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2 Fundamentals of Information, Signal and System Theory

Patient

Measurement

Computer

Doctor

parameters

relevant

physiological

measurand

signal measurand

raw signal

Fig. 2.24: An example of a signal processing chain in biosignal processing.

In the further course of the signal analysis, the aim is often to reduce the amount

of data to a few, but meaningful parameters that can be used for medical decision-

making. For this purpose, the raw signal is usually first freed from disturbing artefacts

and noise by filtering and only then evaluated with regard to diagnostically relevant

parameters such as heart rate and respiratory rate. A mostly graphical representation

of the evaluation is forwarded to the attending physician for a decision on how to pro-

ceed with the patient’s treatment. In this scenario, the final evaluation is carried out

by an experienced doctor, but medical expert systems can also provide important ser-

vices for further evaluation. Here, on the basis of statistically processed expert know-

ledge, a probability-based decision aid is generated with the help of an algorithm,

which is intended to support the doctor in his decision-making.

When creating the signal processing chain, the relevant and unimportant signal

components as well as the methodology for separating them are often still unclear.

The procedure for finding them is more like searching for a needle in a haystack. If

the signals are heavily distorted by filtering, for example, it can happen that the relev-

ant signal components have also become unusable for diagnostic evaluation. For ex-

ample, the heart rate and respiratory rate can be determined as diagnostic parameters

from an ECG signal under optimal conditions. On the other hand, after a correction of

the Baseline in the ECG by a steep high-pass filter above 0.5 Hz, the respiratory rate

will no longer be present in the filtered signal and an evaluation in this respect will

also not provide reliable values. Performing an in-depth spectral analysis of the sig-

nals (cf. subsection 5.3.2) before filtering, on the other hand, allows, for example, to

determine the frequency content of the raw signal in order to identify a possible un-

wanted influence on the useful signal. For this and similar reasons, care must always

be taken during the measurement and processing of the signals to ensure that the

methods used are optimal and signal-preserving. The methods of the above elaborated

processing steps in the signal processing chain are presented in detail in chapter 4, in

chapter 5 and in chapter 6 and discussed on the basis of the selection criteria and

practical application possibilities.

2.6 Post-Reading and Exercises

Information and Information Transmission

1.

What characterises information? What is meant by extended information and

what is it needed for?