TELECOMMUNICATION(DTMF)

                       Dual-tone multi-frequency signaling 
                                          (DTMF)               

Introduction:

Dual-tone multi-frequency signaling (DTMF) is used for telecommunication signaling over analog telephone lines in the voice-frequency band between telephone handsets and other communications devices and the switching center. The version of DTMF that is used in push-button telephones for tone dialing is known as Touch-Tone. It was developed by Western Electric and first used by the Bell System in commerce, using that name as a registered trademark. DTMF is standardized by ITU-T Recommendation Q.23. It is also known in the UK as MF4.Other multi-frequency systems are used for internal signaling within the telephone network.
Introduced by AT&T in 1963, the Touch-Tone system using the telephone keypad gradually replaced the use of rotary dial and has become the industry standard for landline service.
Prior to the development of DTMF, numbers were dialed on automated telephone systems by means of pulse dialing (Dial Pulse or DP in the U.S.) or loop disconnect (LD) signaling, which functions by rapidly disconnecting and re-connecting the calling party's telephone line, imilar to flicking a light switch on and off. The repeated interruptions of the line, as the dial spins, sounds like a series of clicks. The exchange equipment interprets these dial pulses to determine the dialed number. Loop disconnect range was restricted by telegraphic distortion and other technical problems, and placing calls over longer distances required either operator assistance (operators used an earlier kind of multi-frequency dial) or the provision of subscriber trunk dialing equipment.
Multi-frequency signaling  is a group of signaling methods that use a mixture of two pure tone (pure sine wave) sounds. Various MF signaling protocols were devised by the Bell System and CCITT. The earliest of these were for in-band signaling between switching centers, where long-distance telephone operators used a 16-digitkeypad to input the next portion of the destination telephone number in order to contact the next downstream long-distance telephone operator. This semi-automated signaling and switching proved successful in both speed and cost effectiveness. Based on this prior success with using MF by specialists to establish long-distance telephone calls, Dual-tone multi-frequency (DTMF) signaling was developed for the consumer to signal their own telephone-call's destination telephone number instead of talking to a telephone operator.




Touch tone dial telephones : DTMF technology

The touch tone dialing arrangement is as shown below:

The rotary dial is replaced by a push button keyboard. ‘Touching’ a button generates a ‘tone’ which is a combination of two frequencies, one from the lower band and the other from the upper band. The Dtmf application is associated with digital telephony and provides two selected output frequencies (one high band, one low band) for a duration of 100ms. The keypad is arranged as a matrix for selecting high and low frequencies associated with each key. Each key is uniquely referenced by selecting one of the four low band frequencies associated with the matrix rows, coupled with selecting one of the three high band frequencies associated with the matrix column. 

Design Considerations

1.      Choice of bands

2.      Band separation

3.      Choice of frequencies

4.      Choice of power levels

5.      Signaling duration

The choice of code for touch tone signaling should be such that imitation of code signals by speech and music should be difficult. So, some form of multi frequency code is required.

Separating the band of the two frequencies  is needed for the following reasons:

·         Each frequency component can be amplitude regulated separately

·         Before attempting to determine two specific frequencies at the receiving end, band filtering can be used to separate the frequency groups. This renders determination of specific frequencies simpler.

The following fig. shows a simplified block diagram of a touch tone receiver. The limiters attenuate differences in levels between components of an incoming multifrequency signal.

If two frequencies reach the limiter with one of the being relatively stronger , the o/p of the limiter peaks with the stronger signal and weaker signal is further attenuated. If both reach with same strength, limiter o/p is much below full o/p and neither signal dominates. The selective circuitry is designed to recognize signal as  bonafide when it falls within the specified narrow passband and has an amplitude within about 2.5 db of full o/p of the limiter. The limiter and the selective circuitry together reduce the probability mistaking touch signal to audio or vice versa. Band elimination filters are sometimes used in place of band separation filters as they permit a wider spectrum of speech to pass through the filters. 

CCITT Q.23 standards

Consideration of human factors and mechanical design factors include aspects like button size and spacing, stroke lengths, strike force, numbering scheme and button arrangement. User preference and performance studies coupled with design considerations have resulted in following specifications.

·         3/8 inch square buttons,separated by ¼  inch ,1/8  inch stroke length

·         100g force at the bottom of the stroke

·         4*3 array with digits 1,2,3 in top row. 0 in middle of last row. # in third row usually to redial

·         The push button is reserved for special features.

MAJOR ADVANTAGES OF THE TOUCH TONE DIALING :

1.      Potential for data transmission and remote control.

2.      Data in voice answer system – a power application.

3.      Allows a higher dialing rate of more than 10 signals per second.

4.      Can accurately determine the signaling frequencies by testing for very small durations also.