and usage of an efficient DTMF sensing algorithm

Discrimination of DTMF tones from normal telephone and Validating the DTMF signal

Implementation DTMF coevals, encoding, signal sensing and placing the right cardinal imperativeness

Implementing the GUI

3. Scope

The undertaking boundary lines are defined sing the inputs and end products and procedures of the DTMF sensing system. The chief input to the system is continues informations watercourse dwelling the DTMF signal. The system consists of a DTMF generator besides. Main system procedures are noise filtering, speech favoritism and DTMF sensing. The chief end product of the system is the figure which was pressed by the user, in this instance the generated DTMF signal.

4. Overview of the system

Column FrequencyCalculation

Row Frequency Calculation

DTMF signal generator









Finding the right cardinal imperativeness

Band base on balls Filter

Frequency sensing and proof

Output relevant cardinal imperativeness

Noise + address

DTMF decryption system

A DTMF decryption construction chiefly consists of two blocks.

set base on balls filters which will segregate single frequence constituents and cut down noise consequence

sensors to place the frequence constituents separately

To accomplish the ends of the undertaking an efficient algorithm must be selected which is capable of accurately choosing

Specific tones that signify the cardinal imperativenesss separately

Relative signal power at each of the frequences,

The clip period of the tones

Check whether a valid DTMF tone is present sing the signal features and industry criterions

Figure 1.2 Overview of the system ( barricade diagram )

5. Undertaking Plan – Gantt chart

6. DTMF Detection

DTMF sensing procedure requires the ability to place and distinguish from the eight DTMF frequences that are present. Because of the noise introduced to the input signal there is a possibility of misjudging the noise as DTMF tones. To avoid this it is critical to hold a technique of placing and rejecting the false tones green goodss due to resound. One phenomenon which can be used to gauge the public presentation of the DTMF sensing signal is known as the talk-off mistake. Talk-off mistake is defined as the improper sensing of a DTMF tone due to speech or other signifiers of noise in the communicating channel. [ 6 ] One method for know aparting between DTMF tones and address is look intoing for the presence of a 2nd harmonic. If the 2nd harmonic is detected, so the signal can be rejected because true DTMF tones merely contain cardinal tones.

Valid input signal

Check row and column frequences

Check 2nd harmonic

Single altogether and column detect


Second harmonic nowadays


False tone detected



Valid DTMF detect


Figure 1.3 Flow chart of the DTMF sensing [ 6 ]

7. Existing Methodologies for DTMF sensing and Decoding

7.1 Filtering method

Theoretically the filtering method is the simplest and most easy implements method for multi frequence sensing. The overlying DTMF signal is normally first filtered with usage of a brace of low base on balls and high base on balls filters and so the signal is fed in to a set of set base on balls filters.

The overlying signal is separated in to low frequence or high frequence group in mention to the DTMF standard frequence. Eight set base on balls filter blocks are used in the instance of DTMF to recover the frequences in each scope. Due to really high computational denseness package based filters are non used. [ 2 ]









Low base on balls

High base on balls

Signal dwelling DTMF

Digit CalculationThe filters are implemented as hardware blocks specified of making the filtering procedure. Due to the inefficiency of the filtering method for sensing of tones in multiple Pulse Code Modulated channels modern tone sensing strategies do non utilize this method.

Figure 1.4 Frequency filtrating method [ 2 ]

7.2 Fast Fourier Transform ( FFT )

Another methodological analysis to happen being of an single known frequence in a monitored signal is sing an FFT ( Fast Fourier Transform ) and analyse it to see whether there are specific frequences present in the information watercourse. Due to the ground of disregarding most of the computed signals this method is non efficient. [ 3 ]

7.3 Discrete Fourier Transform ( DFT )

The distinct Fourier transform ( DFT ) besides produce the same statistical consequence for a individual frequence of involvement. This makes the DFT a preferable pick for frequence sensing. The Goertzel Algorithm is a DFT in screen up. Goertzel algorithm has numerical methods to extinguish complex figure arithmetic which are present in FFT. The DFT method increases the efficiency and easiness of execution. Even though FFT is fast on Data Acquisition Processor boards, the public presentation in relative to CPU use necessitating a CPU with high treating power.

8. The Goertzel algorithm

The Goertzel algorithm is chiefly suited for sensing of peculiar, pre-defined frequences consist in a signal by agencies of minimum figure of memory and calculation resources. Therefore the Goertzel algorithm has been widely used for observing DTMF frequences contaminated by audio signals. This algorithm is normally implemented in microcontroller devices where both RAM restrictions and treating power prevent the usage of other algorithms such as the Fast Fourier Transformation.

Blocks of informations are processed by the Goertzel algorithm to give in each consequence. When sing the FFT method, before any processing can get down all the informations blocks should be buffered. But in the instance of DFT or Goertzel filter, informations are processed consecutively and expeditiously as they arrive. When the informations sequence end the end product is processed and ready to be forwarded. [ 8 ]

Why usage Goertzel Algorithm?

Most efficient attack for smaller sequence lengths

Reduces the informations memory required well

Requires limited treating power

Best computational burden and memory use efficiency

Real clip processing

9. Design, Coding and execution

To accomplish the aims of the undertaking a system should be implemented to acquire the signal watercourse with DTMF signals, noise and address signals and to know apart DTMF signals and address to end product the specific cardinal imperativenesss harmonizing to the DTMF tones. To implement the system there should be a development environment for the system. Sing the demand of the undertaking MATLAB and Microsoft Visual Studio can be used to code and implement the plan.

Sing that this system is more concerned about frequence sensing and mathematical maps MATLAB package is chosen to go on with the undertaking system execution.

MATLAB is most utile for signal processing and calculation. It is specially designed to make this undertaking easy and good. Sing the demands of this undertaking the maps of MATLAB such as plotting of maps and informations, execution of algorithms, creative activity of user interfaces, and interfacing with plans written in other linguistic communications will be utile. [ 8 ]

10. Testing and Debuging

Attempts are made to prove the system public presentation in a noisy environment. Test instances should be created comprising of different noise and intervention degree. The parametric quantities within the system should be varied till an accurate DTMF sensing procedure is implemented.

The system will besides be tested in a noise free environment to look into whether there are any logical and syntax mistakes within the enforced system.

11. Resources needed for the undertaking

Development Environment – MATLAB R2010b

12. Decision

Sing the facts that Enhances the chance of success by turn toing and extenuating factors early on that could impact the undertaking the feasibleness survey was conducted. Technical and legal feasibleness was besides considered with the economical feasibleness.

In feasibleness the undermentioned issues are taken into consideration.

Whether the aims are logical, approachable, clip efficient and specific

Whether the needed resources are available

Whether the needed engineering is available or non

Handiness of Testers & A ; debuggers

Software and hardware

Sing the facts that leads to the success of this undertaking the work program will be take in to consideration with the intent of accomplishing the aims.