OpenXDF
OpenXDF is an open standard for the digital storage of time-series physiological signals and annotations based on XML. The primary focus is on electroencephalography and polysomnography. OpenXDF was created based on the need for interoperability and free data exchange in polysomnography. OpenXDF uses a two file structure where the time-series data is stored in data blocks in a binary file while the patient information, annotations, and the binary file description is stored in a separate XML header file.
The goal of the OpenXDF initiative is to provide a method by which physiological data can be shared among researchers and clinicians regardless of the device used to acquire the data. This is an on-going initiative supported by the OpenXDF web site.
Two-File Format
Since XML uses text tags to describe content, it creates larger files than comparable binary storage formats. For polygraph recordings this increase in file size would be prohibitive. A typical polysomnogram (digitally stored sleep polygraph) can exceed 300 megabytes in it's raw binary form. If the binary waveform data is stored separately from the descriptive data such as patient information, montages, and annotations, this problem is alleviated. Also multiple waveform files can be linked under a single OpenXDF header file, and existing formats can be adapted without converting the binary waveform data.
EDF (European Data Format)
European data format(EDF) was created in 1992 to provide an open file format for time-series data. It has been the defacto standard for sharing data in the EEG and sleep fields since it's inception. In 2003 an update of EDF (EDF+) allowing coding discontinuous data as well as annotations in UTF-8 format was published. EDF+ is backwards compatibel with EDF.
A free opensource multiplatform viewer for EDF/EDF+ files can be found here EDFbrowser
The EDFgroup
BDF (BioSemi Data Format)
BDF is created by BioSemi. It has the same format as EDF, except for the sample size. Where EDF uses two bytes per sample (16-bits resolution), BDF uses three bytes per sample (24-bits resolution). EDFbrowser accepts BDF files as well.
References
GNU-Sleep
OpenXDF Discussion (Open Format Workshop)
OpenXDF Specifications