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MIT DB records are each 30 minutes in duration, and are annotated throughout; by this we mean that each beat (QRS complex) is described by a label called an annotation. Typically an annotation file for an MIT DB record contains about 2000 beat annotations, and smaller numbers of rhythm and signal quality annotations. AHA DB records are either 35 minutes or 3 hours in duration, and only the last 30 minutes of each record are annotated. ESC DB records are each 2 hours long, and are annotated throughout. The “time” of an annotation is simply the sample number of the sample with which the annotation is associated. Annotations may be associated with a single signal, if desired. Like samples in signals, annotations are kept in time and signal order in annotation files (but see section Annotation Order, for exceptions to this rule). No more than one annotation in a given annotation file may be associated with any given sample of any given signal. There may be many annotation files associated with the same record, however; they are distinguished by annotator names. The annotator name ‘atr’ is reserved to identify reference annotation files supplied by the developers of the databases to document correct beat labels. You may use other annotator names (which may contain letters, digits and underscores, as for record names) to identify annotation files that you create. You may wish to adopt the convention that the annotator name is the name of the file’s creator (a program or a person).

Annotations are visible to the WFDB library user as C structures, the fields of which specify time, beat type, and several user-definable variables. The WFDB library performs efficient conversions between these structures and a compact bit-packed representation used for storage of annotations in annotation files.

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