signal processing Signal processing is an electrical engineering subfield that focuses on analyzing, modifying and synthesizing '' signals'', such as sound, images, and scientific measurements. Signal processing techniques are used to optimize transmissions, ...

, when describing a

periodic function A periodic function is a function that repeats its values at regular intervals. For example, the trigonometric functions, which repeat at intervals of 2\pi radians, are periodic functions. Periodic functions are used throughout science to des ...

in the

time domain Time domain refers to the analysis of mathematical functions, physical signals or time series of economic or environmental data, with respect to time. In the time domain, the signal or function's value is known for all real numbers, for the c ...

, the DC bias, DC component, DC offset, or DC coefficient is the

mean There are several kinds of mean in mathematics, especially in statistics. Each mean serves to summarize a given group of data, often to better understand the overall value ( magnitude and sign) of a given data set. For a data set, the '' ar ...

amplitude of the

waveform In electronics, acoustics, and related fields, the waveform of a signal is the shape of its graph as a function of time, independent of its time and magnitude scales and of any displacement in time.David Crecraft, David Gorham, ''Electro ...

. If the mean amplitude is zero, there is no DC bias. A waveform with no DC bias is known as a ''DC balanced'' or ''DC free'' waveform.

Origin

The term originates in electronics, where ''DC'' refers to a

direct current Direct current (DC) is one-directional flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor such as a wire, but can also flow through semiconductors, insulators, or ev ...

voltage. In contrast, various other non-DC frequencies are analogous to superimposed

alternating current Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct current (DC) which flows only in one direction. Alternating current is the form in whic ...

(AC) voltages or currents, hence called ''AC components'' or ''AC coefficients''.

Applications

In the design of electronic amplifier circuits, every active device has

biasing In electronics, biasing is the setting of DC (direct current) operating conditions (current and voltage) of an active device in an amplifier. Many electronic devices, such as diodes, transistors and vacuum tubes, whose function is processing ...

to set its ''

operating point {{unreferenced, date=April 2013 The operating point is a specific point within the operation characteristic of a technical device. This point will be engaged because of the properties of the system and the outside influences and parameters. In el ...

'', the steady state current and voltage on the device when no signal is applied. In

bipolar transistor biasing Bipolar transistors must be properly biased to operate correctly. In circuits made with individual devices (discrete circuits), biasing networks consisting of resistors are commonly employed. Much more elaborate biasing arrangements are used in ...

, for example, a network of resistors is used to apply a small amount of DC to the base terminal of the transistor. The AC signal is applied at the same terminal and is amplified. The bias network is designed to preserve the applied AC signal. Similarly, amplifiers using

field-effect transistor The field-effect transistor (FET) is a type of transistor that uses an electric field to control the flow of current in a semiconductor. FETs (JFETs or MOSFETs) are devices with three terminals: ''source'', ''gate'', and ''drain''. FETs co ...

s or

vacuum tube A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied. The type known as ...

s also have bias circuits. The operating point of an amplifier greatly affects its characteristics of distortion and efficiency;

power amplifier classes In electronics, power amplifier classes are letter symbols applied to different power amplifier types. The class gives a broad indication of an amplifier's characteristics and performance. The classes are related to the time period that the active ...

are distinguished by the operating point set by the DC bias. DC offset is usually undesirable when it causes

clipping Clipping may refer to: Words * Clipping (morphology), the formation of a new word by shortening it, e.g. "ad" from "advertisement" * Clipping (phonetics), shortening the articulation of a speech sound, usually a vowel * Clipping (publications) ...

or other undesirable change in the operating point of an amplifier. An electrical DC bias will not pass through a

transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...

capacitor A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals. The effect of ...

; thus a simple

isolation transformer An isolation transformer is a transformer used to transfer electrical power from a source of alternating current (AC) power to some equipment or device while isolating the powered device from the power source, usually for safety reasons or to re ...

or series-wired capacitor 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

high-pass filter A high-pass filter (HPF) is an electronic filter that passes signals with a frequency higher than a certain cutoff frequency and attenuates signals with frequencies lower than the cutoff frequency. The amount of attenuation for each frequency ...

. For stored digital signals, subtracting the mean amplitude from each sample will remove the offset. Very low frequencies can look like DC bias but are called "slowly changing DC" or "baseline wander".

Communications systems

DC-balanced signals are used in communications systems to prevent bit errors when passing through circuits with

capacitive coupling Capacitive coupling is the transfer of energy within an electrical network or between distant networks by means of displacement current between circuit(s) nodes, induced by the electric field. This coupling can have an intentional or acciden ...

s. Bit errors can occur when a series of 1's create a DC level that charges the coupling capacitor, bringing the signal input down incorrectly to a 0-level. In order to avoid these kinds of bit errors, most

line code In telecommunication, a line code is a pattern of voltage, current, or photons used to represent digital data transmitted down a communication channel or written to a storage medium. This repertoire of signals is usually called a constrained ...

s are designed to produce DC-balanced signals. The most common classes of DC balanced line codes are constant-weight codes and paired-disparity codes.

Audio

audio recording Sound recording and reproduction is the electrical, mechanical, electronic, or digital inscription and re-creation of sound waves, such as spoken voice, singing, instrumental music, or sound effects. The two main classes of sound recording t ...

, a DC offset is an undesirable characteristic. It occurs in the capturing of sound, before it reaches the recorder, and is normally caused by defective or low-quality equipment. It results in an offset of the center of the recording waveform that can cause two main problems. Either the loudest parts of the signal will be clipped prematurely since the base of the waveform has been moved up, or inaudible low-frequency distortion will occur. Low-frequency distortion may not be audible in the initial recording, but if the waveform is resampled to a compressed or lossy digital format, such as an MP3, those corruptions may become audible. A DC

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 inaud ...

was used in early tape recorders to reduce distortion. A DC bias is applied to the control grid of

s in

power amplifier An audio power amplifier (or power amp) is an electronic amplifier that amplifies low-power electronic audio signals, such as the signal from a radio receiver or an electric guitar pickup, to a level that is high enough for driving louds ...

s in order to regulate power.

Frequency selection

On a

voltage-controlled oscillator A microwave (12–18GHz) voltage-controlled oscillator A voltage-controlled oscillator (VCO) is an electronic oscillator whose oscillation frequency is controlled by a voltage input. The applied input voltage determines the instantaneous oscillat ...

(VCO), such as in a

radio transmitter In electronics and telecommunications, a radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna. The transmitter itself generates a radio frequency alternating current, which is applied to the ...

, selection of the

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. It is usually defined as either the arithmetic mean or the geometri ...

of the

carrier wave In telecommunications, a carrier wave, carrier signal, or just carrier, is a waveform (usually sinusoidal) that is modulated (modified) with an information-bearing signal for the purpose of conveying information. This carrier wave usually has ...

is done with a DC bias. For

frequency modulation Frequency modulation (FM) is the encoding of information in a carrier wave by varying the instantaneous frequency of the wave. The technology is used in telecommunications, radio broadcasting, signal processing, and computing. In analog ...

(FM), the AC component is the

baseband In telecommunications and signal processing, baseband is the range of frequencies occupied by a signal that has not been modulated to higher frequencies. Baseband signals typically originate from transducers, converting some other variable i ...

audio signal An audio signal is a representation of sound, typically using either a changing level of electrical voltage for analog signals, or a series of binary numbers for digital signals. Audio signals have frequencies in the audio frequency range of ro ...

plus any

subcarrier A subcarrier is a sideband of a radio frequency carrier wave, which is modulated to send additional information. Examples include the provision of colour in a black and white television system or the provision of stereo in a monophonic radio broa ...

Frequency-shift keying Frequency-shift keying (FSK) is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier signal. The technology is used for communication systems such as telemetry, weather bal ...

can be done solely by changing the DC bias.

Waveform representation

The concept has been extended to any representation of a

and to two-dimensional transformations like the discrete cosine transform used in

JPEG JPEG ( ) is a commonly used method of lossy compression for digital images, particularly for those images produced by digital photography. The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and imag ...

References

{{reflist Electrical parameters