WFDB Annotation Files
Annotation files contain time-stamped event markers aligned to the signal data. These files provide critical context, such as heartbeat labels, arrhythmias, rhythm changes, and comments.
WFDB supports two annotation file formats:
- MIT format (compact, preferred)
- AHA format (legacy, mainly for interoperability)
MIT Annotation Format
The MIT format is the standard for WFDB annotations. It is binary, compact (averaging slightly over two bytes per annotation), and extensible.
Each annotation in the MIT format consists of:
- A time difference (
I) from the previous annotation, measured in sample intervals. - An annotation type code (
A), describing the event.
The first byte of each annotation pair is the least significant byte. The six most significant bits across the pair encode the annotation type, and the remaining ten bits encode the time difference.
Special Cases in MIT Format
If the annotation type (A) has a special value, the annotation may also contain:
| Code | Meaning | Notes |
|---|---|---|
SKIP (59) | Large skip | A 4-byte absolute time follows. |
NUM (60) | Set num field | Applies to current and following annotations. |
SUB (61) | Set subtyp field | Applies to current annotation only. |
CHN (62) | Set chan field | Applies to current and following annotations. |
AUX (63) | Auxiliary data | I bytes of extra text follow (e.g., rhythm labels). |
A = 0, I = 0 | End of file | No more annotations. |
Auxiliary fields such as subtyp, chan, num, and aux provide additional metadata for selected annotations.
AHA Annotation Format
The AHA format is a fixed-length (16 bytes per annotation) legacy format.
- Originally designed for 9-track tape transfer.
- Used mainly for interoperability, not recommended for modern datasets.
Each 16-byte AHA annotation includes:
- An ASCII annotation code (AHA-specific).
- A timestamp (in sample intervals or milliseconds).
- Optional fields for rhythm descriptions or comments.
WFDB software can automatically distinguish between MIT and AHA formats when reading annotation files.
Summary
| Format | Use Case |
|---|---|
| MIT format | Default for WFDB records; compact, extensible. |
| AHA format | Legacy support; rarely used except for archival data. |
Annotations are essential for interpreting WFDB signals, supporting applications like heartbeat detection, arrhythmia analysis, and rhythm classification.