An audio frequency signal that may be created if two slightly different bias frequencies are combined. The audio frequency is the arithmetic difference between the bias frequencies.
"I received CD's and MP3 CD's yesterday for the 3 reel-to-reel tapes I sent in. These were pretty old tapes (one around 33 years old) that had not been on a player since the 1980's when my reel-to-reel recorder was stolen and never replaced. One of them, in particular, contained unreplaceable studio performance tracks from a record album I had ..."