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Skew refers to time differences between samples having the same sample number in different signals. Skew may arise while digitizing multitrack analog tape recordings, for example, as a result of differences in the azimuth of the recording and playback heads of the recording equipment. It may be possible to measure skew (for example, by applying test signals simultaneously or at known intervals to all input channels, and then by measurement of the digitized test signals). When this is possible, skewedit can then be used to record the skew measurements in the header file.
For
example, assume that a test signal applied simultaneously to all inputs
of record abc is determined to appear on signal 0 at sample 30, on signal
1 at sample 28, on signal 2 at sample 28, and on signal 3 at sample 26.
In this case, skew0 is 4 (30 - 26), skew1 and skew2 are each 2, and skew3
is 0. The command
skewedit abc 4 2 2 0
would apply the proper correction to the header file for the record. (The
final ‘0’ may be omitted from the command.)
Applications built using the WFDB library (version 9.2 or later) are able to correct for skew (the skew correction is performed by the WFDB library and is not visible to the application program). Note that skew correction does not require rewriting the signal file(s).
If you wish to create skew-corrected signal files (for example, to use with applications built using earlier versions of the WFDB library), use xform(1) to do so, using the header file generated by skewedit as input to xform. Note, however, that older applications can generally be updated without source changes simply by recompiling them and linking them with the current WFDB library.
It may be necessary to set and export the shell variable WFDB (see setwfdb(1) ).
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PhysioNetUpdated 8 March 2019