Tape bias

Tape bias is the term for two techniques, AC bias and DC bias, that improve the fidelity of analogue tape recorders. DC bias is the addition of direct current to the audio signal that is being recorded. AC bias is the addition of an inaudible high-frequency signal (generally from 40 to 150 kHz) to the audio signal. Most contemporary tape recorders use AC bias. When recording, magnetic tape has a nonlinear response as determined by its coercivity. Without bias, this response results in poor performance, especially at low signal levels. A recording signal that generates a magnetic field strength less than the tape's coercivity cannot magnetise the tape and produces little playback signal. Bias increases the signal quality of most audio recordings significantly by pushing the signal into more linear zones of the tape's magnetic transfer function. Magnetic recording was proposed as early as 1878 by Oberlin Smith, who on 4 October 1878 filed, with the U.S. patent office, a caveat regarding the magnetic recording of sound and who published his ideas on the subject in the 8 September 1888 issue of The Electrical World as "Some possible forms of phonograph". By 1898, Valdemar Poulsen had demonstrated a magnetic recorder and proposed magnetic tape. Fritz Pfleumer was granted a German patent for a non-magnetic "Sound recording carrier" with a magnetic coating, on 1 January 1928, Years earlier, Joseph O'Neil had created a similar recording medium, yet had not made a working machine that could record sound. The earliest magnetic recording systems simply applied the unadulterated (baseband) input signal to a recording head, resulting in recordings with poor low-frequency response and high distortion. Within short order, the addition of a suitable direct current to the signal, a DC bias, was found to reduce distortion by operating the tape substantially within its linear-response region. The principal disadvantage of DC bias was that it left the tape with a net magnetization, which generated significant noise on replay because of the grain of the tape particles.