There are several reasons why the size of signals may be incorrect. WAVE
determines display scales for signals based on several parameters:
First, the WFDBCAL environment variable specifies the name
of a text file (located in a directory in the WFDB path) containing the
names of many common signals and customary display scales for each
(expressed as physical units per centimeter). For example, ECG
signals are customarily displayed at a scale of 1 mV per centimeter.
If a signal appears at an inappropriately large or small scale, its
name may be missing from the calibration file, the WFDBCAL
variable may not correctly specify the name of the calibration file,
or the WFDBCAL file may not reside in a directory in the WFDB path.
On most systems, the calibration file is `/usr/database/wfdbcal', and the WFDBCAL environment
variable is set by the same command used to set the WFDB path (see the
discussion about the
Instructions for adding additional signal names and scales to the calibration
file are located within the file itself, and may also be found in the
WFDB Applications Guide.
Second, the header file for the record specifies the gain for each signal
(the number of analog-to-digital units per physical unit). If the gain has
not been determined, or is incorrect, the size of the signal as drawn by
WAVE may be incorrect as well. If you know the physical values that
correspond to at least two levels represented in the signal (for example,
the top and bottom of a calibration pulse), you can use the `calsig'
application to determine the gain and to rewrite the header file. See the
WFDB Applications Guide
for details. `calsig' can be most conveniently run from
WAVE 's Analysis window.
- Finally, the display calibration determines the number of pixels per inch
(25.4 mm). WAVE usually gets its information about the display calibration
from the X server, but the X server may not supply the correct information.
``How can I get correct display scales?''
for details on diagnosing and correcting this problem.
George B. Moody (firstname.lastname@example.org)