bias & equalization   

Analogue magnetic recording media (magnetic tape, cassettes, magnetic film, etc.) has a non-linear response to the range of frequencies in the audio spectrum 

The bias is a high frequency signal which shifts the audio into a higher frequency range where the tape is more linear in its response.  Equalization is filtering applied to the signal to correct this non-linearity.

Both must be adjusted for each different brand of media and calibrated often.