DC bias
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In signal processing, 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. A waveform with zero mean or no DC bias is known as a DC balanced or DC free waveform.[1]
Origin
The term originates in electronics, where DC 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.
Applications
In the design of electronic amplifier circuits, every active device has biasing to set its operating point, the steady state current and voltage on the device when no signal is applied. In bipolar 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 using field-effect transistors or vacuum 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 causes clipping or other undesirable change in the operating point of an amplifier. An electrical DC bias will not pass through a transformer or capacitor; thus a simple isolation transformer or series-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 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".
Communications systems
DC-balanced signals are used in communications systems to prevent bit errors when passing through circuits with
Audio
In
A DC tape bias was used in early tape recorders to reduce distortion.
A DC bias is applied to the control grid of
Frequency selection
On a
Waveform representation
The concept has been extended to any representation of a waveform and to two-dimensional transformations like the discrete cosine transform used in JPEG.
See also
- Phantom power
- Root-mean-square amplitude
- Root-mean-square voltage
References
- .
The dc-balanced or dc-free codes, as they are often called, have a long history and their application is certainly not confined to recording practice.
- ^ "DC offset - Audacity Wiki". Archived from the original on 2016-08-23. Retrieved 2014-01-30.
- ^ Randall Aiken. "What Is Biasing?". Aikenamps.com. Archived from the original on 2012-02-13. Retrieved 2012-08-16.