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Programs compiled using the WFDB library (see wfdb(3) ) require calibration data in order to convert between sample values (expressed in analog-to-digital converter units, or adus) and physical units. Calibration files specify the physical characteristics of calibration pulses that may be present in various types of signals, and specify customary scales for plotting these signals. calsig(1) reads the signal file(s) for a record, measures the size of the calibration pulses it finds in adus, and uses specifications from a calibration file to determine adu-to-physical unit conversion parameters, the ‘gain’ and ‘baseline’ fields that it writes back into the header file for the record. Other programs, such as pschart(1) , make use of the ‘gain’ and ‘baseline’ fields from the header file to determine how to convert adus into physical units, and use customary scale specifications from a calibration file to determine how to convert physical units into units of length on a printed page or on-screen. Most users will find that a single calibration file, perhaps a system-wide default, can be used with all of their WFDB records.
Calibration files are line-oriented
text files. Lines are separated by a carriage-return/line-feed pair. Each
type of signal to be calibrated is described by a one-line entry. The format
of each entry is:
DESC<tab>LOW HIGH TYPE SCALE UNITS
where DESC is a string, possibly containing embedded spaces but not tabs,
taken from the signal description field of the header file entry for signals
of the desired type; LOW and HIGH are the physical measurements that correspond
to the low- and high-amplitude phases of the calibration pulse; TYPE specifies
the shape of the calibration pulse (‘sine’, ‘square’, or ‘undefined’); SCALE
specifies the customary scale in physical units per centimeter; and UNITS
is a string (without embedded whitespace) that specifies the physical units
of the signal (e.g., ‘mV’, ‘mmHg’, ‘degrees_Celsius’). If LOW is ‘-’, the signal is
AC-coupled, and HIGH is taken as the peak-to-peak amplitude of the calibration
pulse. LOW must be defined (i.e., must not be ‘-’) for DC-coupled signals. If
HIGH is ‘-’, the size of the calibration pulse is undefined.
Lines that begin with ‘#’, empty lines, and improperly formatted lines are treated as comments and ignored.
The WFDB library function getcal, used by programs such as calsig(1) , psfd(1) , and wave(1) to obtain calibration data from a calibration file, returns the first entry that matches a signal’s description and units. A calibration file entry is considered to match a signal if the DESC field is either an exact match or a prefix of the signal description as given in the header file, and if the UNITS field in the calibration file is an exact match of the units field in the header file. By making use of these two rules, it is possible to write a calibration file that contains entries for several specific cases followed by a ‘catch-all’ case for which the DESC field contains only the common prefix.
Note that SCALE specifications are advisory, not mandatory. The intended use of SCALE is to specify the customary size for signals, and the relative sizes of signals of varying types. When determining a SCALE for a signal type for which there is no customary scale, a good rule of thumb is that the typical short-term range of variation of the plotted signal should be on the order of one centimeter; keep in mind that it may be useful to make measurements on plots, however, and choose a scale that makes such measurements easy to perform. Programs that draw signals at non-standard scales should generally adjust the scales for all signals by the same factor, unless the user specifies otherwise.
An entry
of the form:
ECG lead I - 1 sine 1 mV
matches ‘ECG lead II’ as well as ‘ECG lead I’, because of the prefix rule (see
above). If ‘ECG lead I’ and ‘ECG lead II’ were to require different calibrations
for some reason, an entry of the form:
ECG lead II - 2 sine 1 mV
should be inserted before the entry for ‘ECG lead I’.
Programs that display
time series extracted from annotation files (e.g., wave(1)
, which can display
the sequence of ‘num’ fields in an annotation file as a signal) can use calibration
records to choose an ordinate scale. These records can be included in the
calibration file, with annotator names used in place of the signal type,
and ‘units’ as the units type. An entry with signal type "ann" can be used
as a default for calibrating data from files whose annotator names do not
have entries. For example, the default calibration file contains these
entries:
edr - - undefined 200 units
ann - - undefined 100 units
The first specifies that data from ’edr’ annotation files are to be displayed
at a nominal 200 units per centimeter. The second specifies that files
from other types of annotation files are to be displayed at 100 units per
centimeter.
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PhysioNetUpdated 8 March 2019