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There are two types of phone dials in use around the world. The most common one is called pulse, loop disconnect, or rotary; the oldest form of dialing, it's been with us since the 1920's. The other dialing method, more modern and much loved by Radio Amateurs is called Touch-tone, Dual Tone Multi-Frequency (DTMF) or Multi-Frequency (MF) in Europe. In the U.S. MF means single tones used for system control.

Pulse phone dialing is traditionally accomplished with a rotary phone dial, which is a speed governed wheel with a cam that opens and closes a switch in series with your phone and the phone line. It works by actually disconnecting or "hanging up" the phone at specific intervals. The United States standard is one disconnect per digit, so if you dial a "1," your telephone is "disconnected" once. Dial a seven and you'll be "disconnected" seven times; dial a zero, and you'll "hang up " ten times. Some countries invert the system so "1" causes ten "disconnects" and 0, one disconnect. Some add a digit so that dialing a 5 would cause six disconnects and 0, eleven disconnects. There are even some phone systems in which dialing 0 results in one disconnect, and all other digits are plus one, making a 5 cause six disconnects and 9, ten disconnects.

Although most phone exchanges are quite happy with rates of 6 to 15 Pulses Per Second (PPS), the phone company accepted standard is 8 to 10 PPS. Some modern digital phone exchanges, free of the mechanical inertia problems of older phone systems, will accept a PPS rate as high as 20. 

Besides the PPS rate, the phone dialing pulses have a make/break ratio, usually described as a percentage, but sometimes as a straight ratio. The North American standard is 60/40 percent; most of Europe accepts a standard of 63/37 percent. This is the pulse measured at the phone, not at the phone exchange, where it's somewhat different, having traveled through the phone line with its distributed resistance, capacitance, and inductance. In practice, the make/break ratio does not seem to affect the performance of the phone dial when attached to a normal loop. Bear in mind that each pulse is a switch connect and disconnect across a complex impedance, so the switching transient often reaches 300 Volts. Try not to have your fingers across the phone line when dialing.

Most pulse dialing phones produced today use a CMOS IC and a keyboard. Instead of pushing your finger round in circles, then removing your finger and waiting for the phone dial to return before dialing the next digit, you punch the phone buttons as fast as you want. The IC stores the phone number and pulses it out at the correct rate with the correct make/break ratio and the switching is done with a high-voltage switching transistor. Because the IC has already stored the dialed phone number in order to pulse it out at the correct rate, it's a simple matter for phone designers to keep the memory "alive" and allow the phone to store, recall, and redial the Last Number Dialed (LND). This feature enables you to redial by picking up the handset and pushing just one phone button. 

Because pulse phone dialing entails rapid connection and disconnection of the phone line, you can "dial" a phone that has lost its dial, by hitting the hook-switch rapidly. It requires some practice to do this with consistent success, but it can be done. A more sophisticated approach is to place a Morse key in series with the phone line, wire it as normally closed and send strings of dots corresponding to the digits you wish to dial.

Touch tone, the most modern form of phone dialing, is fast and less prone to error than pulse phone dialing. Compared to pulse, touch tone phone dialing's major advantage is that its audio band signals can travel down phone lines further than pulse, which can travel only as far as your local phone exchange. Touch-tone phone dialing can therefore send signals around the world via the phone lines, and can be used to control phone answering machines and computers. Pulse phone dialing is to touch-tone as FSK or AFSK RTTY is to Switched Carrier RTTY, where mark and space are sent by the presence or absence of DC or unmodulated RF carrier. Most Radio Amateurs are familiar with DTMF for controlling repeaters and for accessing remote and auto phone patches.

Bell Labs developed DTMF in order to have a phone dialing system that could travel across microwave links and work rapidly with computer controlled phone exchanges. Each transmitted digit consists of two separate audio tones that are mixed together (see fig.3). The four vertical columns on the phone keypad are known as the high group and the four horizontal rows as the low group; the digit 8 is composed of 1336 Hz and 852 Hz. The level of each tone is within 3 dB of the other, (the telephone company calls this "Twist"). A complete touch-tone phone pad has 16 digits, as opposed to ten on a pulse phone dial. Besides the numerals 0 to 9, a DTMF "dial" has *, #, A, B, C, and D. Although the letters are not normally found on consumer phones, the IC in the phone is capable of generating them.

The * sign is usually called "star" or "asterisk." The # sign, often referred to as the "pound sign." is actually called an octothorpe. Although many phone users have never used these digits - they are not, after all, ordinarily used in dialing phone numbers - they are used for control purposes, phone answering machines, bringing up remote bases, electronic banking, and repeater control. The one use of the octothorpe that may be familiar occurs in dialing international phone calls from phones in the United States. After dialing the complete phone number, dialing the octothorpe lets the phone exchange know you've finished dialing. It can now begin routing your phone call; without the octothorpe, it would wait and "time out" before switching your call.

When DTMF phone dials first came out they had complicated cams and switches for selecting the digits and used a transistor oscillator with an LC tuning network to generate the tones. Modern phone dials use a matrix switch and a CMOS IC that synthesizes the tones from a 3.57MHz (TV color burst) crystal. This oscillator runs only during dialing, so it doesn't normally produce QRM.

Standard DTMF phone dials will produce a tone as long as a key is depressed. No matter how long you press, the tone will be decoded as the appropriate digit. The shortest duration in which a digit can be sent and decoded is about 100 milliseconds (ms). It's pretty difficult to dial by hand at such a speed, but automatic dialers can do it. A twelve-digit long distance phone number can be dialed by an automatic phone dialer in a little more than a second - about as long as it takes a pulse dial to send a single 0 digit.

The output level of DTMF tones from your phone should be between 0 and -12 dBm. In phones, 0 dB is 1 miliwatt over 600 Ohms. So 0 dB is 0.775 Volts. Because your phone is considered a 600 Ohm load, placing a voltmeter across the line will enable you to measure the level of your tones. 

Click on one of the items below to learn more.

• The Phone Line
• The Speech Network
• The Phone Dial
• The Phone Ringer
• Cellular Networking Perspectives
• How Cellular Phones Work.
• Pager Facts
• Answering Machines
• Cordless Phones

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