Tape bias
Encyclopedia
Tape bias is the term for two phenomena, DC bias
and AC bias, that improve the fidelity of analogue magnetic tape sound recording
s. 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
.
, 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.
) 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.
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 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 German pair received several related patents, including for "high frequency treatment of the sound carrier".
Possibly independently of Weber and Braunmühl, the UK company Boosey & Hawkes
produced a steel wire recorder under government contract during the Second World War
that was equipped with AC bias. 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.
, 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. A constantly, rapidly varying magnetic field also helps overcome magnetic hysteresis
(the tendency of paramagnetic
materials to resist either being magnetized or releasing its magnetism) of both the recording head and the tape medium.
. There is also a level at which the frequency response is a maximum. 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 equalization curve. Consumer equipment, and in particular compact audio 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 known as reducing headroom).
Bang & Olufsen
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. Tandberg
invented the cross-field recording system for tape recorders where a separate head with the bias was used. This produced a better bias than by mixing the two signals in the recording head, but mechanical tolerances for cross-field are tight. The system required frequent readjustment and was largely abandoned.
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 (which does not require bias as the non linearity is not a problem). 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 Ferric layer would be more deeply magnetized, while at higher frequencies only the top Cr-layer was active. In practice, this didn't work well, and some claimed that this thin chrome layer was quickly polished off in heavy use.
DC bias
When describing a periodic function in the frequency domain, the DC bias, DC component, DC offset, or DC coefficient is the mean value of the waveform...
and AC bias, that improve the fidelity of analogue magnetic tape sound recording
Magnetic tape sound recording
The use of magnetic tape for sound recording originated around 1930. Magnetizable tape revolutionized both the radio broadcast and music recording industries. It did this by giving artists and producers the power to record and re-record audio with minimal loss in quality as well as edit and...
s. 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
Magnetic tape
Magnetic tape is a medium for magnetic recording, made of a thin magnetizable coating on a long, narrow strip of plastic. It was developed in Germany, based on magnetic wire recording. Devices that record and play back audio and video using magnetic tape are tape recorders and video tape recorders...
has a nonlinear response at low signal strengths, as measured by its coercivity
Coercivity
In materials science, the coercivity, also called the coercive field or coercive force, of a ferromagnetic material is the intensity of the applied magnetic field required to reduce the magnetization of that material to zero after the magnetization of the sample has been driven to saturation...
. Bias increases the signal quality of most audio recordings significantly by pushing the signal into the linear zone of the tape's transfer function
Transfer function
A transfer function is a mathematical representation, in terms of spatial or temporal frequency, of the relation between the input and output of a linear time-invariant system. With optical imaging devices, for example, it is the Fourier transform of the point spread function i.e...
.
History
Magnetic recording was proposed as early as 1878 by Oberlin SmithOberlin Smith
Oberlin Smith was an American engineer who published one of the earliest works dealing with magnetic recording in 1888. In an article that appeared in the British magazine - Electrical World, he suggested the use of permanent magnetic impressions for the recording of sound...
, who published 1888-09-08 in The Electrical World as "Some possible forms of phonograph". By 1898 Valdemar Poulsen
Valdemar Poulsen
Valdemar Poulsen was a Danish engineer who developed a magnetic wire recorder in 1899.-Biography:He was born on 23 November 1869 in Copenhagen...
had demonstrated a magnetic recorder and proposed magnetic tape. Fritz Pfleumer
Fritz Pfleumer
Fritz Pfleumer was a German-Austrian engineer who invented magnetic tape for recording sound.-Biography:...
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 (basebandBaseband
In telecommunications and signal processing, baseband is an adjective that describes signals and systems whose range of frequencies is measured from close to 0 hertz to a cut-off frequency, a maximum bandwidth or highest signal frequency; it is sometimes used as a noun for a band of frequencies...
) 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
Marvin Camras
Marvin Camras was an electrical engineer and inventor who was widely influential in the field of magnetic recording.Camras built his first recording device, a wire recorder, in the 1930s for a cousin who was an aspiring singer...
(USA) also discovered high frequency (AC) bias independently in 1941 and received .
The 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
Reichs-Rundfunk-Gesellschaft
The Reichs-Rundfunk-Gesellschaft , which can be loosely translated as the State Broadcasting Company, was a national network of German regional public broadcasting companies active from 1925 until 1945...
(RRG). The German pair received several related patents, including for "high frequency treatment of the sound carrier".
Possibly independently of Weber and Braunmühl, the UK company Boosey & Hawkes
Boosey & Hawkes
Boosey & Hawkes is a British music publisher purported to be the largest specialist classical music publisher in the world. Until 2003, it was also a major manufacturer of brass, string and wind musical instruments....
produced a steel wire recorder under government contract during the Second World War
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
that was equipped with AC bias. 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 headTape head
A tape head is a type of transducer used in tape recorders to convert electrical signals to magnetic fluctuations and vice versa.-Principles of operation:...
, 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. A constantly, rapidly varying magnetic field also helps overcome magnetic hysteresis
Hysteresis
Hysteresis is the dependence of a system not just on its current environment but also on its past. This dependence arises because the system can be in more than one internal state. To predict its future evolution, either its internal state or its history must be known. If a given input alternately...
(the tendency of paramagnetic
Paramagnetism
Paramagnetism is a form of magnetism whereby the paramagnetic material is only attracted when in the presence of an externally applied magnetic field. In contrast with this, diamagnetic materials are repulsive when placed in a magnetic field...
materials to resist either being magnetized or releasing its magnetism) of both the recording head and the tape medium.
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 distortionDistortion
A distortion is the alteration of the original shape of an object, image, sound, waveform or other form of information or representation. Distortion is usually unwanted, and often many methods are employed to minimize it in practice...
. There is also a level at which the frequency response is a maximum. 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 equalization curve. Consumer equipment, and in particular compact audio cassette
Compact Cassette
The Compact Cassette, often referred to as audio cassette, cassette tape, cassette, or simply tape, is a magnetic tape sound recording format. It was designed originally for dictation, but improvements in fidelity led the Compact Cassette to supplant the Stereo 8-track cartridge and reel-to-reel...
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 known as reducing headroom).
Bang & Olufsen
Bang & Olufsen
Bang & Olufsen is a Danish company that designs and manufactures audio products, television sets and telephones. It was founded in 1925 by Peter Bang and Svend Olufsen, whose first significant product was a radio that worked with alternating current, when most radios were run from batteries...
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. Tandberg
Tandberg
This article is about the video-conferencing vendor, a Cisco company. Other companies with the same name such as Tandberg Television or Tandberg Data for data storage: see here and here...
invented the cross-field recording system for tape recorders where a separate head with the bias was used. This produced a better bias than by mixing the two signals in the recording head, but mechanical tolerances for cross-field are tight. The system required frequent readjustment and was largely abandoned.
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 (which does not require bias as the non linearity is not a problem). 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 Ferric layer would be more deeply magnetized, while at higher frequencies only the top Cr-layer was active. In practice, this didn't work well, and some claimed that this thin chrome layer was quickly polished off in heavy use.
See also
- Bias (electrical engineering)
- HysteresisHysteresisHysteresis is the dependence of a system not just on its current environment but also on its past. This dependence arises because the system can be in more than one internal state. To predict its future evolution, either its internal state or its history must be known. If a given input alternately...
- Barkhausen effectBarkhausen effectThe Barkhausen effect is a name given to the noise in the magnetic output of a ferromagnet when the magnetizing force applied to it is changed...
- DitherDitherDither is an intentionally applied form of noise used to randomize quantization error, preventing large-scale patterns such as color banding in images...
- Tape recorderTape recorderAn audio tape recorder, tape deck, reel-to-reel tape deck, cassette deck or tape machine is an audio storage device that records and plays back sounds, including articulated voices, usually using magnetic tape, either wound on a reel or in a cassette, for storage...