Based on the need for interoperability and free data exchange in polysomnography, European Data Format (EDF) was designed in 1992. EDF is the current de facto standard for exchanging polygraphic recordings. It is not extensible so there is more than one specification (EDF+). Implementation of EDF is not always easy and compatibility is spotty. Some items must be modified to fit in the format which can lead to lost information. Some additional limitations of EDF are:

• Limited ability to store annotations and scoring information
• Limited patient information
• Text items are limited in length and special characters such as spaces cannot be used
• Limited in language support to US-ASCII
• No provision for data encryption
• Limited to 2 bytes per sample
• Uses 2 digits to represent years

Rather than compete with EDF, OpenXDF will complement and, more importantly, be backward compatible with EDF.

An OpenXDF / EDF viewer (XDFviewer) is available for free and comes with a sample EDF file augmented with OpenXDF features to illustrate compatibility.

Using an extensible format will allow OpenXDF to be both backward and forward compatible and allow for unlimited expansion. Since the protocol has a basis in XML, there will be fewer limitations, making integration into existing systems easier. Some advantages of using XML are:

• Both standard and proprietary information can be stored in the file without affecting compatibility
• No limitations on annotations or scoring information
• No limitations on patient information
• There is no limitation on string lengths, and only loose limitations on sample sizes (1, 2, 4, and 8 bytes) and frequencies
• Text encoding and language support is not limited by XML
• Uses ISO8601 for robust, uniform, Y2K compliant date/time representation
• Allows for data encryption

To make adoption easier, software converters will be written for existing open file formats such as EDF. Converters for other formats can be produced in a short period of time if the existing file format is made available.

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. If the binary waveform data is stored separately from the descriptive data such as patient information, montages, and annotations, this problem is alleviated. By keeping the waveform data in a separate file, there is an added advantage of adapting existing formats without converting the binary waveform data. This would make for easier integration into existing systems and would eliminate the risk of distorting the data upon conversion. For example, it will be possible to create the XML descriptor file from an existing EDF file without changing the original file. This will make OpenXDF backward compatible with EDF and possibly other formats. Separating the descriptive data from the binary waveform data also allows multiple waveform files to be linked together under one OpenXDF header file. XML files are human readable by design, but this feature is not always desirable for security reasons. Thus, software components will be created to facilitate encryption of sensitive patient information and password protect files. This option will be user-configurable for maximum flexibility. The security component will be a standard add-on component that can easily be integrated into existing systems.