1945: Radio And Electronics
Pulse-time modulation (PTM), one of the most revolutionary advances in the technique of radio broadcasting, has been announced by the Federal Telephone and Radio Corporation. This new method of modulation differs as much from the conventional amplitude modulation (AM) as does frequency modulation (FM). Moreover, it is particularly well-adapted to the high-frequency bands toward which it has become increasingly necessary to allocate different radio services because of the crowded conditions of the lower frequency part of the radio spectrum. The system is characterized by being capable of transmitting several sound programs on the same carrier frequency. Each frequency is broken up into a series of triangular high-frequency pulses with a short interval between successive pulses, these pulses occurring at a rate of many thousands per second. Each pulse is free to move within a limited range without interfering with adjacent pulses. Movement of the pulses is controlled by the modulating signals, which in turn are controlled by signals from different channels which are produced by the several programs, such as speech, music or television. In each channel at regular intervals there are synchronizing pulses, some every six intervals, some every four and some every three. The receiver may be synchronized with any set of these synchronizing pulses and thus select any one program
A system of communication employing electromagnetic waves propagated through space. Because of their varying characteristics, radio waves of different lengths are employed for different purposes and are usually identified by their frequency. The shortest waves have the highest frequency, or number of cycles per second; the longest waves have the lowest frequency, or fewest cycles per second. In honor of the German radio pioneer Heinrich Hertz, his name has been given to the cycle per second (hertz, Hz); 1 kilohertz (kHz) is 1000 cycles per sec, 1 megahertz (MHz) is 1 million cycles per sec, and 1 gigahertz (GHz) is 1 billion cycles per sec. Radio waves range from a few kilohertz to several gigahertz. Waves of visible light are much shorter. In a vacuum, all electromagnetic waves travel at a uniform speed of about 300,000 km (about 186,000 mi) per second.
It has three basic parts which are given below:
An abbreviation for wireless fidelity, is a wireless communication technology that can provide connections between portable computers and wired connections to the Internet. To connect users with the Internet, Wi-Fi devices use low-power transmitters and receivers equipped with special computer chips containing radio modems. The chips can be installed in laptop computers, personal digital assistants (PDAs), and cellular telephones.
On the wireless side, some researchers are working on a new local area standard, known as 802.11n, which would double transmission speeds for wireless devices to nearly 200 Mbps. Other researchers are developing a new standard called Unlicensed Mobile Access (UMA), which would allow wireless mobile devices to seamlessly move from local area networks to wide area cellular networks and back again. Currently, there is no way for personal digital assistant (PDA) handhelds and cell phones to move automatically between a wireless LAN and a wireless WAN or vice versa. Service is interrupted, and a manual adjustment must be made on the device for wireless service to continue.