The European Data Format (EDF)1 is a computer file format that was originally designed for archival and exchange of polysomnographic (PSG) sleep recordings. Later, a backward compatible revision (EDF+)2 was developed in order to solve some ambiguities and to also accommodate event data, annotations, and analysis data such as (for instance) sleep scorings. In this way, EDF+ handles most sleep-related data that does not fit into common hospital information systems. The list of EDF(+) compatible manufacturers at the EDF website (www.edfplus.info) shows that EDF and EDF+ are the de facto standard for sleep investigations and are also widely applied in electroencephalography (EEG).
However, EDF(+) did not yet handle video data as recorded for instance in cases of suspected epilepsy or sleep-related behavior disorders. At first sight, EDF(+) does not need to do so because video is usually saved into a common format, for which replay software is widely available. However, video files do not know their own start date and time (that usually differs from the EDF file). Manufacturers save that information at recording time, in different and usually proprietary ways. Therefore, start date and time of the video are lost if replay software of a different manufacturer is involved, for instance in multi-center studies or when replacing equipment. That software may be able to replay video and EDF(+) files but not in a time synchronized way. We have solved this problem by simply coding the time information in the video filename as follows (for example):
NL_012348168_03-MAY-2013_14h45m49.013s_Video.ogv
in which case the video of patient NL-012348168 apparently starts on May 3, 2013 at 0.013 seconds after 14:45:49hr. EDF+ files belonging to the same patient should have filenames with the same initial part, for instance:
NL_012348168_PSG.edf
By this convention, EDF compatible review software can time-synchronize the two files by reading their start date and time from the contents of the PSG file and from the filename of the video file. This proposal was already discussed in 2008 in technical committee TC251 of the European normalization committee (CEN) and in 2009 in Yahoo's EDF usergroup, but never published. In order to demonstrate the practical usefulness and simplicity of this solution, we have now linked the Microsoft Media player into our Polyman EDF reviewer in such a way that video and EDF remain synchronized during replay of either one. Because Polyman is freely downloadable from the EDF website, the only action needed to make this work in other labs is to rename the EDF(+) and video files according to this example. In order to make it work with other review software, that software also would need to adopt this filename convention. We hope that the present communication increases the awareness that simple harmonization of filenames is the only action required to give our video data a much longer and more prosperous life.
In order to avoid ambiguity we conclude here with a detailed specification of the filename convention. The EDF(+) and video filenames should start with the patient identification. After that the EDF(+) file can, but the video filename definitely must, continue with an underscore (_), followed by its start date DD-MMM-YYYY, followed by another underscore, which is followed by the start time HHhMMmSS.XXXXs. The start date days, DD, are 01, 02, 03, … 30 or 31. The months, MMM, are JAN, FEB, MAR, APR, MAY, JUN, JUL, AUG, SEP, OCT, NOV, or DEC. The years, YYYY, obviously are 2013, 2014, and so on. The start time is based on the 24-h clock with the hours, HH, ranging from 00 till 23, so midnight is coded as 00h00m00s. Both minutes, MM, and seconds, SS, range from 00 to 59. The decimal fraction of a second (noted here by .XXXX) can have any length and can also be omitted (for example in 00h37m12s). The addition _Video in the example is not obligatory, and neither do we standardize any video format such as ogv. So, a perfectly OK video filename would also be:
NL_012348168_03-MAY-2013_14h45m49.013s.mpeg
CITATION
Kemp B; Roessen M. European data format now supports video. SLEEP 2013;36(7):1111.
DISCLOSURE STATEMENT
The authors have indicated no financial conflicts of interest.
REFERENCES
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