| 单词 | digital |
| 释义 | digital Pertaining to the representation of data by means of digits, or discrete quantities such as numbers or signals that can be interpreted as numbers. By contrast, analog signals have meaning at all intermediate levels. In telecommunications, digital transmission systems make use of pulses or varying levels of electromagnetic energy, such as electricity, radio waves, or light. Digital communications originates in telegraphy, in which a mechanical key is used to close an electrical circuit for varying lengths of time to send a series of short pulses (dots) and long pulses (dashes) that, in specific combinations, represent specific characters or series of characters. Early mechanical computers used a similar concept for input and output. Contemporary computer systems communicate in binary mode through variations in electrical voltage. Digital signaling in a contemporary electrical transmission system involves a signal that varies in voltage to represent one of two discrete and well-defined states.Two of the simplest approaches are unipolar and bipolar signaling. Unipolar signaling makes use of a positive (+) voltage and a null, or zero (0), voltage. Bipolar signaling, makes use of a positive (+) or a negative (–) voltage. Using a more complex signaling scheme, ISDN Basic Rate Interface (BRI) uses four voltage levels to signal two digital bits per baud.The transmitter creates the signal at a specific carrier frequency and for a specific duration (bit time), and the receiver monitors the signal to determine its state (+ or –).Various data transmission protocols employ different physical signal states and sequences, such as voltage level, voltage transition, or direction of the transition. Because of the discrete nature of each bit transmitted, the bit form is often referred to as a square wave. Digital signaling in an optical transmission system can involve either the pulsing on and off of a light source, or a discrete variation in the intensity of the light signal. Digital transmission over a radio transmission system (e.g., microwave, cellular, or satellite) can be accomplished by discretely varying the amplitude, frequency or phase of the signal. Digital signaling over optical and radio systems also requires careful synchronization of the transmitter and receiver in order to coordinate bit timing. Bandwidth, in digital terms, is measured in bits per second (bps). See also analog, bandwidth, binary, and sine wave. |
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