signal processing
Signal processing is the analysis, interpretation, and manipulation of signals. Signals of interest include sound, images, biological signals such as ECG, radar signals, and many others. Processing of such signals includes filtering, storage and reconstruction, separation of information from noise (for example, aircraft identification by radar), compression (for example, image compression), and feature extraction (for example, speech-to-text conversion).
In communication systems, signal processing only occurs at OSI layer 1, the physical layer (modulation, equalization, multiplexing, radio transmission, etc) in the seven layer OSI model, as well as at OSI layer 6, the presentation layer (source coding, including analog-to-digital conversion and data compression).
Signal classification
Signals are electrical representations of time-varying or spatial-varying physical quantities, either analog or digital, and may come from various sources. In the context of signal processing, arbitrary binary data streams are not considered as signals, but only digital signals that are representations of analog physical quantities.
For analog signals, signal processing may involve the amplification and filtering of audio signals for audio equipment or the modulation and demodulation of signals for telecommunications. For digital signals, signal processing may involve digital filtering and compression of digital signals.
 Analog signal processing — for signals that have not been digitized, as in classical radio, telephone, radar, and television systems
Discrete-time signal processing – for signals that are defined only at discrete points in time, and as such are quantized in time, but not magnitude. This theoretical discipline establishes the mathematical basis for digital signal processing, the technology of processing signals that are quantized in time and magnitude.
Digital signal processing — for signals that have been digitized. Processing is done by general-purpose computers or by digital circuits such as ASICs, FPGAs, or specialized digital signal processors (DSP chips).
Statistical signal processing — analyzing and extracting information from signals based on their statistical properties
Audio signal processing — for electrical signals representing sound, such as speech or music
Speech signal processing — for processing and interpreting spoken words
Image processing — in digital cameras, computers, and various imaging systems
Video processing — for interpreting moving pictures
Array processing — for processing signals from arrays of sensors
History
According to Alan V. Oppenheim and Ronald W. Schafer, the principles of signal processing can be found in the classical numerical analysis techniques of the 17th century. They further state that the "digitalization" or digital refinement of these techniques can be found in the digital control systems of the 1940s and 1950s. [ Digital Signal Processing, Prentice-Hall, 1975, ISBN 0-13-214635-5, page 5. ]
See also
Data acquisition
References
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