The tone duration of DTMF is variable depending on the system used. For amateur radio and commercial two-way radio systems, the keys are used for equipment control, repeater control, remote base operations, and some telephone communications systems. On telephone networks, such keys are used for network control, such as cycling through different carriers. Nowadays, the ABCD keys (as well as * and # ) are used infrequently and are for operations unrelated to their original purpose. This system began operation in 1966 and ended in the early 1990's when a newer system, the Defense Switched Network, replaced Autovon. The highest, Flash Override, would effectively interrupt all ongoing calls and give the speaker full command. These keys were for the urgency of the call. The A,B,C, and D tones are actually originally part of an old US Military telephone system called Autovon. It's most known for telephony dialing, but is in use for many different applications such as DTMF paging for DTMF-enabled VHF Very High Frequency (30-300 MHz)/ UHF Ultra High Frequency (300-3000 MHz) radios and as a marker on broadcasting stations for switching between regional and local programming for events like local news broadcasting.ĭTMF uses the dual-tone method, where each symbol is represented by a combination of two distinct tones. It's most known for telephony dialing, but is in use for many different applications such as DTMF paging for DTMF-enabled VHF/UHF radios.ĭTMF is a signaling mode used for a variety of purposes. DTMF is a signaling mode used for a variety of purposes.

Dtmf frequency