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.
"CVC Productions: Thank you very much for the rapid service and excellent work in restoration of my January 16, 1949 Wilcox-Gay Corporation Coin Recordio-Gram and transcribing it to a current CD version. I cannot begin to tell you how thrilled I was to hear the very clear speaking and singing voice of my then 16 year old but now deceased wife. ..."