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. A waveform without a DC component is known as a DC-balanced or DC-free waveform.

The term originated in electronics, where it refers to a direct current voltage. In contrast, various other non-DC frequencies are analogous to superimposed alternating current (AC) voltages or currents, hence called AC components or AC coefficients.

The concept has been extended to any representation of a waveform and to two-dimensional transformations like the discrete cosine transform used in JPEG.

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; 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. 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 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.

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