Data Acquisition Process - Signal Processing of Random Physiological Signals - page 32

In Out

Collection Recording Preparation Qualification Analysis

FIGURE 5.1 : Data acquisition and analysis process

5.2 DATA RECORDING/TRANSMISSION
As in the past, magnetic storage systems are the most convenient and desirable type
of data storage. In the 1950s through the 1980s, magnetic tape recordings were the
only way to store large quantities of data. If a magnetic tape recorder was used,
the most widely used method was referred to as “Frequency Modification” (FM)
for analog type recordings. Beyond the limitation inherent in the magnetization-
reproduce and modulation-demodulation operations, there are a number of miscellaneous
problems.
Among the more important of these problems are the errors associated with
variations in the speed at which the tape passes over the record and/or reproduce
heads. These errors were called “Time Based Errors,” with the most common being
“Flutter,” which is the variation of tape velocity from the normal. For the frequency
modulation (FM) recording systems, the time based (frequency) is used to represent
the amplitude variations of the signal. The error in good high-quality FM recorders is
about 0.25%.
Currently, few laboratories still use magnetic tape recordings. The large “Giga-
bite” disk memories and optical disc with compression techniques are the most common
approach to storage of data; however, the data must be in digital format, and thus must
be converted from an analog format to a digital format. The advantage of storing the
raw data is that different signal-processing methods may be used on the same data and
the results compared.

Mechanical
conversion

Electrical
conversion

Physical
quantity

Pickoff

Mech Qty. Elec.

Internal

FIGURE 5.2 : General model of a mechanical transducer for data collection, that is, a blood
pressure transducer