Sample MEF 3.0 data can be found in this github repository (sample_data.mefd.tar file).
This dataset contains four EEG time-series data channels in MEF 3.0 format. The files are unencrypted. The dataset is a two-hour segment of 256 Hz samples. It also contains a record (annotation) file which has two text notes.
The data were converted to MEF 3.0 using the meflib and mefwriter code repositories.
(This repository updated September 2021, replacing a previous dataset with this one).
Below is a screenshot of what the first ~30 seconds looks like when plotted. (note the timestamp is shown as GMT -7, or Pacific daylight time)
