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Name

xform - sampling frequency, amplitude, and format conversion for WFDB records

Synopsis

xform -i input-record [ options ... ]

Description

xform copies the signal files (and, optionally, annotation files) of the specified input-record. By default, all signals are copied in their entirety; using appropriate options, xform can be used to copy only a portion of the record, or only a subset of the signals, or both. Options are:
-a annotator
Copy the specified annotator as well as the signal files. Two or more annotator arguments, separated by spaces, can follow -a. An annotator supplied via the standard input may be specified using ‘-’, but only immediately after -a; in this case only, annotations are copied to the standard output.
-c
Clip the output (set any sample values that would fall outside of the range supported by the selected format to the maximum or minimum supported values). By default, the output is not clipped; rather, the values are wrapped around modulo the supported range (i.e., the excess high-order bits are simply discarded). Use of wrap-around can result in bizarre artifacts, but has the advantage that the affected portions of the output signals can (usually) be interpreted properly. Clipping mode is appropriate for testing algorithms or devices that must operate using a more restricted amplitude range than was used when digitizing the original record.
-d
Dither the input by adding a pseudo-random value to each sample. The pseudo-random values are selected from a triangular probability density function between -1 and +1. Dithering is appropriate whenever the output has a lower resolution than the input, as may occur when changing the sampling frequency or gain. The -d option has no effect unless the sampling frequency or gain are changed in the output record. Note that the RNG used to generate the pseudo-random values is started with a fixed seed, so that xform’s output is strictly reproducible. Change the seed in the source and recompile to obtain a different realization of dither if desired.
-f time
Begin at the specified time in the input record (default: the beginning of the record).
-h
Print a usage summary.
-H
Read the signals in high-resolution mode (default: standard mode). These modes are identical for ordinary records. For multifrequency records, the standard decimation of oversampled signals to the frame rate is suppressed in high-resolution mode (rather, all other signals are resampled at the highest sampling frequency).
-M
Read the signals in multifrequency mode. Each signal (in a multifrequency record) is copied to the output record without changing its sampling frequency. In an ordinary record, this option has no effect other than to force the input and output sampling frequencies to be equal.
-n new-record
Create a new-record for the output signal files.
-N new-record
As above, but copy the signal descriptions from the header file for the record specified using the -o option (see below) rather than from the input record.
-o output-record
The header file for output-record (which must exist before running xform) determines the names, sampling frequency, formats (see signal(5) ), gains, and ADC zero levels of the output signals. If the -o option is absent, xform prompts the user for the output specifications.
-s signal-list
Write only the signals named in the signal-list (one or more input signal numbers or names, separated by spaces; default: write all signals). This option may be used to re-order or duplicate signals.
-S script
Take answers to prompts from the specified script (a text file).
-t time
Process until the specified time in the input record (default: continue to the end of the record).
-u
Adjust annotation times as needed so that they are unique. If the output sampling frequency is less than that of the input, the times of closely-spaced annotations may coincide in the output, which may cause problems for some older WFDB applications. The -u option avoids this.

If a new-record is specified, a new header file is created after the signal file transformation is complete. The new header file, if created, contains the correct sample counts and checksums for the new signal files. Any output annotation files that are created as a result of using -a are associated with new-record if it has been specified, or with output-record otherwise. To process only a segment of the input-record, specify the starting and ending times using the -f and -t options.

Sampling frequency changes are performed by linear interpolation; any combination of input and output sampling frequencies is permissible. This interpolation method has the advantage of being reasonably fast, an important consideration since it is often necessary to operate on a million or more samples. Resampling noise is not a significant problem for the typical applications of xform (changing the sampling frequency by factors of five or less). Aliasing can be a problem, however, when the input sampling frequency is greater than the output sampling frequency. In such cases, if the input signals contain frequency components at or above half of the output sampling frequency, the input signals should be low-pass filtered (using, for example fir(1) ) to remove these components before processing them with xform. Conversely, if the output sampling frequency is substantially greater than the input sampling frequency, resampling noise introduced at frequencies in excess of half of the input sampling frequency can be removed by low-pass filtering the output signals.

Normally, the ADC resolution fields in the header files are ignored, and scaling is determined by the ratios of the gain fields. An undefined (0) gain is considered equivalent to a gain of 200 ADC units per physical unit. An exception to this rule occurs if both input and output gains are undefined; in this case, scaling is determined by the difference in the ADC resolution fields, if any.

Also note that xform writes over any existing data files named in the header file for output-record; thus output-record should not be the name of an ordinary database record. Normally, the database signal files are read-only, and attempts to overwrite them are futile. For many applications the "piped records" 8 and 16 and the "local records" 8l and 16l will be found useful as output records.

If signal selection, scaling, and sampling frequency conversion are not needed, snip(1) is recommended as a faster alternative to xform.

Environment

It may be necessary to set and export the shell variable WFDB (see setwfdb(1) ).

Diagnostics

As xform runs, it prints a ‘.’ on the standard error output for each minute processed. If any of the output samples fall outside the range of values that can be properly represented using the specified output format, xform issues warnings but continues to process the record.

See Also

fir(1) , setwfdb(1) , snip(1) , signal(5)

Author

George B. Moody (george@mit.edu)

Source

http://www.physionet.org/physiotools/wfdb/app/xform.c


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