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Each annotation occupies an even number of bytes. The first byte in each pair is the least significant byte. The six most significant bits (A) of each byte pair are the annotation type code, and the ten remaining bits (I) specify the time of the annotation, measured in sample intervals from the previous annotation (or from the beginning of the record for the first annotation). If 0 < A <= ACMAX, then A is defined in <wfdb/ecgcodes.h>. Several other possibilities exist:
All annotations occupy exactly 16 bytes. Within each block, the first byte is unused, the second byte contains the AHA annotation code (an ASCII character; see <wfdb/ecgmap.h>), the third through sixth bytes contain the time (see below) in PDP-11 long integer format as above, and the seventh and eighth bytes contain an annotation serial number.
In annotation files taken directly from the AHA database distribution tapes, the last eight bytes in each annotation are unused, and the time is given in milliseconds measured from the beginning of the annotated segment of the record. In AHA-format annotation files generated by WFDB library applications, annotation times are given in sample intervals from the beginning of the record, and the last eight bytes of each annotation contain the MIT annotation subtype (in the ninth byte), the MIT annotation code (in the tenth byte), and up to six ASCII characters (in the remaining bytes) used to describe RHYTHM and NOTE annotations.
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Updated 8 March 2019