In signal processing, white noise is a random signal having equal intensity at different frequencies, giving it a constant power spectral density. The term is used, with this or similar meanings, in many scientific and technical disciplines, including physics, acoustical engineering, telecommunications, statistical forecasting. White noise refers to a statistical model for signals and signal sources, rather than to any specific signal. White noise draws its name from white light, although light that appears white does not have a flat power spectral density over the visible band. In discrete time, white noise is a discrete signal whose samples are regarded as a sequence of serially uncorrelated random variables with zero mean and finite variance. Depending on the context, one may require that the samples be independent and have identical probability distribution. In particular, if each sample has a normal distribution with zero mean, the signal is said to be Additive white Gaussian noise; the samples of a white noise signal may be sequential in time, or arranged along one or more spatial dimensions.
In digital image processing, the pixels of a white noise image are arranged in a rectangular grid, are assumed to be independent random variables with uniform probability distribution over some interval. The concept can be defined for signals spread over more complicated domains, such as a sphere or a torus. An infinite-bandwidth white noise signal is a purely theoretical construction; the bandwidth of white noise is limited in practice by the mechanism of noise generation, by the transmission medium and by finite observation capabilities. Thus, random signals are considered "white noise" if they are observed to have a flat spectrum over the range of frequencies that are relevant to the context. For an audio signal, the relevant range is the band of audible sound frequencies; such a signal is heard by the human ear as a hissing sound, resembling the /sh/ sound in "ash". In music and acoustics, the term "white noise" may be used for any signal that has a similar hissing sound; the term white noise is sometimes used in the context of phylogenetically based statistical methods to refer to a lack of phylogenetic pattern in comparative data.
It is sometimes used analogously in nontechnical contexts to mean "random talk without meaningful contents". Any distribution of values is possible. A binary signal which can only take on the values 1 or –1 will be white if the sequence is statistically uncorrelated. Noise having a continuous distribution, such as a normal distribution, can of course be white, it is incorrectly assumed that Gaussian noise refers to white noise, yet neither property implies the other. Gaussianity refers to the probability distribution with respect to the value, in this context the probability of the signal falling within any particular range of amplitudes, while the term'white' refers to the way the signal power is distributed over time or among frequencies. We can therefore find Gaussian white noise, but Poisson, etc. white noises. Thus, the two words "Gaussian" and "white" are both specified in mathematical models of systems. Gaussian white noise is a good approximation of many real-world situations and generates mathematically tractable models.
These models are used so that the term additive white Gaussian noise has a standard abbreviation: AWGN. White noise is the generalized mean-square derivative of the Wiener Brownian motion. A generalization to random elements on infinite dimensional spaces, such as random fields, is the white noise measure. White noise is used in the production of electronic music either directly or as an input for a filter to create other types of noise signal, it is used extensively in audio synthesis to recreate percussive instruments such as cymbals or snare drums which have high noise content in their frequency domain. A simple example of white noise is a nonexistent radio station. White noise is used to obtain the impulse response of an electrical circuit, in particular of amplifiers and other audio equipment, it is not used for testing loudspeakers as its spectrum contains too great an amount of high frequency content. Pink noise, which differs from white noise in that it has equal energy in each octave, is used for testing transducers such as loudspeakers and microphones.
To set up the equalization for a concert or other performance in a venue, a short burst of white or pink noise is sent through the PA system and monitored from various points in the venue so that the engineer can tell if the acoustics of the building boost or cut any frequencies. The engineer can adjust the overall equalization to ensure a balanced mix. White noise is used as the basis of some random number generators. For example, Random.org uses a system of atmospheric antennae to generate random digit patterns from white noise. White noise is a common synthetic noise source used for sound masking by a tinnitus masker. White noise machines and other white noise sources are sold as privacy enhancers and sleep aids and to mask tinnitus. Alternatively, the use of an FM radio tuned to unused frequencies is a simpler and more cost-effective source of white noise. However, white noise generated from a common commercial radio receiver tuned to an unused frequency is vulnerable to being contaminated with spurious signals, such as adjacent radio stations, harmonics f
The decibel is a unit of measurement used to express the ratio of one value of a power or field quantity to another on a logarithmic scale, the logarithmic quantity being called the power level or field level, respectively. It can be used to express a change in an absolute value. In the latter case, it expresses the ratio of a value to a fixed reference value. For example, if the reference value is 1 volt the suffix is "V", if the reference value is one milliwatt the suffix is "m". Two different scales are used when expressing a ratio in decibels, depending on the nature of the quantities: power and field; when expressing a power ratio, the number of decibels is ten times its logarithm to base 10. That is, a change in power by a factor of 10 corresponds to a 10 dB change in level; when expressing field quantities, a change in amplitude by a factor of 10 corresponds to a 20 dB change in level. The decibel scales differ by a factor of two so that the related power and field levels change by the same number of decibels in, for example, resistive loads.
The definition of the decibel is based on the measurement of power in telephony of the early 20th century in the Bell System in the United States. One decibel is one tenth of one bel, named in honor of Alexander Graham Bell. Today, the decibel is used for a wide variety of measurements in science and engineering, most prominently in acoustics and control theory. In electronics, the gains of amplifiers, attenuation of signals, signal-to-noise ratios are expressed in decibels. In the International System of Quantities, the decibel is defined as a unit of measurement for quantities of type level or level difference, which are defined as the logarithm of the ratio of power- or field-type quantities; the decibel originates from methods used to quantify signal loss in telegraph and telephone circuits. The unit for loss was Miles of Standard Cable. 1 MSC corresponded to the loss of power over a 1 mile length of standard telephone cable at a frequency of 5000 radians per second, matched the smallest attenuation detectable to the average listener.
The standard telephone cable implied was "a cable having uniformly distributed resistance of 88 Ohms per loop-mile and uniformly distributed shunt capacitance of 0.054 microfarads per mile". In 1924, Bell Telephone Laboratories received favorable response to a new unit definition among members of the International Advisory Committee on Long Distance Telephony in Europe and replaced the MSC with the Transmission Unit. 1 TU was defined such that the number of TUs was ten times the base-10 logarithm of the ratio of measured power to a reference power. The definition was conveniently chosen such that 1 TU approximated 1 MSC. In 1928, the Bell system renamed the TU into the decibel, being one tenth of a newly defined unit for the base-10 logarithm of the power ratio, it was named the bel, in honor of the telecommunications pioneer Alexander Graham Bell. The bel is used, as the decibel was the proposed working unit; the naming and early definition of the decibel is described in the NBS Standard's Yearbook of 1931: Since the earliest days of the telephone, the need for a unit in which to measure the transmission efficiency of telephone facilities has been recognized.
The introduction of cable in 1896 afforded a stable basis for a convenient unit and the "mile of standard" cable came into general use shortly thereafter. This unit was employed up to 1923 when a new unit was adopted as being more suitable for modern telephone work; the new transmission unit is used among the foreign telephone organizations and it was termed the "decibel" at the suggestion of the International Advisory Committee on Long Distance Telephony. The decibel may be defined by the statement that two amounts of power differ by 1 decibel when they are in the ratio of 100.1 and any two amounts of power differ by N decibels when they are in the ratio of 10N. The number of transmission units expressing the ratio of any two powers is therefore ten times the common logarithm of that ratio; this method of designating the gain or loss of power in telephone circuits permits direct addition or subtraction of the units expressing the efficiency of different parts of the circuit... In 1954, J. W. Horton argued that the use of the decibel as a unit for quantities other than transmission loss led to confusion, suggested the name'logit' for "standard magnitudes which combine by addition".
In April 2003, the International Committee for Weights and Measures considered a recommendation for the inclusion of the decibel in the International System of Units, but decided against the proposal. However, the decibel is recognized by other international bodies such as the International Electrotechnical Commission and International Organization for Standardization; the IEC permits the use of the decibel with field quantities as well as power and this recommendation is followed by many national standards bodies, such as NIST, which justifies the use of the decibel for voltage ratios. The term field quantity is deprecated by ISO 80000-1. In spite of their widespread use, suffixes are not recognized by the IEC or ISO. ISO 80000-3 describes definitions for units of space and time; the decibel for use in acoustics is defined in ISO 80000-8. The major difference from the article below is that for acoustics the decibel has no
FM broadcasting is a method of radio broadcasting using frequency modulation technology. Invented in 1933 by American engineer Edwin Armstrong, wide-band FM is used worldwide to provide high-fidelity sound over broadcast radio. FM broadcasting is capable of better sound quality than AM broadcasting, the chief competing radio broadcasting technology, so it is used for most music broadcasts. Theoretically wideband AM can offer good sound quality, provided the reception conditions are ideal. FM radio stations use the VHF frequencies; the term "FM band" describes the frequency band in a given country, dedicated to FM broadcasting. Throughout the world, the FM broadcast band falls within the VHF part of the radio spectrum. 87.5 to 108.0 MHz is used, or some portion thereof, with few exceptions: In the former Soviet republics, some former Eastern Bloc countries, the older 65.8–74 MHz band is used. Assigned frequencies are at intervals of 30 kHz; this band, sometimes referred to as the OIRT band, is being phased out in many countries.
In those countries the 87.5–108.0 MHz band is referred to as the CCIR band. In Japan, the band 76–95 MHz is used; the frequency of an FM broadcast station is an exact multiple of 100 kHz. In most of South Korea, the Americas, the Philippines and the Caribbean, only odd multiples are used. In some parts of Europe and Africa, only multiples are used. In the UK odd or are used. In Italy, multiples of 50 kHz are used. In most countries the maximum permitted frequency error is specified, the unmodulated carrier should be within 2000 Hz of the assigned frequency. There are other unusual and obsolete FM broadcasting standards in some countries, including 1, 10, 30, 74, 500, 300 kHz. However, to minimise inter-channel interference, stations operating from the same or geographically close transmitter sites tend to keep to at least a 500 kHz frequency separation when closer frequency spacing is technically permitted, with closer tunings reserved for more distantly spaced transmitters, as interfering signals are more attenuated and so have less effect on neighboring frequencies.
Frequency modulation or FM is a form of modulation which conveys information by varying the frequency of a carrier wave. With FM, frequency deviation from the assigned carrier frequency at any instant is directly proportional to the amplitude of the input signal, determining the instantaneous frequency of the transmitted signal; because transmitted FM signals use more bandwidth than AM signals, this form of modulation is used with the higher frequencies used by TV, the FM broadcast band, land mobile radio systems. The maximum frequency deviation of the carrier is specified and regulated by the licensing authorities in each country. For a stereo broadcast, the maximum permitted carrier deviation is invariably ±75 kHz, although a little higher is permitted in the United States when SCA systems are used. For a monophonic broadcast, again the most common permitted. However, some countries specify a lower value for monophonic broadcasts, such as ±50 kHz. Random noise has a triangular spectral distribution in an FM system, with the effect that noise occurs predominantly at the highest audio frequencies within the baseband.
This can be offset, to a limited extent, by boosting the high frequencies before transmission and reducing them by a corresponding amount in the receiver. Reducing the high audio frequencies in the receiver reduces the high-frequency noise; these processes of boosting and reducing certain frequencies are known as pre-emphasis and de-emphasis, respectively. The amount of pre-emphasis and de-emphasis used is defined by the time constant of a simple RC filter circuit. In most of the world a 50 µs time constant is used. In the Americas and South Korea, 75 µs is used; this applies to both stereo transmissions. For stereo, pre-emphasis is applied to the left and right channels before multiplexing; the use of pre-emphasis becomes a problem because of the fact that many forms of contemporary music contain more high-frequency energy than the musical styles which prevailed at the birth of FM broadcasting. Pre-emphasizing these high frequency sounds would cause excessive deviation of the FM carrier. Modulation control devices are used to prevent this.
Systems more modern than FM broadcasting tend to use either programme-dependent variable pre-emphasis. Long before FM stereo transmission was considered, FM multiplexing of other types of audio level information was experimented with. Edwin Armstrong who invented FM was the first to experiment with multiplexing, at his experimental 41 MHz station W2XDG located on the 85th floor of the Empire State Building in New York City; these FM multiplex transmissions started in November 1934 and consisted of the main channel audio program and three subcarriers: a fax program, a synchronizing signal for the fax program and a telegraph “order” channel. These original FM multiplex subcarriers were amplitude modulated. Two musical programs, consisting of both the Red and Blue Network program feeds of the NBC Radio Network, were transmitted using the same system of subcarrier modulation as part of a studio-to-transmitter link system. In April 1935, the AM subcarriers were replaced with much improved results.
The first FM subcarrier transmissions emanating from Major Armstrong's experimental station KE2XCC at Alpine, New Jersey occurred in 1948. These transmissions consisted of two-cha
Telefunken was a German radio and television apparatus company, founded in Berlin in 1903, as a joint venture of Siemens & Halske and the Allgemeine Elektricitäts-Gesellschaft. The name "Telefunken" appears in: the product brand name "Telefunken". H. System Telefunken, founded 1903 in Berlin as a subsidiary of Siemens & Halske. H.. KG" in Heilbronn, Germany. L." The company Telefunken USA was incorporated in early 2001 to provide restoration services and build reproductions of vintage Telefunken microphones. Around the start of the 20th century, two groups of German researchers worked on the development of techniques for wireless communication; the one group at AEG, led by Adolf Slaby and Georg Graf von Arco, developed systems for the Kaiserliche Marine. Their main competitor was the British Marconi Company; when a dispute concerning patents arose between the two companies, Kaiser Wilhelm II urged both parties to join efforts, creating Gesellschaft für drahtlose Telegraphie System Telefunken joint venture on 27 May 1903, with the disputed patents and techniques invested in it.
On 17 April 1923, it was renamed The Company for Wireless Telegraphy. Telefunken was the company's telegraphic address; the first technical director of Telefunken was Count Georg von Arco. Telefunken became a major player in the radio and electronics fields, both civilian and military. During World War I, they supplied radio sets and telegraphy equipment for the military, as well as building one of the first radio navigation systems for the Zeppelin force; the Telefunken Kompass Sender operated from 1908 to 1918, allowing the Zeppelins to navigate throughout the North Sea area in any weather. Starting in 1923, Telefunken built broadcast transmitters and radio sets. In 1928, Telefunken made history by designing the V-41 amplifier for the German Radio Network; this was the first two-stage, "Hi-Fi" amplifier. Over time, Telefunken perfected their designs and in 1950 the V-72 amplifier was developed; the TAB V-72 soon became popular with recording facilities. The V-72S was the only type of amplifier found in the REDD.37 console used by the Beatles at Abbey Road Studios on many of their early recordings.
In 1932, record players were added to the product line. In 1941, Siemens transferred its Telefunken shares to AEG as part of the agreements known as the "Telefunken settlement", AEG thus became the sole owner and continued to lead Telefunken as a subsidiary. During the Second World War, Telefunken was a supplier of vacuum tubes and radio relay systems, developed Funkmess facilities and directional finders, as part of the German air defence against aerial bombing. During the war, manufacturing plants were developed in west of Germany or relocated. Thus, under AEG, turned into the smaller subsidiary, with the three divisions realigning and data processing technology, elements as well as broadcast and phono. Telefunken was the originator of the FM radio broadcast system. Telefunken, through the subsidiary company Teldec, was for many decades one of the largest German record companies, until Teldec was sold to WEA in 1988. In 1959, Telefunken established a modern semiconductor works in Heilbronn, where in April 1960 production began.
The works was expanded several times, in 1970 a new 6-storey building was built at the northern edge of the area. At the beginning of the 1970s it housed 2,500 employees. In 1967, Telefunken was merged with AEG, renamed to AEG-Telefunken. In the beginning of the 1960s, Walter Bruch developed the PAL-colour television system for the company, in use by most countries of the western Hemisphere. PAL is established i.e. in the United Kingdom and, except France, many other European countries - in Brazil, South Africa and Australia. The mainframe computer TR 4 was developed at Telefunken in Backnang, the TR 440 model was developed at Telefunken in Konstanz, including the first ball-based mouse named Rollkugel in 1968; the computers were in use at many German university computing centres from the 1970s to around 1985. The development and manufacture of large computers was separated in 1974 to the Konstanz Computer Company; the production of mini- and process computers was integrated into the automatic control engineering division of AEG.
When AEG was bought by Daimler in 1985, "Telefunken" was dropped from the company name. In 1995, Telefunken was sold to Tech Sym Corporation for $9 million. However, Telefunken remained a German company. In the 1970s and early 1980s, Telefunken was instrumental in the development of high quality audio noise reduction sy
Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to the noise power expressed in decibels. A ratio higher than 1:1 indicates more signal than noise. While SNR is quoted for electrical signals, it can be applied to any form of signal, for example isotope levels in an ice core, biochemical signaling between cells, or financial trading signals. Signal-to-noise ratio is sometimes used metaphorically to refer to the ratio of useful information to false or irrelevant data in a conversation or exchange. For example, in online discussion forums and other online communities, off-topic posts and spam are regarded as "noise" that interferes with the "signal" of appropriate discussion; the signal-to-noise ratio, the bandwidth, the channel capacity of a communication channel are connected by the Shannon–Hartley theorem. Signal-to-noise ratio is defined as the ratio of the power of a signal to the power of background noise: S N R = P s i g n a l P n o i s e, where P is average power.
Both signal and noise power must be measured at the same or equivalent points in a system, within the same system bandwidth. Depending on whether the signal is a constant or a random variable, the signal to noise ratio for random noise N with expected value of zero becomes: S N R = s 2 σ N 2 or S N R = E σ N 2 where E refers to the expected value, i.e. in this case the mean of S 2. If the signal and the noise are measured across the same impedance, the SNR can be obtained by calculating the square of the amplitude ratio: S N R = P s i g n a l P n o i s e = 2, where A is root mean square amplitude; because many signals have a wide dynamic range, signals are expressed using the logarithmic decibel scale. Based upon the definition of decibel and noise may be expressed in decibels as P s i g n a l, d B = 10 log 10 and P n o i s e, d B = 10 log 10 . In a similar manner, SNR may be expressed in decibels as S N R d B = 10 log 10 . Using the definition of SNR S N R d B = 10 log 10 . Using the quotient rule for logarithms 10 log 10 = 10
Compact disc is a digital optical disc data storage format, co-developed by Philips and Sony and released in 1982. The format was developed to store and play only sound recordings but was adapted for storage of data. Several other formats were further derived from these, including write-once audio and data storage, rewritable media, Video Compact Disc, Super Video Compact Disc, Photo CD, PictureCD, CD-i, Enhanced Music CD; the first commercially available audio CD player, the Sony CDP-101, was released October 1982 in Japan. Standard CDs have a diameter of 120 millimetres and can hold up to about 80 minutes of uncompressed audio or about 700 MiB of data; the Mini CD has various diameters ranging from 60 to 80 millimetres. At the time of the technology's introduction in 1982, a CD could store much more data than a personal computer hard drive, which would hold 10 MB. By 2010, hard drives offered as much storage space as a thousand CDs, while their prices had plummeted to commodity level. In 2004, worldwide sales of audio CDs, CD-ROMs and CD-Rs reached about 30 billion discs.
By 2007, 200 billion CDs had been sold worldwide. From the early 2000s CDs were being replaced by other forms of digital storage and distribution, with the result that by 2010 the number of audio CDs being sold in the U. S. had dropped about 50% from their peak. In 2014, revenues from digital music services matched those from physical format sales for the first time. American inventor James T. Russell has been credited with inventing the first system to record digital information on an optical transparent foil, lit from behind by a high-power halogen lamp. Russell's patent application was filed in 1966, he was granted a patent in 1970. Following litigation and Philips licensed Russell's patents in the 1980s; the compact disc is an evolution of LaserDisc technology, where a focused laser beam is used that enables the high information density required for high-quality digital audio signals. Prototypes were developed by Sony independently in the late 1970s. Although dismissed by Philips Research management as a trivial pursuit, the CD became the primary focus for Philips as the LaserDisc format struggled.
In 1979, Sony and Philips set up a joint task force of engineers to design a new digital audio disc. After a year of experimentation and discussion, the Red Book CD-DA standard was published in 1980. After their commercial release in 1982, compact discs and their players were popular. Despite costing up to $1,000, over 400,000 CD players were sold in the United States between 1983 and 1984. By 1988, CD sales in the United States surpassed those of vinyl LPs, by 1992 CD sales surpassed those of prerecorded music cassette tapes; the success of the compact disc has been credited to the cooperation between Philips and Sony, which together agreed upon and developed compatible hardware. The unified design of the compact disc allowed consumers to purchase any disc or player from any company, allowed the CD to dominate the at-home music market unchallenged. In 1974, Lou Ottens, director of the audio division of Philips, started a small group with the aim to develop an analog optical audio disc with a diameter of 20 cm and a sound quality superior to that of the vinyl record.
However, due to the unsatisfactory performance of the analog format, two Philips research engineers recommended a digital format in March 1974. In 1977, Philips established a laboratory with the mission of creating a digital audio disc; the diameter of Philips's prototype compact disc was set at 11.5 cm, the diagonal of an audio cassette. Heitaro Nakajima, who developed an early digital audio recorder within Japan's national public broadcasting organization NHK in 1970, became general manager of Sony's audio department in 1971, his team developed a digital PCM adaptor audio tape recorder using a Betamax video recorder in 1973. After this, in 1974 the leap to storing digital audio on an optical disc was made. Sony first publicly demonstrated an optical digital audio disc in September 1976. A year in September 1977, Sony showed the press a 30 cm disc that could play 60 minutes of digital audio using MFM modulation. In September 1978, the company demonstrated an optical digital audio disc with a 150-minute playing time, 44,056 Hz sampling rate, 16-bit linear resolution, cross-interleaved error correction code—specifications similar to those settled upon for the standard compact disc format in 1980.
Technical details of Sony's digital audio disc were presented during the 62nd AES Convention, held on 13–16 March 1979, in Brussels. Sony's AES technical paper was published on 1 March 1979. A week on 8 March, Philips publicly demonstrated a prototype of an optical digital audio disc at a press conference called "Philips Introduce Compact Disc" in Eindhoven, Netherlands. Sony executive Norio Ohga CEO and chairman of Sony, Heitaro Nakajima were convinced of the format's commercial potential and pushed further development despite widespread skepticism; as a result, in 1979, Sony and Philips set up a joint task force of engineers to design a new digital audio disc. Led by engineers Kees Schouhamer Immink and Toshitada Doi, the research pushed forward laser and optical disc technology. After a year of experimentation and discussion, the task force produced the Red Book CD-DA standard. First published in 1980, the stand
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 a direct current to the audio signal, being recorded. AC bias is the addition of an inaudible high-frequency signal 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 at low signal levels. A recording signal which generates a magnetic field strength less than tape's coercivity is unable to magnetise the tape and produces little playback signal. Bias increases the signal quality of most audio recordings 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 proposed magnetic tape. Fritz Pfleumer was granted a German patent for a non-magnetic "Sound recording carrier" with a magnetic coating, on 1928-01-31, but it was overturned in favor of an earlier US patent by Joseph A. O'Neill; the earliest magnetic recording systems applied the unadulterated 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 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. Some early DC-bias systems used a permanent magnet, placed near the record head, it had to be swung out of the way for replay. DC bias was replaced by AC bias but was re-adopted by some low-cost cassette recorders.
Although the improvements with DC bias were significant, an better recording is possible if an AC bias is used instead. While several people around the world rediscovered AC bias, it was the German developments that were used in practice and served as the model for future work; the original patent for AC bias was filed by Wendell L. Carlson and Glenn L. Carpenter in 1921 resulting in a patent in 1927; the value of AC bias was somewhat masked by the primitive state of other aspects of magnetic recording and Carlson and Carpenter's achievement was ignored. The first rediscovery seems to have been by Dean Wooldrige at Bell Telephone Laboratories, around 1937, but the BTL lawyers found the original patent, kept silent about their rediscovery of AC bias. Teiji Igarashi, Makoto Ishikawa, Kenzo Nagai of Japan published a paper on AC biasing in 1938 and received a Japanese patent in 1940. Marvin Camras rediscovered high-frequency bias independently in 1941 and received a patent in 1944; the reduction in distortion and noise provided by AC bias was rediscovered in 1940 by Walter Weber while working at the Reichs-Rundfunk-Gesellschaft.
The German pair received several related patents, including DE 743411 for "high-frequency treatment of the sound carrier". Independently of Weber and Braunmühl, the UK company Boosey & Hawkes produced a steel-wire recorder under government contract during the Second World War, equipped with AC bias. Examples still surface from time to time, many having been disposed of as government surplus stock. After the war and Hawkes produced a "Reporter" tape recorder in the early 1950s using magnetic tape, rather than wire, based on German wartime technology; as the tape leaves the trailing edge of the gap in the tape head, the oscillating magnetic field due to the applied AC bias is reduced to the average magnetic field of the much slower-changing audio signal, the tape particles are therefore left in this magnetic condition. The non-linearity of the magnetic particles in the tape coating is overcome by having the AC bias field greater by at least an order of magnitude, which saturates these particles in both magnetic directions while they pass the gap in the recording head.
The AC bias level is quite critical and, after being adjusted for a particular tape formulation with a specific recording machine, is left unchanged. The mechanism is similar to the demagnetizing signal, used to erase the tape except that the desired audio signal is retained on the tape during the recording process; the large AC bias acts as a demagnetizing signal which decays exponentially as the tape moves beyond the head, while the audio signal is the residual field that remains imprinted on the magnetic medium. A quantitative explanation of AC bias has been given by Bertram; the characteristics of the recording system change quite markedly as the level of the bias current is changed. There is a level. There is a level at which the high-frequency response is at maximum; these conditions do not occur at the same bias level. Professional reel-to-reel and cassette recorders are always set up for minimal distortion. Consumer equipment, in particular compact-cassette recorders have the bias set at a compromise level to give good frequency response and acceptably low distortion.
Bang & Olufsen invented and patented the so-called Do