Key switches[ edit ] In the first electronic keyboards in the early s, the key switches were individual switches inserted into holes in metal frames. The most popular switch types were reed switches contacts enclosed in a vacuum in a glass capsule, affected by a magnet mounted on the switch plunger. This became more acceptable, however, for use in computer terminals at the time, which began to see increasingly shorter model lifespans as they advanced. As the key was depressed, the capacitance between the plunger pad and the patterns on the PCB below changed, which was detected by integrated circuits IC.
The information carried by the signal has a certain bandwidth associated with it, and the carrier must have a channel width at least as great as the information bandwidth.
For regular amplitude-modulated AM broadcasting the radio-frequency bandwidth must be twice the information-frequency bandwidth. Teleprinter and telex operation requires only a small bandwidth, on the order of hertz, depending on the maximum speed of the pulses forming the information code.
Telephonic The wireless technology applied to computer processing must have high intelligibility, but naturalness high fidelity is not of great importance. Tests have shown that the main components of speech lie between about and 3, hertz, and telephonic channels carried by radio are therefore normally confined to a bandwidth of about four kilohertz.
The smaller the information bandwidth employed, the more speech channels can be carried in a given carrier bandwidth and the more economical the system will be.
Young people can hear audio frequencies ranging from about 30 hertz to 18 kilohertz, but, as they grow older, hearing ranges from about hertz to 10 kilohertz. For high-quality high-fidelity reproduction of voice or speech, the range should be not less than about 30 hertz the lowest frequency of a large organ pipe to 15 kilohertz piccolo, cymbal, triangle.
Acceptable audio quality under certain circumstances may be achieved with a bandwidth as small as five kilohertz, as in AM radio; a much larger bandwidth is needed for transmitting a moving picture because it is necessary to convey the overall average light content of a picture as well as the picture detail.
The average light content requires frequencies as low as 20 hertz to be transmitted, and picture detail demands frequencies up to five megahertz for a standard television picture.
Modulators and demodulators A carrier wave is a radio-frequency wave that carries information. The information is attached to the carrier wave by means of a modulation process that involves the variation of one of the carrier-frequency characteristics, such as its amplitude, its frequency, or its duration.
All of these processes are discussed in greater detail in the article telecommunication system. In amplitude modulation the information signal varies the amplitude of the carrier wave, a process that produces a band of frequencies known as sidebands on each side of the carrier frequency.
These sidebands a pair to each modulation frequency cover a range of frequencies equal to the sum and difference between the carrier frequency and the information signal. Frequency modulation involves varying the frequency the number of times the wave passes through a complete cycle in a given period of time, measured as cycles per second of the carrier in accordance with the amplitude of the information signal.
The amplitude of the carrier wave is unaffected by the variation; only its frequency changes. Frequency modulation produces more often many more than one pair of side frequencies for each modulation frequency. The greater the frequency deviation the greater is the effective modulation.
Though theoretically its maximum value need not be limited to 75 kilohertz, any increase beyond this value requires a wider channel, which adds to the cost of reception and reduces the number of transmitters that can be accommodated in the band. The total channel width is approximately twice the sum of the maximum deviation frequency and modulating frequency.
If channel width is restricted in either transmitter or receiver circuits, distortion of the information signal occurs. A radio broadcast normally consists of only one information signal. The listener hears what he would hear at the microphone position if only one of his ears was functioning; i.
In such a system it is not possible to gain any impression of the position of the instrument groupings in an orchestra, nor can lateral movement be indicated, though movement toward or away from the microphone is conveyed by a change in sound volume.
Stereophonic broadcasting requires two microphones, one to collect sounds from the left and one from the right; the two sets of information must be separable in the receiver and be fed to loudspeakers on the left and on the right at the listening position.
For high-fidelity reproduction the full audio range up to 15 kilohertz is transmitted; this can only be achieved satisfactorily at very high frequencies with frequency modulation.Applied Computer Science. Mercer County Community College. Computer Science / Data Processing.
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The Department of Computer Science at Montclair State University supports the Bachelor of Science degrees in Computer Science, Information Technology and Science Informatics. WELCOME TO THE INFORMATION TECHNOLOGY LABORATORY. The Information Technology Laboratory (ITL), one of seven research laboratories within the National Institute of Standards and Technology (NIST), is a globally recognized and trusted source of high-quality, independent, and unbiased research and data.