I just found out that the 30 seconds previews (played back by Deezer for non-signed-in users) do not start from the beginning of the original, but somewhere halfway. What is the exact offset of these previews? Are they the same for all songs?
Solution
For songs lasting more than 1 minute, the extract is from 30sec to 1 minute,
For songs lasting less than 1 minute, the extract is the last 30 sec from the song,
For songs lasting less than 30sec, the extract is the full song.