|This article does not cite any references or sources. (August 2007)|
A digital signal is a type of continuous signal that is a representation of a sequence of discrete values (a quantified signal, for example of an arbitrary bit stream, or of a digitized (sampled and analog-to-digital converted) analog signal. Digital signals are often electronic, but may be optical or other forms.
Digital signals are present in all digital electronics, notably computing equipment and telecommunications.
Although digital signals are strictly a subset of analog signals, they are created and processed differently. With digital signals, system noise, provided it is not too great, will not change the quantification whereas with signals handled using analog processing, noise always degrades the operation to some degree.
In computer architecture and other digital systems, a waveform that switches between two voltage levels representing the two states of a Boolean value (0 and 1) is referred to as a digital signal, even though it is an analog voltage waveform, since it is interpreted in terms of only two levels.
The clock signal is a special digital signal that is used to synchronize many (but not all) digital circuits. The image shown can be considered the waveform of a clock signal. Logic changes are triggered either by the rising edge or the falling edge.
The given diagram is an example of the practical pulse and therefore we have introduced two new terms that are:
- Rising edge: the transition from a low voltage (level 1 in the diagram) to a high voltage (level 2).
- Falling edge: the transition from a high voltage to a low one.
Although in a highly simplified and idealized model of a digital circuit we may wish for these transitions to occur instantaneously, no real world circuit is purely resistive and therefore no circuit can instantly change voltage levels. This means that during a short, finite transition time the output may not properly reflect the input, and indeed may not correspond to either a logically high or low voltage.
Logic voltage levels
The two states of a wire are usually represented by some measurement of an electrical property: Voltage is the most common, but current is used in some logic families. A threshold is designed for each logic family. When below that threshold, the wire is "low", when above "high." Digital circuits establish a "no man's area" or "exclusion zone" that is wider than the tolerances of the components. The circuits avoid that area, in order to avoid indeterminate results.
It is usual to allow some tolerance in the voltage levels used; for example, 0 to 2 volts might represent logic 0, and 3 to 5 volts logic 1. A voltage of 2 to 3 volts would be invalid, and occur only in a fault condition or during a logic level transition. However, few logic circuits can detect such a condition and most devices will interpret the signal simply as high or low in an undefined or device-specific manner. Some logic devices incorporate schmitt trigger inputs whose behavior is much better defined in the threshold region, and have increased resilience to small variations in the input voltage.
The levels represent the binary integers or logic levels of 0 and 1. In active-high logic, "low" represents binary 0 and "high" represents binary 1. Active-low logic uses the reverse representation.
|Technology||L voltage||H voltage||Notes|
|CMOS||0 V to VDD/2||VDD/2 to VDD||VDD = supply voltage|
|TTL||0 V to 0.8 V||2 V to VCC||VCC is 4.75 V to 5.25 V|
|ECL||-1.175 V to VEE||0.75 V to 0 V||VEE is about -5.2 V. VCC=Ground|
To create a digital signal, an analog signal must be modulated with a control signal to produce it. As we have already seen, the simplest modulation, a type of line coding is simply to switch on and off a DC signal, so that high voltages are a '1' and low voltages are '0'.
In digital radio schemes one or more carrier waves are amplitude or frequency or phase modulated with a signal to produce a digital signal suitable for transmission. Similar methods are used in Asymmetric Digital Subscriber Line over telephone wires.