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- AC-coupled signal
A signal, such as an ECG, for which only variations in level, rather
than absolute levels, are significant. Such signals are usually passed
through high-pass filters before they are digitized, in order to remove
any DC component (baseline offset), so that the gain can be chosen
optimally for the range of variation in the signal.
- ADC resolution
The number of significant bits per sample. Typical ADCs yield between 8
and 16 bits of resolution.
- ADC zero
The value produced by the ADC given a 0 volt input. For bipolar ADCs,
this value is usually 0, but for the unipolar (offset binary) converter
used for the MIT DB, the ADC zero was 1024.
The unit of amplitude for samples.
- AHA DB
The American Heart Association Database for the Evaluation of Ventricular
Arrhythmia Detectors, consisting of 80 records identified by four-digit
- AHA format
The format used for interchange of AHA DB and MIT DB records on 9-track
tape between institutions, not used for on-line files because it is
relatively wasteful of storage space compared to MIT format
A label, associated with a particular sample, which describes a feature
of the signal at that time. Most annotations are QRS annotations and
indicate the QRS type (normal, PVC, SVPB, etc.). Annotations are
putann and read by
- Annotation code
An integer in the range of 1 to
ACMAX (a constant defined in
`<ecg/ecgcodes.h>') inclusive, which denotes an event type.
- Annotation file
A set of annotations in time order.
- Annotator name
A name associated with an annotation file. On writable UNIX and
Macintosh file systems, the annotation file name is constructed from the
annotator name by appending a `.' and the record name. On CD-ROMs
and MS-DOS file systems, the annotator name is restricted to three
characters, and the annotation file name is constructed from the record
name by appending a `.' and the annotator name. UNIX and Macintosh
versions of the DB library can locate and read annotation files named
using either convention.
- Annotator [number]
An integer by which an annotation file, once opened, is known. Input
annotators and output annotators each have their own series of annotator
numbers assigned in serial order beginning with 0. Since at most
DB_MAXANN (defined in `<ecg/db.h>') input or output
annotation files may be open at once, the largest valid annotator number
DB_MAXANN - 1.
- Application program
In this guide, a program that uses the DB library to do
The annotator name for the reference annotation files. On CD-ROMs and
MS-DOS file systems, `atruth' is truncated within the names of the
reference annotation files to `atr';
annopen accepts either
`atruth' or `atr' as the annotator name for these files.
- Base counter value
The counter value (q.v.) that corresponds to sample 0. The base
counter value is read by
getbasecount, and set by
(or by any of the functions that read `header' files). If not
defined explicitly, the base counter value is taken to be 0.
- Base time
The time of day that corresponds to sample 0 in a given record. For
MIT, AHA, and ESC DB records, the base time was not recorded and is taken to
be 0:0:0 (midnight).
- Baseline [amplitude]
The sample value that corresponds to the baseline (isoelectric level or
physical zero level) in the signal. This quantity may drift during the
record for a variety of reasons, in which case the
DB_Siginfo object that describes the signal is only an
approximation. The baseline is not the same as the ADC zero
(q.v.), which is a fixed characteristic of the digitizer.
- Calibration file
A file containing data used to build a calibration list (q.v.).
- Calibration list
A memory-resident linked list of
(see section Calibration Information Structures).
Each such structure specifies the size and type of the calibration pulse,
and the customary plotting scale, for a particular type of signal.
A read-only medium used for distribution of the MIT-BIH and ESC
databases, among others. CD-ROMs are physically identical in appearance
to audio compact disks. CD-ROM readers are available for most
microcomputers and for many larger systems.
- Closing [a record]
The process of completing I/O associated with a record.
- Counter frequency
The difference between counter values (q.v.) that are separated
by an interval of one second. The counter frequency is constant
throughout any given record. It may be undefined, in which case it is
treated as equivalent to the sampling frequency (q.v.) by the DB
library. The counter frequency is read by
and set by
setcfreq (or by any of the functions that read
- Counter value
A number that serves as a time reference, in a record for which a
counter frequency is defined. A counter value may be converted to the
time in seconds from the beginning of the record by subtracting the base
counter value (q.v.) and dividing the remainder by the counter
frequency. The units of `c'-prefixed
strtim arguments are
- Database files
Those files (annotation files, `header' files, signal files, and
calibration files) that are accessed via the DB library.
- Database path
The names of the directories in which `header', annotation, and
calibration files are kept. The environment variable
DB must be
set by the user and exported accordingly.
- DB library
A set of functions (subroutines), able to read and write database files,
callable by C and C++ programs, and described in this guide.
- DC-coupled signal
A signal, such as a blood pressure signal, for which absolute levels are
significant. Such signals must be digitized without being passed
through high-pass filters, in order to preserve absolute levels.
- ESC DB
The European ST-T Database, consisting of 90 records identified by
`e'-prefixed four-digit record names.
A set of samples, containing all samples that occur within a given
frame interval. For an ordinary record, a frame contains exactly one
sample of each signal; for a multi-frequency record, a frame contains
at least one sample of each signal, and more than one sample of
each oversampled signal (q.v.).
- Frame interval
A time interval during which at least one sample exists for each signal.
For an ordinary record, the frame interval and the sampling interval are
identical. For a multi-frequency record, the frame interval is chosen
to be an integer multiple of each sampling frequency used.
- Frame rate
The basic sampling frequency defined for a multi-frequency record; the
reciprocal of the frame interval. The frame rate is usually the lowest
sampling frequency used for any signal included in the record.
In this context, the number of adus (q.v.) per physical unit, referred to the
original analog signal. Gain in this sense is directly proportional to the
degree of amplification (the usual meaning of the word) of the analog
signal prior to digitization. Gain may vary between signals in a
- `header' file
A file accessible via the DB library that describes the signal files
associated with a given database record. On writable UNIX and Macintosh
file systems, `header' files have names of the form
header.record', where record is the record name
(q.v.). On CD-ROMs and MS-DOS file systems, `header' files have
names of the form `
record.hea'. UNIX and Macintosh
versions of the DB library can locate and read `header' files named
using either convention.
- High-resolution mode
An alternative mode for reading a multi-frequency record using
getvec, that can be selected using
getvec replicates samples of signals
digitized at less than the maximum sampling frequency, so that each
sample of any oversampled signals appear in at least one sample vector.
- Info string
Free text within a `header' file. Info strings can be read using
getinfo and written using
- Local record
A record for which the signal files reside in the current directory,
typically used for user-created signals. Records `8l' and
`16l' are local records.
- Low-resolution mode
The default mode for reading a multi-frequency record using
getvec. In low-resolution mode,
getvec returns one
sample per signal per frame, by decimating any oversampled signals
to the frame rate.
- MIT DB
The Massachusetts Institute of Technology--Beth Israel Hospital
Arrhythmia Database, consisting of 48 records identified by three-digit
- MIT format
The standard format for storage of DB records on CD-ROMs, used on the
MIT, ESC, and MGH DB CD-ROMs, among others.
- Modification label
An "invisible" annotation at the beginning of an annotation file. A
modification label defines an annotation mnemonic and a corresponding
dbinit) opens an annotation
file that contains modification labels, it automatically calls
setanndesc to add the mnemonics and
descriptions to the translation tables used by
dbinit) creates an annotation file, it automatically generates
modification labels, for each annotation code that has been (re)defined
setanndesc. For this reason, you
should normally make all of your calls to
setanndesc before calling
(An exception is if you are simply translating mnemonics and
descriptions into another language, rather than redefining them.)
Version 5.3 and later versions of the DB library support reading and
writing modification labels; earlier versions read modification labels
- Multi-frequency record
A record containing signals sampled at two or more sampling frequencies.
Version 9.0 and later versions of the DB library support reading and
writing multi-frequency records.
- Multi-segment record
A composite record that is the concatenation of two or more ordinary
(single-segment) records. Multi-segment records do not have their own
signal files (the signal files of their constituent segments are read
when it is necessary to read signals of multi-segment records), but they
have their own `header' files (created using
and may have their own annotation files as well (annotation files for
the constituent segments of a multi-segment record are not
concatenated automatically when the record is read). The DB Software
dbcollate (under MS-DOS,
application that can create multi-segment records from sets of
single-segment records. Version 9.1 and later versions of the DB
library support reading and writing multi-segment records.
- Multiplexed signal file
A set of vectors in time order, each consisting of two or more integer
samples, thus representing an equal number of signals.
- 9-track tape
A medium used for archival storage of DB records, which was once nearly
universally available on minicomputers and larger systems. The
important parameters are tape density (typically 800 or 1600 bpi) and
block size (typically some multiple of 512 bytes). Higher tape density
and larger block size permit more data to be stored on a tape.
- Opening [a database record or a file]
The process of making a database record or a file accessible, if
necessary by creating it.
- Oversampled signal
In a multi-frequency record, any signal recorded at a sampling frequency
greater than the frame rate (q.v.).
- Physical unit
The natural unit of measurement of the original analog signal (e.g.,
millivolts, liters per second, degrees). To convert samples into
physical units, subtract the ADC zero and divide the remainder by the
- Physical zero
The level (in physical units) that corresponds to the baseline (in adu),
normally zero physical units. For example, physical zero for a pressure
signal with units of mmHg is 0 mmHg.
- Piped record
A database record for which a signal file is designated as `-',
signifying that it is to be read from the standard input or written to
the standard output. Records `8' and `16' are piped
Extraneous bytes at the beginning of a signal file that are not to be
read as samples. Signal files created using the DB library do not
contain prologs, but signal files created using other means may contain
prologs. To read such a signal file using the DB library, provided that
the sample data is in a supported format, it is sufficient to record the
length of the prolog (in bytes) in the appropriate locations in a
`header' file that names the signal file. If you need to create
such a `header' file, refer to the description of the byte offset
field in header(5) (the specification of the `header' file
format in the ECG Database Applications Guide, or
see section dbsetstart.
An extensible set of files that may include signal files, annotation
files, and a `header' file, all of which are associated with the
same original signals. Only the `header' file is mandatory.
Although records are sometimes called tapes for historical reasons,
records are now more commonly maintained on optical or magnetic disks
than on tape.
- Record name
A character string that identifies a database record. Record names of
MIT DB records are 3-digit numerals, those of AHA DB records are 4-digit
numerals, and those of ESC DB records are 4-digit numerals with a
prefixed `e'. Record names may contain up to
(defined in `<ecg/db.h>') characters, including any combination of
letters, digits, and underscores. Case (the difference between `e'
and `E', for example) is significant in record names, even under
operating systems such as MS-DOS that do not treat case as significant
in file names.
- Reference annotation file
An annotation file supplied by the creator of a record to document its
contents as accurately and thoroughly as possible. The annotator name
`atruth' is reserved for reference annotation files.
An integer (of at least 16 bits) that corresponds to a voltage measured
at a given instant by an analog-to-digital converter. Samples are
putvec and read by
- Sample interval
The unit of time; the interval between consecutive samples of a given
- Sample number
An attribute of a sample defined as the number of samples of the same
signal that precede it; thus the first sample of any signal has sample
number 0. Sample numbers are long integers (32 bits). Samples that
have the same sample number in different signals of a given record may
be treated as having been observed simultaneously.
- Sampling frequency
The number of samples of a given signal that represent one second of
the original analog signal. The sampling frequency is constant
throughout a signal file, and is the same for all signals in a given
A continuously varying function of time that is approximated by
- Signal file
A set of samples in time order, which represent a signal or signal
- Signal group
A set of signals that are multiplexed together and stored in the same
file. It is possible to reset input pointers for all signals in a
given signal group
(see section isgsettime),
but not independently for individual signals within a signal group.
- Signal group number
A number by which a signal file, once opened, is known.
- Signal number
An integer by which a signal, once opened, is known. Input and output
signals each have their own series of signal numbers assigned in serial
order beginning with 0. Since at most
DB_MAXSIG (defined in
`<ecg/db.h>') input or output signals may be open at once, the
largest valid signal number is
DB_MAXSIG - 1.
The time difference between samples having the same sample number but
belonging to different signals. Ideally the skew is zero (or less than
one sample interval), but in some cases this is not so. For example, if
the signals were originally recorded on multitrack analog tape, very
small differences in the azimuth of the recording and playback heads may
result in measurable skew among signals. If the skew can be measured
(for example, by reference to features of two signals with a known time
difference), it can be recorded in the `header' file for a record;
once this has been done,
getframe correct for
skew automatically. If you need to correct for skew, see
skewedit(1) and header(1) (in the ECG Database
Applications Guide), or see section dbsetskew. Prospectively, if you
anticipate that skew may be a problem, it is a good idea to apply an
easily identifiable synchronization pulse to all your inputs
simultaneously while recording; you can then locate this pulse in
each digitized signal and use these measurements to correct for skew.
- Standard time format
Any string format legal as an argument for
(see section [ms]timstr).
A database record.
In this guide, synonymous with sample number (q.v.). Thus the "time of
an annotation" is the sample number of the sample to which the
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George B. Moody (firstname.lastname@example.org)