DC bias

When describing a periodic function in the frequency domain, the DC bias, DC component, 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.

A waveform with a zero DC component is known as a DC-balanced waveform. DC-balanced waveforms are useful in communications systems, since they can be used on AC-coupled electrical connections to avoid voltage imbalance problems between connected systems or components. For this reason, 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.

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

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.

Practical applications

Tape bias has been used on magnetic tape. A DC bias is also applied to the control grid vacuum tubes in power amplifiers in order to regulate power.[1] There are also several other uses for a DC bias.

Remote power

There are several methods in which a DC bias is used to provide low-voltage power to a remote location, without batteries, and without having to run separate electrical cables or having to comply with higher-voltage electrical codes.

For microphones, this is known as phantom power, supplying 9 to 48 volts DC, while the returning audio is AC. A similar concept is used for power over Ethernet, though that may also or instead be provided by the two extra pairs of wires not used for data. Another difference in this system, which works with DC (not AC) signals that reverse polarity within each pair, is that the power is instead supplied between pairs rather than within them.

On modern satellite dishes, especially in Ku band and Ka band 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 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.