DC bias

When describing a periodic function in the time domain, the DC bias, DC component, DC offset, or DC coefficient is the mean value of the waveform. If the mean amplitude is zero, there is no DC offset. In contrast, various other frequencies are analogous to superimposed AC voltages or currents, hence called AC components or AC coefficients.

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. The term's use is extended to two-dimensional transformations like the discrete cosine transform used in JPEG.

A waveform without a DC component is known as a DC-balanced or DC-free waveform.^[1] DC-balanced waveforms are useful in communications systems to avoid voltage imbalance problems between connected systems or components.

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 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" or "baseline wander".

Practical applications

A DC tape bias was used in early tape recorders to reduce distortion.

A DC bias is applied to the control grid of vacuum tubes in power amplifiers in order to regulate power.^[2]

DC-balanced signals are used in communications systems to prevent bit errors when passing through circuits with Capacitive coupling or transformers. Bit errors can occur when a 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 codes and paired-disparity codes.

Audio Recording

In audio recording, a DC offset is an undesirable characteristic of a recording sound. It occurs in the capturing of sound, before it reaches the recorder, and is normally caused by defective or low-quality equipment. The offset causes the center of the recording waveform to not be at 0, but at a higher value, for example, +1. This 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.^[3]

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 microphones.

On modern satellite dishes, 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).

Frequency selection

On a voltage-controlled oscillator (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 subcarriers. Frequency-shift keying can be done solely by changing the DC bias.

Electrical grid

The electrical grid, which is normally three-phase AC, can be severely disrupted by the presence of a large DC bias. This is caused by strong solar flares hitting the Earth's atmosphere, which in turn creates strong electromagnetic fields. This induces voltages in long-distance electrical lines, which can be strong enough to arc across transformers. (Even pipelines, 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 latitudes 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.

See also

• AC bias

References