burosch1.mpg and burosch2.mpg are (obviously artificial) samples with interlaced content. Both are bottom-field-first, which is correctly indicated by libavcodec. There are not field-encoded, meaning the samples contain frames that constist of interlaced fields if analyzed visually. Look at V.VOB for a field-encoded sample. The samples are very useful for testing deinterlacers, but be careful: With these samples, vf tfields=1 looks superior (as does vdpau:deint=2), while this is often not true for interlaced dvb recordings.