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Insignal processing, when describing aperiodic function in thetime domain, theDC bias,DC component,DC offset, orDC coefficient is themean value of thewaveform. A waveform with zero mean or no DC bias is known as aDC balanced orDC free waveform.[1]
The term originates in electronics, whereDC refers to adirect current voltage. In contrast, various other non-DC frequencies are analogous to superimposedalternating current (AC) voltages or currents, hence calledAC component orAC coefficients.
In the design of electronic amplifier circuits, every active device hasbiasing to set itsoperating point, the steady state current and voltage on the device when no signal is applied. Inbipolar 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 usingfield-effect transistors orvacuum tubes 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 causesclipping or other undesirable change in the operating point of an amplifier. An electrical DC bias will not pass through atransformer orcapacitor; thus a simpleisolation transformer orseries-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 ahigh-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".
DC-balanced signals are used in communications systems to prevent bit errors when passing through circuits withcapacitive coupling ortransformers. 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, mostline codes are designed to produce DC-balanced signals. The most common classes of DC balanced line codes areconstant-weight codes andpaired-disparity codes.
Inaudio 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.[2]
A DCtape bias was used in early tape recorders to reduce distortion.
A DC bias is applied to the control grid ofvacuum tubes inpower amplifiers in order to regulate power.[3]
On avoltage-controlled oscillator (VCO), such as in aradio transmitter, selection of thecenter frequency of thecarrier wave is done with a DC bias. Forfrequency modulation (FM), the AC component is thebasebandaudio signal plus anysubcarriers.Frequency-shift keying can be done solely by changing the DC bias.
The concept has been extended to any representation of awaveform and to two-dimensional transformations like thediscrete cosine transform used inJPEG.
Thedc-balanced ordc-free codes, as they are often called, have a long history and their application is certainly not confined to recording practice.
