.TH XFORM 1 "7 January 2009" "WFDB 10.4.12" "WFDB Applications Guide"
.SH NAME
xform \- sampling frequency, amplitude, and format conversion for WFDB records
.SH SYNOPSIS
\fBxform -i\fR \fIinput-record\fR [ \fIoptions\fR ... ]
.SH DESCRIPTION
\fBxform\fR copies the signal files (and, optionally, annotation files) of the
specified \fIinput-record\fR. By default, all signals are copied in their
entirety; using appropriate options, \fBxform\fR can be used to copy only
a portion of the record, or only a subset of the signals, or both.
\fIOptions\fR are:
.TP
\fB-a\fR \fIannotator\fR
Copy the specified \fIannotator\fR as well as the signal files. Two or more
\fIannotator\fR arguments, separated by spaces, can follow \fB-a\fR. An
annotator supplied via the standard input may be specified using `-', but only
immediately after \fB-a\fR; in this case only, annotations are copied to the
standard output.
.TP
\fB-c\fR
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.
.TP
\fB-d\fR
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
\fB-d\fR 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 \fBxform\fR's
output is strictly reproducible. Change the seed in the source and recompile
to obtain a different realization of dither if desired.
.TP
\fB-f\fR \fItime\fR
Begin at the specified \fItime\fR in the input record (default: the
beginning of the record).
.TP
\fB-h\fR
Print a usage summary.
.TP
\fB-H\fR
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).
.TP
\fB-M\fR
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.
.TP
\fB-n\fR \fInew-record\fR
Create a \fInew-record\fR for the output signal files.
.TP
\fB-N\fR \fInew-record\fR
As above, but copy the signal descriptions from the header file for the
record specified using the \fB-o\fR option (see below) rather than from
the input record.
.TP
\fB-o\fR \fIoutput-record\fR
The header file for \fIoutput-record\fR (which must exist before running
\fBxform\fR) determines the names, sampling frequency, formats (see
\fBsignal\fR(5)), gains, and ADC zero levels of the output signals.
If the \fB-o\fR option is absent, \fBxform\fR prompts the user for the
output specifications.
.TP
\fB-s\fR \fIsignal-list\fR
Write only the signals named in the \fIsignal-list\fR (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.
.TP
\fB-S\fR \fIscript\fR
Take answers to prompts from the specified \fIscript\fR (a text file).
.TP
\fB-t\fR \fItime\fR
Process until the specified \fItime\fR in the input record (default: continue
to the end of the record).
.TP
\fB-u\fR
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 \fB-u\fR option avoids this.
.PP
If a \fInew-record\fR 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 \fB\-a\fR are associated with \fInew-record\fR if it has been
specified, or with \fIoutput-record\fR otherwise.
To process only a segment of the \fIinput-record\fR, specify the starting and
ending times using the \fB-f\fR and \fB-t\fR options.
.PP
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 \fBxform\fR (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 \fBfir\fR(1)) to remove these components before processing them with
\fBxform\fR. 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.
.PP
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.
.PP
Also note that \fBxform\fR writes over any existing data files named in
the header file for \fIoutput-record\fR; thus \fIoutput-record\fR 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" \fI8\fR and \fI16\fR and the "local records"
\fI8l\fR and \fI16l\fR will be found useful as output records.
.PP
If signal selection, scaling, and sampling frequency conversion are not needed,
\fBsnip\fR(1) is recommended as a faster alternative to \fBxform\fR.
.SH ENVIRONMENT
.PP
It may be necessary to set and export the shell variable \fBWFDB\fR (see
\fBsetwfdb\fR(1)).
.SH DIAGNOSTICS
.PP
As \fBxform\fR 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,
\fBxform\fR issues warnings but continues to process the record.
.SH SEE ALSO
\fBfir\fR(1), \fBsetwfdb\fR(1), \fBsnip\fR(1), \fBsignal\fR(5)
.SH AUTHOR
George B. Moody (george@mit.edu)
.SH SOURCE
http://www.physionet.org/physiotools/wfdb/app/xform.c