Most new radio systems are digital, including digital television, satellite radio and digital audio broadcasting. The earliest form of digital broadcasting was a telegraph spark, used by pioneers such as Marconi. By pressing the key, the operator can send messages in Morse code by activating a rotating sizzle switch. The rotary switch is a tone in the receiver, where a single spark would produce a hiss, indistinguishable from static. Sparks transmitters are now illegal, because their transmissions cover several hundred megahertz. This is highly consumed both RF and power.
The next advance was continuous wave telegraphy, or CW (continuous wave), in which a pure radio frequency produced by a vacuum tube of the electronic oscillator was activated and deactivated by a key. A receiver with a local oscillator would be “heterodyne” with pure frequency radio, creating a whistling audio tone. CW uses less bandwidth of 100 Hz. CW is still used, these days mainly by amateur radio operators (hams). Strictly speaking, the on-off of a medium will be called “Interrupted Continuous Wave” or ICW or on-off keying (OOK) coding.
Radiotelegraph printer normally operates on short waves (HF) and is highly appreciated by the military because it creates writing without information from the specialized operator. They send out a bit like one of the two tones using frequency shifting. Groups of five or seven bits become a character printed by a ticker. From 1925-1975, the type of radiotelety was how the most commercial messages were sent to less developed countries. These are still used by military and meteorological services.
Aircraft use a 1200 baud VHF radioteletype service to send their ID, altitude and position, and get data from the gate and connecting flight. Microwave dishes on satellites, telephone exchanges and television stations generally use Quadrature Amplitude Modulation (QAM). MAQ sends data by changing the phase and amplitude of the radio signal. Engineers like QAM, as it packs the most bits into a radio signal when they receive a (non-shared) exclusive band with fixed narrow band. Usually, bits are sent in repeating “frames”. Special bit pattern is used to locate the start of a frame.
Communication systems that are limited to a narrow band interference fixed range vulnerable. Initially, a variety of techniques distributed jams resistant for military use, the most famous satellite transmissions Global Positioning System Spectrum developed. Commercial use of the spread spectrum began in the 1980s. Bluetooth, most cell phones, and Wi-Fi version 802.11b use every different form of spread spectrum.
Systems that require reliability, or who share their frequency with other services, can use “orthogonal frequency division coded multiplexing” or COFDM.CoFDM breaks a digital signal as long as several hundred slower subchannels. Digital signal is sent subsequently QAM on sub-channels Modern COFDM systems use a small computer to make and decode signal with digital signal processing which is more flexible and much less expensive than older systems that have applied electronic channels Distinct.
COFDM resists fading and ghosting because narrow QAM channel signals can be sent slowly. An adaptive system that sends or error correction codes can also withstand interference, since most interference can affect only some of the QAM channels. COFDM is used for Wi-Fi, some cell phones, Digital Radio World, Eureka 147, and many other local networks, digital television and radio standards.