I read with interest Hal Davis' item on this subject. It is my
understanding that magnetic recording techniques were indeed known to
the Allies before the Second World War.  During the 1930s the BBC in
London was using a high-speed magnetic strip recorder called the
Blattnerphone.  I believe also that the Allies became aware of the
Germans use of magnetic recording because of repeated extended
broadcasts of opera in the early years of the war.

Michael Woolf
New Zealand