bias

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(n.) In audio engineering, a high-frequency signal in the 100 kHz range, recorded on magnetic tape along with the audio signal to compensate for the tape characteristics. The characteristic frequency response would be very narrow if just the audio signal were recorded on tape, and low-frequency distortion would be high. With the bias adjusted properly, the frequency response is relatively flat across the audible range. With bias set too low, bass distortion will be audible, and with too much bias, the high frequency response will drop off. Engineers usually set the bias so that the reproduction of a 10 kHz tone (at 15 ips) is as high as possible, then to increase the bias until the reproduced level drops a small amount. This is called overbias. The amount of optimal overbias differs from one tape to another. Bias is necessary because of the phenomenon of inertia. A tape contains many small magnetic particles called domains. These domains are exposed to a magnetic field from the record head and oscillate in polarity as the AC signal voltage changes. These small domains have their own inertia. Each time the analog signal moves from positive to negative and back again, the voltage passes the zero point for an instant. At this moment, the domain is at rest, and there is a short period of bias inertia before it starts moving again. The high frequency of a bias signal ensures that the domains are always kept in motion, negating the effect of inertia at audio frequencies.