A signal is formally defined as a function of one or more independent variables that conveys information on the nature of a physical phenomenon. For example- any physical quantity that varies with time, space or any other independent variables such as voice, image, light etc .
Any signal generation is usually associated with a system that responds to a stimulus or force. The stimulus (like human voice) in combination with the system (like carbon microphone) is called a signal source (like sound signal source ).
After generating when we pass a signal through a system (physical device), it performs an operation on the signal for proper information transmission. Such operations are usually referred to as signal processing (DSP).If the system is digital in nature , the operations are called as a digital signal processing.
A digital system can be constructed as a combination of digital hardware and software, each of which performs its own set of specified rules. The method or set of rules for implementing the system is called an algorithm.
In practice we have an interest in devising algorithms that are computationally efficient, fast and easily implemented. Thus a major topic in our study of digital signal processing is the discussion of efficient algorithms for performing such operations as filtering, correlation, and spectral analysis.
The main applications of DSP are audio signal processing, audio compression, digital image processing, video compression, speech processing, speech recognition, digital communications, radar, sonar, financial signal processing, seismology and bio-medicine. Specific examples are speech compression and transmission in digital mobile phones, room correction of sound in hi-fi and sound reinforcement applications, weather forecasting, economic forecasting, seismic data processing, analysis and control of industrial processes, medical imaging such as CAT scans and MRI, MP3 compression, computer graphics, image manipulation, hi-fi loudspeaker crossovers and equalization, and audio effects for use with electric guitar amplifiers.