Establish exact equivalence of FFT processing between HRV Studio and Kubios

Establish whether HRV Studio’s FFT processing is exactly equivalent to Kubios HRV’s FFT processing, including the spectral window and related implementation details, to determine the causes of residual cross-platform discrepancies in frequency-domain HRV estimates.

Background

The paper reports a targeted comparison of FFT-based HRV estimates from HRV Studio and Kubios under nominally matched Smoothness Priors preprocessing, interpolation, and frequency-band definitions. The comparison nevertheless showed larger discrepancies than the corresponding autoregressive comparison, especially for very-low-frequency and absolute-power measures.

A key unresolved issue is that Kubios did not expose its FFT window function through either its graphical interface or exported reports, while HRV Studio used whole-signal FFT estimation and Kubios reported segmented FFT settings. Without access to these implementation details, the contribution of windowing, segmentation, and other FFT-specific procedures to the residual disagreement cannot be isolated.