In magnetic tape recording, adaptive biasing is the technique of continuously varying the bias current to a recording head in accordance with the level of high-frequency audio signals. With adaptive biasing, high levels of high-frequency audio signals cause a proportionate decrease in bias current using either feedforward or preferably a negative feedback control system. Compared with the use of fixed bias current, adaptive biasing provides a higher maximum output level and higher dynamic range at the upper end of the audible spectrum and to a lesser extent, mid-range frequencies. The effect of adaptive biasing is most pronounced in compact cassette and low-speed reel-to-reel media. The first commercial implementation, the feedforward system Dolby HX was developed by Dolby Laboratories by 1979 and was rejected by the industry. The subsequent negative-feedback system Dolby HX Pro was developed by Bang & Olufsen and marketed by Dolby, and became the de facto standard of the consumer high fidelity industry in the mid-1980s.
Fixed and adaptive biasing
Tape bias is a strong, high-frequency, alternating current that is fed to a tape recording head along with the audio signal with the purpose of making more linear the inherently non-linear response of the magnetic particles in the tape's magnetic coating. The frequency of the bias signal in consumer cassette decks is usually fixed at between 80 and 100 kHz. The quality of the bias signal is critical because noise, hum and direct current in the bias severely degrade audio fidelity. The level of the bias signal defines the slope and shape of the resulting magnetization curve.
The optimal bias level for each tape formulation is a compromise between maximum output levels, noise, distortion and frequency response. Nominal bias, corresponding to maximum sensitivity and/or maximum output at 10 kHz, is less desirable for mid-range frequencies. Over-biasing is better suited for mid-range and low frequencies but it reduces tape sensitivity at higher frequencies and degrades the signal-to-noise ratio. As a side benefit, optimum bias improves the response to tape dropouts because stronger magnetic fields penetrate more deeply into the magnetic coating. Under-biasing causes excessive distortion and modulation noise, and raises the susceptibility to dropouts, and is thus unwanted. In practice, tape is always slightly over biased; the optimal bias current is set at two or three decibels (dB) above the nominal value. This optimal setting improves linearity at mid-range frequencies but reduces dynamic range and causes a drop in high-frequency response, which is offset with pre-emphasis in the recording chain.
Recording very low wavelengths at tape speeds of and presents another challenge. Audible high-frequency components of the recorded signal act as biasing currents, resulting in excessive over-biasing that manifests itself in dynamic range compression and early onset of saturation at high frequencies, especially when recording on low quality tapes with low saturation levels. In the 1970s, music typically published on vinyl records or transmitted on FM radio did not contain much high-frequency energy and usually could not drive the tape into saturation. The digitally mastered, direct-to-disc and disco recordings of the late 1970s and early 1980s, however, often contain enough high-frequency information, or "hot" treble, to trigger tape overload.
In the late 1970s, the recording industry proposed three solutions to the problem. Metal particle tapes had very high maximum output levels and treble saturation levels but were prohibitively expensive for most home users. The early metal tapes had high absolute level of hiss and there were fears metal tape would quickly degrade but this did not happen. The second solution was developed independently by Tandberg and Akai, and relied on limiting recording levels. The patented Tandberg Dyneq and Akai ADRS circuits electronically compressed the signal before it could overload the tape. In 1979, Kenneth James Gundry of Dolby Laboratories proposed the third alternative; adapting the bias current to the treble content of the source signal. The increase in high-frequency energy, which effectively overbiases the tape, would be compensated for with a reciprocal decrease in the output of the bias generator.