Signal

A signal is defined as defined as any physical quantity that varies with time, space or any other independent variable or variables. Mathematically, we describe a signal as a function of one or more independent variables. For example, the functions

  describe two signals, one that varies  linearity with the independent variable t (time) and a second that varies quadratically with t. As another example, consider the function
  
This function describes a signal of two independent variables x and y.

Analog- to-Digital and Digital-to-analog Conversion

Most signals of practical interest, such as speech, biological signals, seismic signals.radar signal, sonar signals, and various communications signals such as audio and video signals, are analog.To process analog signal by digital means, it is first necessary to converted them to digital form, that is, to convert them to a sequence of numbers having finite precision. This procedure is called analog -to-digital(A/D) conversion, and the corresponding devices are called A/D converters(ADCs).

Conceptually, we view A/D conversion as a three-step process.this process is illustrated in Figure

 

Fig: Block diagram of a digital signal processing system

Advantage of Digital Over Analog Signal Processing

1.  The main advantage of digital signals over analog signals is that the precise signal level of the digital signal is not vital. This means that digital signals are fairly immune to the imperfections of real electronic systems which tend to spoil analog signals. As a result, digital CD's are much more robust than analog LP's.

 2. Codes are often used in the transmission of information. These codes can be used either as a means of keeping the information secret or as a means of breaking the information into pieces that are manageable by the technology used to transmit the code, e.g. The letters and numbers to be sent by a Morse code are coded into dots and dashes.

 3. Digital signals can convey information with greater noise immunity, because each information component (byte etc) is determined by the presence or absence of a data bit (0 or one). Analog signals vary continuously and their value is affected by all levels of noise.
4. Digital signals can be processed by digital circuit components, which are cheap and easily produced in many components on a single chip. Again, noise propagation through the demodulation system is minimized with digital techniques.

 5. Digital signals do not get corrupted by noise etc. You are sending a series of numbers that represent the signal of interest (i.e. audio, video etc.)
6.  Digital signals typically use less bandwidth. This is just another way to say you can cram more information (audio, video) into the same space.
7. Digital can be encrypted so that only the intended receiver can decode it (like pay per view video, secure telephone etc.)

 8. Enables transmission of signals over a long distance.
9. Transmission is at a higher rate and with a wider broadband width.

10. It is more secure.
11. It is also easier to translate human audio and video signals and other messages into machine language.
12. There is minimal electromagnetic interference in digital technology.
13. It enables multi-directional transmission simultaneously.