Signal transmission using electronic signal processing.
convert signals from other physical
waveforms, which then are processed, transmitted as
, received and converted by another transducer to final form.
The signal on the left looks like noise, but the signal processing technique known as the
(right) shows that it contains five well-defined frequency components.
Signal processing is an electrical engineering subfield that focuses on analysing, modifying, and synthesizing signals such as sound, images, and scientific measurements. Signal processing techniques can be used to improve transmission, storage efficiency and subjective quality and to also emphasize or detect components of interest in a measured signal.  
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 digital refinement of these techniques can be found in the digital control systems of the 1940s and 1950s. 
Claude Shannon wrote the influential paper " A Mathematical Theory of Communication" which was published in the Bell System Technical Journal. The paper laid the groundwork for later development of information communication systems and the processing of signals for transmission.  
Signal processing matured and flourished in the 1960s and 1970s, and digital signal processing became widely used with specialized
digital signal processor chips in the 1980s. 
Analog signal processing is for signals that have not been digitized, as in most 20th-century radio, telephone, radar, and television systems. This involves linear electronic circuits as well as nonlinear ones. The former are, for instance,
passive filters, active filters, additive mixers, integrators, and delay lines. Nonlinear circuits include compandors, multipliers ( frequency mixers, voltage-controlled amplifiers), voltage-controlled filters, voltage-controlled oscillators, and phase-locked loops.
Continuous time Continuous-time signal processing is for signals that vary with the change of continuous domain (without considering some individual interrupted points).
The methods of signal processing include
time domain, frequency domain, and complex frequency domain. This technology mainly discusses the modeling of linear time-invariant continuous system, integral of the system's zero-state response, setting up system function and the continuous time filtering of deterministic signals
Discrete time Discrete-time signal processing is for sampled signals, defined only at discrete points in time, and as such are quantized in time, but not in magnitude.
Analog discrete-time signal processing is a technology based on electronic devices such as sample and hold circuits, analog time-division multiplexers, analog delay lines and analog feedback shift registers. This technology was a predecessor of digital signal processing (see below), and is still used in advanced processing of gigahertz signals.
The concept of discrete-time signal processing also refers to a theoretical discipline that establishes a mathematical basis for digital signal processing, without taking
quantization error into consideration.
Digital signal processing is the processing of digitized discrete-time sampled signals. Processing is done by general-purpose
computers or by digital circuits such as ASICs, field-programmable gate arrays or specialized digital signal processors (DSP chips). Typical arithmetical operations include fixed-point and floating-point, real-valued and complex-valued, multiplication and addition. Other typical operations supported by the hardware are circular buffers and lookup tables. Examples of algorithms are the fast Fourier transform (FFT), finite impulse response (FIR) filter, Infinite impulse response (IIR) filter, and adaptive filters such as the Wiener and Kalman filters.
Nonlinear signal processing involves the analysis and processing of signals produced from nonlinear systems and can be in the time, frequency, or spatio-temporal domains.
 Nonlinear systems can produce highly complex behaviors including  bifurcations, chaos, harmonics, and subharmonics which cannot be produced or analyzed using linear methods.
Statistical signal processing is an approach which treats signals as
stochastic processes, utilizing their statistical properties to perform signal processing tasks. Statistical techniques are widely used in signal processing applications. For example, one can model the  probability distribution of noise incurred when photographing an image, and construct techniques based on this model to reduce the noise in the resulting image.
Seismic signal processing
Audio signal processing – for electrical signals representing sound, such as speech or music 
Image processing – in digital cameras, computers and various imaging systems
Video processing – for interpreting moving pictures
Wireless communication – waveform generations, demodulation, filtering, equalization
Array processing – for processing signals from arrays of sensors
Process control – a variety of signals are used, including the industry standard 4-20 mA current loop
Financial signal processing – analyzing financial data using signal processing techniques, especially for prediction purposes.
Feature extraction, such as image understanding and speech recognition. Quality improvement, such as
noise reduction, image enhancement, and echo cancellation. Source coding including
audio compression, image compression, and video compression. Genomic signal processing 
In communication systems, signal processing may occur at:
Mathematical methods applied
Sengupta, Nandini; Sahidullah, Md; Saha, Goutam (August 2016). "Lung sound classification using cepstral-based statistical features". Computers in Biology and Medicine. 75 (1): 118-129. doi: 10.1016/j.compbiomed.2016.05.013. PMID 27286184.
Alan V. Oppenheim and Ronald W. Schafer (1989). Discrete-Time Signal Processing. Prentice Hall. p. 1. ISBN . 0-13-216771-9
Oppenheim, Alan V.; Schafer, Ronald W. (1975). Digital Signal Processing. Prentice Hall. p. 5. ISBN . 0-13-214635-5
"A Mathematical Theory of Communication - CHM Revolution". Computer History . Retrieved .
^ a b
Fifty Years of Signal Processing: The IEEE Signal Processing Society and its Technologies, 1948-1998. The IEEE Signal Processing Society. 1998.
^ a b
Billings, S. A. (2013). Nonlinear System Identification: NARMAX Methods in the Time, Frequency, and Spatio-Temporal Domains. Wiley. ISBN . 978-1119943594
Slawinska, J., Ourmazd, A., and Giannakis, D. (2018). "A New Approach to Signal Processing of Spatiotemporal Data". 2018 IEEE Statistical Signal Processing Workshop (SSP). IEEE Xplore. pp. 338-342. doi: 10.1109/SSP.2018.8450704. ISBN . 978-1-5386-1571-3 S2CID 52153144. CS1 maint: uses authors parameter ( link)
^ a b
Scharf, Louis L. (1991). Statistical signal processing: detection, estimation, and time series analysis. Boston: Addison-Wesley. ISBN . 0-201-19038-9 OCLC 61160161.
Sarangi, Susanta; Sahidullah, Md; Saha, Goutam (September 2020). "Optimization of data-driven filterbank for automatic speaker verification". Digital Signal Processing. 104: 102795. arXiv: . 2007.10729 doi: 10.1016/j.dsp.2020.102795. S2CID 220665533.
Anastassiou, D. (2001). "Genomic signal processing". IEEE Signal Processing Magazine. IEEE. 18 (4): 8-20. doi: 10.1109/79.939833.
Patrick Gaydecki (2004). . IET. pp. 40-. Foundations of Digital Signal Processing: Theory, Algorithms and Hardware Design ISBN . 978-0-85296-431-6
Shlomo Engelberg (8 January 2008). . Springer Science & Business Media. Digital Signal Processing: An Experimental Approach ISBN . 978-1-84800-119-0
Boashash, Boualem, ed. (2003). Time frequency signal analysis and processing a comprehensive reference (1 ed.). Amsterdam: Elsevier. ISBN . 0-08-044335-4
Stoica, Petre; Moses, Randolph (2005). Spectral Analysis of Signals (PDF). NJ: Prentice Hall.
Peter J. Schreier; Louis L. Scharf (4 February 2010). . Cambridge University Press. Statistical Signal Processing of Complex-Valued Data: The Theory of Improper and Noncircular Signals ISBN . 978-1-139-48762-7
Max A. Little (13 August 2019). . OUP Oxford. Machine Learning for Signal Processing: Data Science, Algorithms, and Computational Statistics ISBN . 978-0-19-102431-3
Steven B. Damelin; Willard Miller, Jr (2012). . Cambridge University Press. The Mathematics of Signal Processing ISBN . 978-1-107-01322-3
Daniel P. Palomar; Yonina C. Eldar (2010). . Cambridge University Press. Convex Optimization in Signal Processing and Communications ISBN . 978-0-521-76222-9
P Stoica, R Moses (2005). Spectral Analysis of Signals (PDF). NJ: Prentice Hall.
Kay, Steven M. (1993). Fundamentals of Statistical Signal Processing. Upper Saddle River, New Jersey: Prentice Hall. ISBN . 0-13-345711-7 OCLC 26504848.
Papoulis, Athanasios (1991). Probability, Random Variables, and Stochastic Processes (third ed.). McGraw-Hill. ISBN . 0-07-100870-5 Kainam Thomas Wong
: Statistical Signal Processing lecture notes at the University of Waterloo, Canada.
Ali H. Sayed, Adaptive Filters, Wiley, NJ, 2008, ISBN 978-0-470-25388-5. Thomas Kailath, Ali H. Sayed, and Babak Hassibi, Linear Estimation, Prentice-Hall, NJ, 2000, ISBN 978-0-13-022464-4.