Remote Control using Mobile Telephone
Prior to DTMF, phone systems used a system known as pulse (Dial Pulse or DP in the USA) or loop disconnect (LD) signalling to dial numbers, which works by rapidly disconnecting and connecting the calling party’s phone line, like flicking a light switch on and off. The repeated connection and disconnection, as the dial spins, sounds like a series of clicks. The exchange equipment counts those clicks or dial pulses to determine the called number. LD range was restricted by telegraphic distortion and other technical problems, and placing calls over longer distances required either operator assistance
(operators used an earlier kind of multi-frequency dial) or the provision of subscriber trunk dialling equipment.
DTMF was developed at Bell Labs in order to allow dialing signals to dial long-distance numbers, potentially over nonwire links such as microwave radio relay links or atellites. For a few non crossbar offices, encoder/decoders were added that would convert the older pulse signals into DTMF tones and play them down the line to the emote end office. At the remote site another encoder/decoder could decode the tones and perform pulse dialing, for example for Strowger switches. It was as if you were onnected directly to that end office, yet the signaling would work over any sort of link. This idea of using the existing network for signaling as well as the message is nown as n-band signaling.
It was clear even in the late 1950s when DTMF was being developed that the future of switching lay in electronic switches, as opposed to the electromechanical crossbar systems then in use. Either switching system could use either dial system, but DTMF promised shorter holding times, which was more important in the larger and more complex registers used in crossbar systems. In this case pulse dialing made no sense at any point in the circuit, and plans were made to roll DTMF out to end users as soon as possible. Tests of the system occurred in the early 1960s, where DTMF became known as Touch Tone. Though Touch Tone phones were already in use in a few places, they were vigorously promoted at the 1964 New York World’s Fair.
Download Seminar Report on Mobile Remote Controll using DTMF tone – Electronics Project
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Electronics Engineering | Admin September 25, 2009