We’ve had a few inSync readers inquire about the bias settings on cassette decks and what they really do. This can be a very deep subject so for the sake of brevity we’ll limit the discussion to some of the practical aspects.
The purpose of the bias control on tape decks is to allow some flexibility in optimizing your tape machine for some of the various tape formulations in use today. Many users are confused about the function of the bias selector on decks because they have “no effect on the sound.” Bias (see WFTD archive bias) is only applied during recording, so a true bias switch will not affect the way a tape plays back at all. Different tape formulations (Cr02, FeCR, Metal, Normal, etc.) each require different amounts of bias to be applied during recording to make a well-balanced recording. Applying the incorrect amount of bias will cause non-linear high frequency response, distortion, or both.
ALL tape decks have bias and controls to set bias. Most cassette decks have a two or three position switch for recalling factory preset bias settings that correspond to an approximation of the most frequently used high and low bias tapes. These controls are generic and designed to be close enough to make a decent recording on most any kind of brand name tape one comes into contact with. Professionals who require a higher level of precision find it necessary to tweak their bias settings to suit them. On most decks, however, these controls are internal and must be set by a qualified technician. Some more expensive decks (such as the TASCAM 122MKIII) add a set of external bias controls so that the adjustment can be accomplished without opening the unit. While most of us don’t have very high expectations of the cassette format these days, it should be noted that a well calibrated cassette with a high quality tape can make a surprisingly good recording. I have personally been able to make cassette tapes which listeners cannot distinguish from a source CD.