Pulse dialing:
1. Generated through make and break contact.
2. DC Current pulse is generated.
3. Each number is separated by a short pause to prevent overlapping.
4. Codes are unary except for 0.
5. Dial is rotary.
6. Dial is slow moving.
DTMF Dialing
1. Uses push buttons
2. Uses 8 different frequencies in pairs.
3. 16 different characters can be represented.
4. The frequencies used prevent generating of harmonics.
5. Each number is represented by a high signal & low signal
Dual Tone Multi Frequency (DTMF) was first introduced in 1963 with 10 buttons in Western Electric 1500 –type telephones. DTMF was originally called TouchTone. DTMF is a more efficient means than dial pulsing for transferring telephone numbers from a subscriber’s location to the central office switching machine. DTMF is a simple two-of –eight encoding scheme where each digit is represented by the linear addition of two frequencies. DTMF is strictly for signaling between a subscriber’s location and the nearest telephone office or message switching center. DTMF is sometimes confused with another two-tone signaling system called multi frequency signaling (Mf), which is a two-of-six code designed to be used only to convey information between two electronic switching machines. Fig. shows the four-row-by-four column keypad matrix used with a DTMF keypad. As the figure shows, the keypad is comprised of 16 keys and eight frequencies. Most household telephones, however, are not equipped with the special-purpose keys located in the fourth column (i.e. , the A,B,C, and D keys). Therefore, most household telephones actually use two-of-seven tone encoding scheme. The four vertical frequencies (called the low group frequencies) are 697 Hz, 852 Hz, and 941 Hz, and the four horizontal frequencies (called the high group frequencies) are 1209 Hz, 1336 Hz, 1477 Hz, and 1633 Hz. The frequency tolerance of the oscillators is + .5%. As shown in Fig., the digits 2 through 9 can also be used to represent 24 of the 26 letters (Q and Z are omitted). The letters were originally used to identify one local telephone exchange from another.
Dial Pulses: Dial pulsing (sometimes called rotary dial pulsing) is the method originally used to transfer digits from a telephone set to the local switch. Pulsing digits from a rotary switch began soon after the invention of the automatic switching machines. The concept of dial pulsing is quite simple and is depicted in Fig. The process begins when the telephone set is lifted off hook, completing a path for current through the local loop. When the switching machine detects the off-hook condition, it responds with dial tone. After hearing the dial tone, the subscriber begins the dial pulsing digits by rotating a mechanical dialing mechanism and then letting it returns to its rest position. As the rotary switch returns to its rest position, it outputs a series of dial pulses corresponding to digit dialed.
When a digit is dialed, the loop circuit alternately opens (breaks) and closes (makes) a prescribed number of times. The number of switch make/break sequences corresponds to the digit dialed (i.e., the digit 3 produces three switch openings and three switch closers). Dial pulses occur at 10 make/break cycles per second (i.e., a period of 100 ms per pulse cycle). For example, the digit 5 corresponds to five make/break cycles lasting a total of 500ms. The switching machine senses and counts the number make/break pairs in the sequence.
The break time is nominally 61 ms, and the make time is nominally 39ms. Digits are separated by idle period of 300 ms called the inter digit time. It is essential that the switching machine recognize the inter digit time so that it can separate the pulses from successive digits. The central office switch incorporates a special timeout circuit to ensure that the break part of dialing pulse is not misinterpreted as the phone being returned to its on-hook (idle) condition.