Analysis of ECG bandwidth gap as a possible carrier for supplementary digital data

This paper presents the use of bandwidth gap for embedding supplementary digital data into the ECG record without changing its ability to diagnose. The gap results from overestimation of the constant sampling frequency which is greater than the local bandwidth of cardiac components in large parts of the ECG record. The proposed algorithm starts with conventional beat detection and wave delineation procedures. Next the time-frequency representation is determined and a standard ECG bandwidth function is used to separate the area occupied by cardiac components. The remaining parts, roughly extending from QRS-end to QRS-ons in the 1-st scale and from T-end to P-ons in the 2-nd scale, contain noise and artifacts of given statistical properties. Analysis of these properties opens the opportunity to hide the supplementary digital data stream into variable-size sectors of values having similar statistical properties. The usability of the method is twofold: (1) for transmission (or storage) of multimodal data through an ECG channel and (2) for hiding sensitive data into a standard record. In the latter case, the record may be recovered (up to 78 characters per heartbeat) or not, depending only on reader's privileges.