Toward Sensor to Reproduce Human Sense - The Electronic Nose Artificial Olfaction Technology - page 132

unpredictable noise adds some unpredictable change in the parameters. It is very
often convenient to approximate or model the signal by means of a deterministic
function(s).
An important family of deterministic signals is the periodic family. A periodic
signal is a deterministic signal that may be expressed by

S ðÞ¼ st þ nT ð Þ

Where n is an integer, and T is the period.
Under some conditions, the blood pressure signal may be modeled by a com-
plex periodic signal, with the heart rate as its period and the blood pressure wave
shape as its basic wave shape.
Most deterministic functions are non periodic. It is sometimes worthwhile to
consider an 'almost periodic' type of signal. Example: The ECG signal can
sometimes be considered almost periodic.
The most important class of signals is the stochastic class. Stochastic signals
cannot be expressed accurately; they can be described only in terms of probabil-
ities. Stationary stochastic processes are processes whose statistics do not change
in time. The expectation and the variance (as with any other statistical mean) of a
stationary process will be time-independent. Unfortunately, almost all signals are
non-stationary e.g., sleep Electroencephalograph (EEG) signal.
According to origin bio-signals may be classified as follows:

1. Bioelectric signals: Nerve cells and muscle cells generate a signal which is
called bioelectric signal. Membrane potential excited in certain conditions and
generates action potential which is source of the bioelectric signal. A simple
transducer is required for measurement of the bioelectric signal. ECG and
EEGs are these types of signals which are shown in Figs. 5. 10 and 5.11 .
2. Bioimpedance signals: The Bioimpedance signal is generated as the
muscle/body tissue place under test sinusoidal currents with frequency range

Fig. 5.9 Classification of signals