PROBADO – A Digital Library System for Heterogeneous Non-textual Documents Berndt R. Blümel I. Clausen M. Damm D. Diet J. Fellner D. Klein R. Scherer M. Sens I. Thomas V. Wessel R. e-learning digital library multimedia documents 370 periodical academic journal eleed 8 1 2012 1860-7470 urn:nbn:de:0009-5-32740 http://nbn-resolving.de/urn:nbn:de:0009-5-32740 berndt2012