signal processing Signal processing is an electrical engineering subfield that focuses on analyzing, modifying and synthesizing ''signals'', such as audio signal processing, sound, image processing, images, Scalar potential, potential fields, Seismic tomograph ...

, when describing a

periodic function A periodic function, also called a periodic waveform (or simply periodic wave), is a function that repeats its values at regular intervals or periods. The repeatable part of the function or waveform is called a ''cycle''. For example, the t ...

in the

time domain In mathematics and signal processing, the time domain is a representation of how a signal, function, or data set varies with time. It is used for the analysis of mathematical functions, physical signals or time series of economic or environmental ...

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

mean A mean is a quantity representing the "center" of a collection of numbers and is intermediate to the extreme values of the set of numbers. There are several kinds of means (or "measures of central tendency") in mathematics, especially in statist ...

value of the

waveform In electronics, acoustics, and related fields, the waveform of a signal is the shape of its Graph of a function, graph as a function of time, independent of its time and Magnitude (mathematics), magnitude Scale (ratio), scales and of any dis ...

. A waveform with zero mean or 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 electric current, flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor (material), conductor such as a wire, but can also flow throug ...

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

alternating current Alternating current (AC) is an electric current that 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 w ...

(AC) voltages or currents, hence called AC component or AC coefficients.

Applications

In the design of electronic amplifier circuits, every active device has biasing to set its '' operating point'', the steady state current and voltage on the device when no signal is applied. In bipolar transistor biasing, 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 current through a semiconductor. It comes in two types: junction FET (JFET) and metal-oxide-semiconductor FET (MOSFET). FETs have three termi ...

s or

vacuum tube A vacuum tube, electron tube, thermionic 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 voltage, potential difference has been applied. It ...

s also have bias circuits. The operating point of an amplifier greatly affects its characteristics of distortion and efficiency; power amplifier classes are distinguished by the operating point set by the DC bias. DC offset is usually undesirable when it causes clipping or other undesirable change in the operating point of an amplifier. An electrical DC bias will not pass through a

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

capacitor In electrical engineering, a capacitor is a device that stores electrical energy by accumulating electric charges on two closely spaced surfaces that are insulated from each other. The capacitor was originally known as the condenser, a term st ...

; thus a simple isolation transformer 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. 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 (electronics), coupling is the transfer of energy within an electrical network or between distant networks by means of displacement current between circuit(s) node (circuits) , nodes, induced by the electric field. This coup ...

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 telecommunications, a line code is a pattern of voltage, current, or photons used to represent digital data transmission (telecommunications), transmitted down a communication channel or written to a storage medium. This repertoire of signal ...

s are designed to produce DC-balanced signals. The most common classes of DC balanced line codes are

constant-weight code In coding theory, a constant-weight code, also called an ''m''-of-''n'' code or ''m''-out-of-''n'' code, is an error detection and correction code where all codewords share the same Hamming weight. The one-hot code and the balanced code are two ...

s and paired-disparity codes.

Audio

In audio recording, 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 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) 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 loudspeakers or headphones. Audio power a ...

s in order to regulate power.

Frequency selection

On a voltage-controlled oscillator (VCO), such as in a radio transmitter, selection of the center frequency of the

carrier wave In telecommunications, a carrier wave, carrier signal, or just carrier, is a periodic waveform (usually sinusoidal) that conveys information through a process called ''modulation''. One or more of the wave's properties, such as amplitude or freq ...

is done with a DC bias. For

frequency modulation Frequency modulation (FM) is a signal modulation technique used in electronic communication, originally for transmitting messages with a radio wave. In frequency modulation a carrier wave is varied in its instantaneous frequency in proporti ...

(FM), the AC component is the baseband

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 signal (signal processing), digital signals. Audio signals have frequencies i ...

plus any subcarriers.

Frequency-shift keying Frequency-shift keying (FSK) is a frequency modulation scheme in which digital information is encoded on a carrier signal by periodically shifting the frequency of the carrier between several discrete frequencies. The technology is used fo ...

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 A discrete cosine transform (DCT) expresses a finite sequence of data points in terms of a sum of cosine functions oscillating at different frequency, frequencies. The DCT, first proposed by Nasir Ahmed (engineer), Nasir Ahmed in 1972, is a widely ...

used in

JPEG JPEG ( , short for Joint Photographic Experts Group and sometimes retroactively referred to as JPEG 1) is a commonly used method of lossy compression for digital images, particularly for those images produced by digital photography. The degr ...

References

{{reflist Electrical parameters