Dual-Tone Multi-Frequency (DTMF) dialing is used to
initiate phone calls by sending tone sequences from a phone to a local phone
company central office. The version of DTMF used for telephone dialing is known
as Touch-Tone, and is standardized in most parts of the world. Other
multi-frequency systems are used for signaling within the telephone network.
In the time before the development of Touch-Tone,
telephone systems used rotary dials that generated a series of pulses ("clicks")
that were generated by rapidly disconnecting and connecting the caller's
telephone line, similar to flicking a light switch on and off.
The repeated connection and disconnection, as the dial
spins, sounds like a series of clicks. The central office exchange equipment
counts those clicks to determine the called number. Placing long distance calls
required either operator assistance (operators used an earlier kind of
multi-frequency dial) or the provision of subscriber trunk dialing equipment.
The DTMF dialing system traces its roots to a technique
AT&T developed in the 1950s called MF (Multi-Frequency) which was deployed
within the AT&T telephone network to direct calls between switching facilities
using in-band signaling. In the early 1960s, a derivative technique was offered
by AT&T as a "modern" to place calls. The consumer product was marketed by AT&T
under the registered trade name Touch-Tone®. Other vendors of compatible
telephone equipment used other names such as "Tel-Touch."
The DTMF system uses eight different frequency signals
transmitted in pairs to represent sixteen different numbers, symbols and letters
- as shown below.
The keypad is laid out in a 4×4 matrix, with each
row representing a low frequency, and each column representing a high frequency.
Pressing a single key (such as '1' ) will send a sinusoidal tone of the two
frequencies (697 and 1209 Hz. The original keypads had levers inside, so each
button activated two contacts. The multiple tones are the reason for calling the
system multifrequency. These tones are then decoded by the switching center to
determine which key was pressed.
The tone frequencies, as defined by the Precise Tone Plan,
are selected such that harmonics and intermodulation products will not cause an
unreliable signal. No frequency is a multiple of another, the difference between
any two frequencies does not equal any of the frequencies, and the sum of any
two frequencies does not equal any of the frequencies. The frequencies were
initially designed with a ratio of 21/19, which is slightly less than a whole
tone. The frequencies may not vary more than ±1.5% from their nominal frequency,
or the switching center will ignore the signal. The high frequencies may be the
same volume or louder as the low frequencies when sent across the line. The
loudness difference between the high and low frequencies can be as large as 3 dB
and is referred to as "twist". The minimum duration of the tone should be at
least 70 msec, although in some countries and applications DTMF receivers must
be able to reliably detect DTMF tones as short as 45ms.
The engineers had envisioned phones being used to access
computers, and surveyed a number of companies to see what they would need for
this role. This led to the addition of the number sign (#) and star (*) keys, as
well as a group of keys for menu selection: A, B, C and D. In the end, the
lettered keys were dropped from most phones, and it was many years before the
star became widely used for service codes such as star-67 to suppress caller ID.
The U.S. military also used the letters, relabeled, in
their now defunct Autovon phone system. Here they were used before dialing the
phone in order to give some calls priority, cutting in over existing calls if
necessary. The idea was to allow important traffic to get through every time.
The levels of priority available were Flash Override (A), Flash (B), Immediate
(C), and Priority (D), with Flash Override being the highest priority. Pressing
one of these keys gave your call priority, overriding other conversations on the
network. Pressing C, Immediate, before dialing would make the switch first look
for any free lines, and if all lines were in use, it would disconnect any
non-priority calls, and then any priority calls. Flash Override will kick every
other call off the trunks between the origin and destination. Consequently, it
is limited to the White House Communications Agency. Precedence dialing is still
done on the military phone networks, but using number combinations rather than
the separate tones.
Present-day uses of the A, B, C and D keys on telephone
networks are few, and exclusive to network control. For example, the A key is
used on some networks to cycle through different carriers at will (thereby
listening in on calls). Their use is probably prohibited by most carriers. The
A, B, C and D tones are used in amateur radio phone patch and repeater
operations to allow, among other uses, control of the repeater while connected
to an active phone line.
(info from Wikipedia)