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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 waveform. 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. 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「DC bias」の詳細全文を読む スポンサード リンク
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