Physical nature of different biosignals

Definition
Biological signal is a summarizing term for all kinds of signals that can be (continually) measured and monitored from biological beings. It shows chemical or physical quantities that characterize the property or state of human biological condition. The term biosignal is often used to mean bio-electrical signal but in fact, biosignal refers to both electrical* and non-electrical signals.Biosignal can be defined as a description of a physiological phenomenon.

*The term electrical Biosignals refers to changes in electric currents produced by the sum of electrical potential differences across a specialized tissue, organ or cell system.

Describing Biosignals

 * Continuous Signals
 * are described by a continuous function s (t) which will show information at any time. *Most biomedical signals are continuous


 * Descrete Signals
 * are described by a sequence s (m) which will show information exactly at a particular time.


 * Determenistic Signals
 * are signals which can be determined and described exactly using mathematics or gaphics. Real world biosignals are never deterministic. Periodic signals belong to this group and are expressed by s(t)= s(t+nT) n is integer and T is period. Blood pressure could be characterized as a complex periodic signal.


 * Stochastic Signals
 * cannot be expressed exactly but only in terms of probabilities. Stationary stochastic processes wil not change in time. The expectations of such a process is time independant. Most of them are non stationary. An example would be EEG (Electroencephalography)

Classifications
Since there are many Biosignals it is very hard to find a unique way of classifying them.

Classification by existence:
 * Permanent biosignals
 * They can exist without an artificial trigger and are available at any time. The source for those Biosignals is 	already inside the body.


 * Induced biosignals
 * are artifically triggered or induced and last only during the time of excitation.

Classification by dynamic nature:
 * A static biosignal carries information in its steadystate level which may exhibit relatively slow changes over time.
 * Dynamic biosignals yield extensive changes in the time domain, with dynamic processes conveying the physiological information of interest.

Classification by origin:
 * Magnetic biosignals
 * These signals include motion and displacement signals, pressure and tension and flow signals, and others.


 * Optic biosignals
 * are the result of optical functions of the biologic system, occurring naturally or induced by the measurement (Blood oxigination)


 * Acoustic biosignals
 * Many physiological phenomina create noise like the flow of blood in the heart or throuhg blood vessels also the flow or air through the airways creates acoustic sounds.


 * Chemical biosignals
 * reflect chemical composition and its temporal changes in body solids, liquids, and gases. Examples are measuring the concentration of various ions and vicinity of a cell by means of specific ion electrodes


 * Thermal Biosignals
 * Temperature measurement shows physical and biochemical processes proceeded in organism.(Heat loss, heat absorption)