DC bias
Encyclopedia
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
(possibly scaled according to the norm of the corresponding basis function
of the frequency analysis
filter bank
). The name comes from the middle 20th century design of electrical line codes for use with transmission channels unable to transmit a DC
voltage
or current. In such usage, this coefficient
represents the useless DC, whilst the coefficients representing various other frequencies are analogous to superimposed AC
voltages or currents, hence called AC components or AC coefficients.
A waveform with a zero DC component (that is, the average D.C. component should be 0) is known as a DC-balanced waveform. DC-balanced waveforms are useful in communications systems, because they can be used on AC-coupled electrical connections to avoid voltage imbalance problems between connected systems or components. To be specific, bit errors can occur when a (relatively) long series of 1's create a DC level that charges the capacitor of the high-pass filter used as the AC coupler, bringing the signal input down incorrectly to a 0-level. In order to avoid these kinds of bit errors, most line codes are designed to produce DC-balanced waveforms. The most common classes of DC balanced line codes are constant-weight code
s and paired-disparity codes.
The term's use is extended to two-dimensional transformations like the discrete cosine transform
used in JPEG
.
The term originated in electronics, where it refers to a direct current
voltage, but the concept has been extended to any representation of a waveform
. DC offset is the mean amplitude of the waveform; if the mean amplitude is zero, there is no DC offset.
DC offset is usually undesirable when it causes saturation or change in the operating point of an amplifier. An electrical DC bias will not pass through a transformer
; thus a simple isolation transformer
can be used to block or remove it, leaving only the AC component on the other side. In signal processing terms, DC offset can be reduced in real-time by a one-pole one-zero high-pass filter
. When one already has the entire waveform, subtracting the mean amplitude from each sample will remove the offset. Often, very low frequencies are called "slowly changing DC". While not technically accurate, a high-pass filter can remove such a "changing offset" better because its cutoff does not extend to as low a bandwidth as the above method.
has been used on magnetic tape
. A DC bias is also applied to the control grid vacuum tube
s in power amplifiers in order to regulate power.http://www.aikenamps.com/WhatIsBiasing.htm There are also several other uses for a DC bias.
s.
On modern satellite dish
es, used for direct-broadcast
satellite TV, a DC bias is used to provide electrical power to the feedhorn. Changing the bias between approximately 12 and 18 volts also allows selection of the correct polarization (horizontal and vertical on older systems, clockwise
and counterclockwise on newer circular polarization
systems).
(VCO), such as in a radio transmitter, selection of the center frequency
of the carrier wave
is done with a DC bias. For frequency modulation
(FM), the AC component is the baseband
audio signal
plus any subcarrier
s. Frequency-shift keying
can be done solely by changing the DC bias.
AC, can be severely disrupted by the presence of a large DC bias. This is caused by strong solar flare
s hitting the Earth
's atmosphere
, which in turn creates strong electromagnetic field
s. This induce
s voltages in long-distance electrical lines, which can be strong enough to arc across transformers. (Even pipeline
s, such as the mostly above-ground Alaska Pipeline, are prone to this, and must be tied to electrical ground with zinc
sacrificial anodes.) This is a rare but serious problem, mostly for far northern locations like Canada
and Scandinavia
, where a strong aurora borealis will cover much lower latitude
s than normal during such a situation. Space weather
forecasts are used to predict when these geomagnetic storm
events might occur. High-voltage direct current
systems have their own control gear at conversion stations and can adapt somewhat better to such conditions, however large and often widely-fluctuating voltages can still cause problems like harmonics.
Periodic function
In mathematics, a periodic function is a function that repeats its values in regular intervals or periods. The most important examples are the trigonometric functions, which repeat over intervals of length 2π radians. Periodic functions are used throughout science to describe oscillations,...
in the frequency domain
Frequency domain
In electronics, control systems engineering, and statistics, frequency domain is a term used to describe the domain for analysis of mathematical functions or signals with respect to frequency, rather than time....
, the DC bias, DC component, DC offset, or DC coefficient is the mean
Mean
In statistics, mean has two related meanings:* the arithmetic mean .* the expected value of a random variable, which is also called the population mean....
value of the waveform
Waveform
Waveform means the shape and form of a signal such as a wave moving in a physical medium or an abstract representation.In many cases the medium in which the wave is being propagated does not permit a direct visual image of the form. In these cases, the term 'waveform' refers to the shape of a graph...
(possibly scaled according to the norm of the corresponding basis function
Basis function
In mathematics, a basis function is an element of a particular basis for a function space. Every continuous function in the function space can be represented as a linear combination of basis functions, just as every vector in a vector space can be represented as a linear combination of basis...
of the frequency analysis
Frequency analysis
In cryptanalysis, frequency analysis is the study of the frequency of letters or groups of letters in a ciphertext. The method is used as an aid to breaking classical ciphers....
filter bank
Filter bank
In signal processing, a filter bank is an array of band-pass filters that separates the input signal into multiple components, each one carrying a single frequency subband of the original signal. One application of a filter bank is a graphic equalizer, which can attenuate the components...
). The name comes from the middle 20th century design of electrical line codes for use with transmission channels unable to transmit a DC
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
or current. In such usage, this coefficient
Coefficient
In mathematics, a coefficient is a multiplicative factor in some term of an expression ; it is usually a number, but in any case does not involve any variables of the expression...
represents the useless DC, whilst the coefficients representing various other frequencies are analogous to superimposed AC
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
voltages or currents, hence called AC components or AC coefficients.
A waveform with a zero DC component (that is, the average D.C. component should be 0) is known as a DC-balanced waveform. DC-balanced waveforms are useful in communications systems, because they can be used on AC-coupled electrical connections to avoid voltage imbalance problems between connected systems or components. To be specific, bit errors can occur when a (relatively) long series of 1's create a DC level that charges the capacitor of the high-pass filter used as the AC coupler, bringing the signal input down incorrectly to a 0-level. In order to avoid these kinds of bit errors, most line codes are designed to produce DC-balanced waveforms. The most common classes of DC balanced line codes are constant-weight code
Constant-weight code
In coding theory, a constant-weight code, also called an m of n code, is an error detection and correction code where all codewords share the same Hamming weight. The theory is closely connected to that of designs...
s and paired-disparity codes.
The term's use is extended to two-dimensional transformations like the discrete cosine transform
Discrete cosine transform
A discrete cosine transform expresses a sequence of finitely many data points in terms of a sum of cosine functions oscillating at different frequencies. DCTs are important to numerous applications in science and engineering, from lossy compression of audio and images A discrete cosine transform...
used in JPEG
JPEG
In computing, JPEG . The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and image quality. JPEG typically achieves 10:1 compression with little perceptible loss in image quality....
.
The term originated in electronics, where it refers to a direct current
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
voltage, but the concept has been extended to any representation of a waveform
Waveform
Waveform means the shape and form of a signal such as a wave moving in a physical medium or an abstract representation.In many cases the medium in which the wave is being propagated does not permit a direct visual image of the form. In these cases, the term 'waveform' refers to the shape of a graph...
. DC offset is the mean amplitude of the waveform; if the mean amplitude is zero, there is no DC offset.
DC offset is usually undesirable when it causes saturation or change in the operating point of an amplifier. An electrical DC bias will not pass through a transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
; thus a simple isolation transformer
Isolation transformer
An isolation transformer is a transformer used to transfer electrical power from a source of alternating current power to some equipment or device while isolating the powered device from the power source, usually for safety...
can be used to block or remove it, leaving only the AC component on the other side. In signal processing terms, DC offset can be reduced in real-time by a one-pole one-zero high-pass filter
High-pass filter
A high-pass filter is a device that passes high frequencies and attenuates frequencies lower than its cutoff frequency. A high-pass filter is usually modeled as a linear time-invariant system...
. When one already has the entire waveform, subtracting the mean amplitude from each sample will remove the offset. Often, very low frequencies are called "slowly changing DC". While not technically accurate, a high-pass filter can remove such a "changing offset" better because its cutoff does not extend to as low a bandwidth as the above method.
Practical applications
Tape biasTape 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 to the audio...
has been used on 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...
. A DC bias is also applied to the control grid vacuum tube
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
s in power amplifiers in order to regulate power.http://www.aikenamps.com/WhatIsBiasing.htm There are also several other uses for a DC bias.
Remote power
A DC bias can be used to power microphones or other devices over the same wires as required for the signal. This is commonly used, for example, in professional audio microphoneMicrophone
A microphone is an acoustic-to-electric transducer or sensor that converts sound into an electrical signal. In 1877, Emile Berliner invented the first microphone used as a telephone voice transmitter...
s.
On modern satellite dish
Satellite dish
A satellite dish is a dish-shaped type of parabolic antenna designed to receive microwaves from communications satellites, which transmit data transmissions or broadcasts, such as satellite television.-Principle of operation:...
es, used for direct-broadcast
Direct broadcast satellite
Direct broadcast satellite is a term used to refer to satellite television broadcasts intended for home reception.A designation broader than DBS would be direct-to-home signals, or DTH. This has initially distinguished the transmissions directly intended for home viewers from cable television...
satellite TV, a DC bias is used to provide electrical power to the feedhorn. Changing the bias between approximately 12 and 18 volts also allows selection of the correct polarization (horizontal and vertical on older systems, clockwise
Clockwise
Circular motion can occur in two possible directions. A clockwise motion is one that proceeds in the same direction as a clock's hands: from the top to the right, then down and then to the left, and back to the top...
and counterclockwise on newer circular polarization
Circular polarization
In electrodynamics, circular polarization of an electromagnetic wave is a polarization in which the electric field of the passing wave does not change strength but only changes direction in a rotary type manner....
systems).
Frequency selection
On a voltage-controlled oscillatorVoltage-controlled oscillator
A voltage-controlled oscillator or VCO is an electronic oscillator designed to be controlled in oscillation frequency by a voltage input. The frequency of oscillation is varied by the applied DC voltage, while modulating signals may also be fed into the VCO to cause frequency modulation or phase...
(VCO), such as in a radio transmitter, selection of the center frequency
Center frequency
In electrical engineering and telecommunications, the center frequency of a filter or channel is a measure of a central frequency between the upper and lower cutoff frequencies...
of the carrier wave
Carrier wave
In telecommunications, a carrier wave or carrier is a waveform that is modulated with an input signal for the purpose of conveying information. This carrier wave is usually a much higher frequency than the input signal...
is done with a DC bias. For frequency modulation
Frequency modulation
In telecommunications and signal processing, frequency modulation conveys information over a carrier wave by varying its instantaneous frequency. This contrasts with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant...
(FM), the AC component is the baseband
Baseband
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...
audio signal
Audio signal
An audio signal is an analog representation of sound, typically as an electrical voltage. Audio signals may be synthesized directly, or may originate at a transducer such as a microphone, musical instrument pickup, phonograph cartridge, or tape head. Loudspeakers or headphones convert an electrical...
plus any subcarrier
Subcarrier
A subcarrier is a separate analog or digital signal carried on a main radio transmission, which carries extra information such as voice or data. More technically, it is an already-modulated signal, which is then modulated into another signal of higher frequency and bandwidth...
s. Frequency-shift keying
Frequency-shift keying
Frequency-shift keying is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier wave. The simplest FSK is binary FSK . BFSK uses a pair of discrete frequencies to transmit binary information. With this scheme, the "1" is called...
can be done solely by changing the DC bias.
Electrical grid
The electrical grid, which is normally three-phaseThree-phase
In electrical engineering, three-phase electric power systems have at least three conductors carrying voltage waveforms that are radians offset in time...
AC, can be severely disrupted by the presence of a large DC bias. This is caused by strong solar flare
Solar flare
A solar flare is a sudden brightening observed over the Sun surface or the solar limb, which is interpreted as a large energy release of up to 6 × 1025 joules of energy . The flare ejects clouds of electrons, ions, and atoms through the corona into space. These clouds typically reach Earth a day...
s hitting the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
's atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
, which in turn creates strong electromagnetic field
Electromagnetic field
An electromagnetic field is a physical field produced by moving electrically charged objects. It affects the behavior of charged objects in the vicinity of the field. The electromagnetic field extends indefinitely throughout space and describes the electromagnetic interaction...
s. This induce
Electromagnetic induction
Electromagnetic induction is the production of an electric current across a conductor moving through a magnetic field. It underlies the operation of generators, transformers, induction motors, electric motors, synchronous motors, and solenoids....
s voltages in long-distance electrical lines, which can be strong enough to arc across transformers. (Even pipeline
Pipeline transport
Pipeline transport is the transportation of goods through a pipe. Most commonly, liquids and gases are sent, but pneumatic tubes that transport solid capsules using compressed air are also used....
s, such as the mostly above-ground Alaska Pipeline, are prone to this, and must be tied to electrical ground with zinc
Zinc
Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...
sacrificial anodes.) This is a rare but serious problem, mostly for far northern locations like Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
and Scandinavia
Scandinavia
Scandinavia is a cultural, historical and ethno-linguistic region in northern Europe that includes the three kingdoms of Denmark, Norway and Sweden, characterized by their common ethno-cultural heritage and language. Modern Norway and Sweden proper are situated on the Scandinavian Peninsula,...
, where a strong aurora borealis will cover much lower latitude
Latitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...
s than normal during such a situation. Space weather
Space weather
Space weather is the concept of changing environmental conditions in near-Earth space or thespace from the Sun's atmosphere to the Earth's atmosphere. It is distinct from the concept ofweather within the Earth's planetary atmosphere...
forecasts are used to predict when these geomagnetic storm
Geomagnetic storm
A geomagnetic storm is a temporary disturbance of the Earth's magnetosphere caused by a disturbance in the interplanetary medium. A geomagnetic storm is a major component of space weather and provides the input for many other components of space weather...
events might occur. High-voltage direct current
High-voltage direct current
A high-voltage, direct current electric power transmission system uses direct current for the bulk transmission of electrical power, in contrast with the more common alternating current systems. For long-distance transmission, HVDC systems may be less expensive and suffer lower electrical losses...
systems have their own control gear at conversion stations and can adapt somewhat better to such conditions, however large and often widely-fluctuating voltages can still cause problems like harmonics.