Introduction to Biomedical Signals

Living organisms are made up of many component systems -- the human body, for

example, includes the nervous system, the cardiovascular system, and the musculoskeletal

system, among others. Each system is made up of several subsystems that

carry on many physiological processes. For example, the cardiac system performs

the important task of rhythmic pumping of blood throughout the body to facilitate

the delivery of nutrients, as well as pumping blood through the pulmonary system

for oxygenation of the blood itself.

Physiological processes are complex phenomena, including nervous or hormonal

stimulation and control; inputs and outputs that could be in the form of physical

material, neurotransmitters, or information; and action that could be mechanical,

electrical, or biochemical. Most physiological processes are accompanied by or

manifest themselves as signals that reflect their nature and activities. Such signals

could be of many types, including biochemical in the form of hormones and neurotransmitters,

electrical in the form of potential or current, and physical in the form of

pressure or temperature.

Diseases or defects in a biological system cause alterations in its normal physiological

processes, leading to pathological processes that affect the performance,

health, and general well-being of the system. A pathological process is typically

associated with signals that are different in some respects from the corresponding

normal signals. If we possess a good understanding of a system of interest, it becomes

possible to observe the corresponding signals and assess the state of the system. The

task is not very difficult when the signal is simple and appears at the outer surface of

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2 INTRODUCTION TO BIOMEDICAL SIGNALS

the body. For example, most infections cause a rise in the temperature of the body,

which may be sensed very easily, albeit in a relative and qualitative manner, via the

palm of one's hand. Objective or quantitative measurement of temperature requires

an instrument, such as a simple thermometer.

A single measurement x of temperature is a scalar, and represents the thermal state

of the body at a particular or single instant of time t (and a particular position). If we

record the temperature continuously in some form, say a strip-chart record, we obtain

a signal as a function of time; such a signal may be expressed in continuous-time or

analog form as x(t). When the temperature is measured at discrete points of time,

it may be expressed in discrete-time form as x(nT) or x(n), where n is the index

or measurement sample number of the array of values, and T represents the uniform

interval between the time instants of measurement. A discrete-time signal that can

take amplitude values only from a limited list of quantized levels is called a digital