I understand that the purpose of AC-bias is to improve the quality of a tape recording by countering some otherwise adverse physical properties of practical magnetic materials.
I've had this explained to me in a number of different ways, often contradictory, always vague. Online sources like Wikipedia don't help because they're more muddled and vague than most!
Could someone who is eloquent and knowledgeable give the definitive answer as to the physical processes which are involved and how AC bias helps? Actual equations preferred to hokey analogies.
Best Answer
It's to overcome the hysteresis of the magnetic tape.
The record head is a small electromagnet pressed against the tape. If a current is applied, it magnetises the oxide particles on the tape. the stronger the current, the more magnetic domains are re-aligned and the stronger the signal recorded.
However, a very tiny current will have no effect on the tape at all. This would mean that weak signals would end up distorted (as only the peaks would be recorded). Even weaker signals would not be recorded at all.
Adding a high-frequency bias ensures that there is always enough magnetic field to magnetise the tape. However, the frequency is higher than the tape is capable of recording, so it doesn't affect the result.