An electrical signal of relatively high frequency applied to magnetic tape during the recording process, along with the audio signal, to linearize the transfer characteristic of magnetic recording tape. The bias frequency is generally in the range of 70 to 120 kHz (HF bias).
"I wanted to thank you for the wonderful work that you did on the 5 home-recorded 78 records that I sent you. These were made by my Grandfather in the late 1940's and 1950's. No one in the family had heard them in almost 50 years. They varied in what shape they were in, and we really thought that they would not be playable after so many years. ..."