Michael Woolf wrote:

> 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.

This is true; indeed the first magnetic recordings were made by Valdemar
Poulsen around 1900. Pre WW2 machines all suffered from high distortion
and noise, and could never equal the quality of a disc recording.  The
Germans stumbled on the use of high-frequency bias, which when added to
the recorded signal greatly improved the fidelity of the machines; so
much so that it was impossible to discern any difference between a live
or recorded broadcast. Disc recordings could be spotted by the surface
noise of the record, but the german tape recorders had the allies
fooled for a time.

After the war, captured Magnetophons were analyzed, and the cat was out
of the bag. The first Ampex machines were patterned on these, and were
some of the first post-war tape machines capable of really high quality
recordings.

John Ward