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Tape bias

Tape bias is the term for two phenomena, DC bias and AC bias, that improve the fidelity of analogue magnetic tape sound recordings. DC bias is the addition of a 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. Magnetic tape has a nonlinear response at low signal strengths, as measured by its coercivity. Bias increases the signal quality of most audio recordings significantly by pushing the signal into the linear zone of the tape's transfer function.

History

Magnetic recording was proposed as early as 1878 by Oberlin Smith, who published 1888-09-08 in 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 for a "Sound recording carrier" on 1928-01-31, but it was later overturned in favour of the earlier by Joseph A. O'Neill.

DC bias

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 was found to reduce distortion by operating the head substantially within its linear response region. The principal disadvantage of DC bias was that it left the tape with a net magnetisation which, because of the grain of the tape particles, generated significant noise on replay. Some early DC bias systems used a permanent magnet that was placed near the record head. It had to be swung out of the way for replay. DC bias was re-adopted by some very low cost cassette recorders.

AC bias

Although the improvements are marked with such DC bias, even more dramatic improvement results if an alternating current bias is used instead. While several people around the world discovered AC bias, it was the German developments that were widely used in practice and served as the model for future work.

The first patent for AC bias was filed by W. L. Carlson and Glenn L. Carpenter in 1921, eventually resulting in . The value of AC bias was somewhat masked by the primitive state of other aspects of magnetic recording, however, and Carlson and Carpenter's achievement was largely ignored. Teiji Igarishi, Mokoto Ishikawa, and Kenzo Nagai of Japan published a paper on AC biasing in 1938 and received a Japanese patent in 1940. Marvin Camras (USA) also discovered High Frequency (AC) Bias independently in 1941 and received .

The striking improvement in distortion and noise provided by AC bias was rediscovered in 1940 by Walter Weber (1907-1944), while working for Hans Joachim von Braunmühl at the Reichs-Rundfunk-Gesellschaft (RRG). The pair received several related patents, including for "high frequency treatment of the sound carrier".

The UK company Boosey & Hawkes produced a steel wire recorder under Government contract during the Second World War that was equipped with AC bias . It is unlikely that they were aware of the German developments (otherwise they would probably have copied the tape system in its entirety). Examples still surface from time to time, many having been disposed of as Government surplus stock. After the war, Boosey and Hawkes also produced a "Reporter" tape recorder in the early 1950s using magnetic tape, rather than wire, which was based on German wartime technology.

Theory

As the tape leaves the tape head, the applied bias partially offsets the tape's field and the remaining net induction is essentially the difference between the positive and negative half-cycles of the previously recorded signal. This differencing operation further cancels some of the nonlinearity.

Note: that this is a very simplistic explanation of AC bias. In practice, the full modus operandi of AC bias is not adequately understood. There are several prevailing theories, though none of them fully explains the operation, and in particular, what happens if too little or too much bias is applied.

Practice

The characteristics of the recording system change quite markedly as the level of the bias current is changed. There is a level at which the system gives the minimum distortion. There is also a level at which the frequency response is a maximum. Unfortunately, these conditions do not occur at the same bias level. Professional recorders are invariably set up for minimum distortion, the necessary frequency response being achieved by choosing an appropriate tape speed and equalisation curve. Consumer equipment, and in particular Compact Cassette recorders have the bias set at a compromise level to give good frequency response and acceptably low distortion. Also the higher audio frequencies are then weakened more (also know as reducing headroom).

B&O invented and patented the so-called Dolby HX (Headroom eXtension) Pro principle for combining bias control with the Dolby system for better frequency response in cassette recorders. Akai invented the Crossfield system for tape recorders where during recording a separate head with the bias frequency modulating it was held very close to the recording head, producing a better bias, than by mixing the two signals in the recording head. But mechanical tolerances for Crossfield are tight, requiring frequent readjustment and this system was largely replaced.

Different amplitudes of bias field are optimal for different types of tape, so most recorders offer a bias setting switch on the control panel, or, in the case of the compact audio cassette, may switch automatically according to cutouts on the cassette shell. Ferric based tapes require the lowest bias field, with Chrome based tapes (including the pseudo chromes) requiring a higher level. Metal particle requires even more. Metal Evaporated tape accepts the highest level of bias, but it is mostly used for digital recording purposes. The same is valid for a combination cassette tape, the FeCr-variant, on which a thicker Ferric layer was covered by a thinner chrome layer. The idea behind this was that at lower frequencies and higher head currents the Fe layer would be deeper magnetised, while at higher frequencies only the top Cr-layer was active. In practice this proved disappointing and some claimed that this thin chrome layer was quickly polished off in heavy use.

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