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)
Maybe Bell didn't invent the telephone
Determining the true inventor of a specific invention can be tricky.
Often credit goes to the inventor of the most practical or best working invention or the first person to secure a patent, rather than to the original inventor.
There is a lot of controversy and intrigue surrounding the invention of the telephone, with court cases, books, articles and websites dealing with the subject.
Alexander Graham Bell's design that was first one patented (in 1876), but he was not the first inventor to come up with the idea of a telephone.
Antonio Meucci, an Italian immigrant to the US, began developing the design of a telephone in 1849. In 1871, he filed a caveat (an announcement of an invention) for his design.
Due to financial hardship, Meucci could not renew his caveat, and his role in the invention of the telephone was overlooked until the United States House of Representatives passed a Resolution in 2002, honoring Meucci's contributions and work.
That resolution, "Sense of the House Honoring the Life and Achievements of 19th Century Italian-American Inventor Antonio Meucci" was sponsored by Italian-American Congressman Vito Fossella who represented Staten Island and parts of Brooklyn, NY. Fossella said, "Antonio Meucci was a man of vision whose enormous talents led to the invention of the telephone. Meucci began work on his invention in the mid-1800s, refining and perfecting the telephone during his many years living on Staten Island."
Elisha Gray, a professor at Oberlin College, applied for a caveat of the telephone on the same day Bell applied for his patent of the telephone.
In Historical First Patents: The First United States Patent for Many Everyday Things, Travis Brown, reports that Bell got to the patent office first, on February 14, 1876. He was the fifth entry of that day, while Gray was 39th. The US Patent Office awarded Bell the patent, Number 174,465 rather than honor Gray's caveat.
On the morning of Monday February 14, 1876, Gray signed and had notarized the caveat that described a telephone that used a liquid microphone. His attorney submitted it to the Patent Office.
That same morning a lawyer for Alexander Graham Bell submitted Bell's patent application. The caveat allowed an inventor to delay filing the more expensive application, while still establishing priority of invention.
If a patent application for the same invention was later filed by a different person, the patent office would declare an interference and contact the first person and allow him or her to file a substitute application within three months.
When Gray was notified through Baldwin, his lawyer, of this interference, Baldwin advised Gray to abandon his caveat because he said Bell had invented it first and had it notarized earlier than Gray. When Gray agreed to abandon his caveat, the examiner granted the patent to Bell.
Gray's caveat was taken to the US Patent Office a few hours before Bell's application. But the filing fee for Gray's caveat was entered on the cash blotter hours after Bell's filing fee which led to the myth that Bell had arrived at the Patent Office earlier.
Bell was in Boston on February 14 and did not know this was happening until he arrived in Washington on February 26. Whether Bell's application was filed before or after Gray's caveat no longer mattered, because Gray abandoned his caveat and that opened the door to Bell being granted the patent.
Although Gray had abandoned his caveat, Gray applied for a patent for the same invention in late 1877. This put him in interference with Bell's patents. The patent examiner held "while Gray was undoubtedly the first to conceive of and disclose the [variable resistance] invention, as in his caveat of February 14, 1876, his failure to take any action amounting to completion until others had demonstrated the utility of the invention deprives him of the right to have it considered." Gray challenged Bell's patent anyway, and after two years of litigation, Bell was awarded rights to the invention, and as a result, Bell is credited as the inventor.
In 1869, prior to his telephone "inventing," Elisha Gray and partner Enos M. Barton had founded Gray & Barton Co. in Cleveland, Ohio to supply telegraph equipment to Western Union. In 1872, the partnership became Western Electric. Ironically, despite Gray's patent loss to Bell, Gray's Western Electric became the exclusive manufacturer of telephone equipment for the Bell System. By the early 1900s, Western became of the largest manufacturers in the world.
Western Electric also had a thriving electrical distribution business, furnishing customers with non-telephone products made by other manufacturers. The distribution business was spun off as Graybar Electric Company in 1925. Graybar is now a major distributor of telephone equipment, including products made by Western Electric successors Lucent and Avaya.
Other claimants as inventors of the telephone, challenging the universal fame gained by Alexander Graham Bell, are Johann Philipp Reis, Innocenzo Manzetti, and Charles Bourseul in Europe, and Amos Dolbear, Sylvanus D. Cushman, Daniel Drawbaugh, Edward Farrar, and James McDonough in the United States.
(info from the Library of Congress, Wikipedia and esanet.it)
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