【正文】 naling, DTMF tones were also used by cable television broadcasters to indicate the start and stop times of local mercial insertion points during station breaks for the benefit of cable panies. Until better outofband signaling equipment was developed in the 1990s, fast, unacknowledged, and loud DTMF tone sequences could be heard during the mercial breaks of cable channels in the United States and elsewhere., *, A, B, C, and DThe engineers had envisioned phones being used to access puters, and surveyed a number of panies to see what they would need for this role. This led to the addition of the number sign (, sometimes called 39。octothorpe39。 or 39。pound39。 in this context 39。hash39。 or 39。gate39。 in the UK) and asterisk or 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 these keys became widely used for vertical service codes such as *67 in the United States of America and Canada to suppress caller ID.Public payphones that accept credit cards use these additional codes to send the information from the magnetic strip.The . 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 need be. 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 nonpriority calls, and then any priority calls. Flash Override will kick every other call off the trunks between the origin and destination. Consequently, it was limited to the White House Communications Agency.Precedence dialing is still done on the military phone networks, but using number binations (Example: Entering 93 before a number is a priority call) rather than the separate tones and the Government Emergency Telemunications Service has superseded Autovon for any civilian priority telco access.Presentday uses of the A, B, C and D keys on telephone networks are few, and exclusive to network control.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.DTMF tones are also used by some cable television networks and radio networks to signal the local cable pany/network station to insert a local advertisement or station identification. These tones were often heard during a station ID preceding a local ad insert. Previously, terrestrial television stations also used DTMF tones to shut off and turn on remote transmitters.DTMF signalling tones can also be heard at the start or end of some VHS (Video Home System) cassette tapes. Information on the master version of the video tape is encoded in the DTMF tone. The encoded tone provides information to automatic duplication machines, such as format, duration and volume levels, in order to replicate the original video as closely as possible.DTMF tones are sometimes used in caller ID systems to transfer the caller ID information, however in the USA only Bell 202 modulated FSK signaling is used to transfer the data. A DTMF can be heard on most Whelen Outdoor Warning systems.KeypadThe DTMF keypad is laid out in a 44 matrix, with each row representing a low frequency, and each column representing a high frequency. Pressing a single key (such as 39。139。 ) will send a sinusoidal tone for each of the two frequencies (697 and 1209 hertz (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.DTMF keypad frequencies (with sound clips)1209Hz1336Hz1477Hz1633Hz697Hz123A770Hz456B852Hz789C941Hz*0DSpecial tone frequenciesNational telephone systems define additional tones to indicate the status of lines, equipment, or the result of calls with special tones. Such tones are standardized in each country and may consist of single or multiple frequencies. Most European countries use a single frequency, where the United States uses a dual frequency system presented in the following table.EventLow frequencyHigh frequencyBusy signal480Hz620HzRingback tone (US)440Hz480HzDial tone350Hz440HzThe 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 177。% from their nominal frequency, or the switching center will ignore the signal. The high frequencies may be the same volume as – or louder than – the low frequencies when sent across the line. The loudness difference between the high and low frequencies can be as large as 3 decibels (dB) and is referred to as twist. The duration of the tone should be at least 70 ms, although in some countries and applications DTMF receivers must be able to reliably detect DTMF tones as short as 45ms.As with other multifrequency receivers, DTMF was originally decoded