Binary vs digital signaling
This value is referred to as the period and it has units of seconds. There is a second method of measuring the periodic waveform and it is directly related to the period. This measurement is called frequency and it has units of cycles per seconds , also referred to as Hertz Hz. To convert the measurement of time for a period to the measurement of frequency in hertz, simply invert the period. If it takes 0. The last measurement of a periodic waveform is the duty cycle.
The duty cycle represents the percentage of time that a periodic signal is a logic '1'. Somewhere in the middle is where most periodic signals fall. The measurements for the period and the pulse duration are represented with T and t h respectively as shown in the figure below. No signal is truly digital.
A close examination of a digital signal reveals gradual transitions between logic 1 and logic 0 and vice versa. There are many ways to represent a digital signal over a period of time. The figure below represents a single line a single switch with only two possible values, logic 1 and logic 0. The area between the horizontal hash marks on the rising and falling edges of the signal represent the period where the signal is undefined and in transition. Sometimes, digital lines are grouped together to perform a single function.
This circumstance may be represented with figures such as the one below. Alternatively, these multiple lines can be combined into a more abstract representation such as the one below. Each of these symbols has one or more inputs lines coming in from the left side and one output line exiting from the right. The symbols can be added together to create complex circuits.
For example, adding a small circle to an input or an output of a logic symbol is identical to adding an inverter at that input or output. Logic 1's or logic 0's are sent into the inputs of these symbols, and by definition, a specific logic 1 or logic 0 is expected on the output. The following section describes which symbols have which outputs based on a certain set of inputs.
Now, we need a method to represent how to combine two or more binary signals to produce an output or function. In general, a truth table shows the relationship between columns of inputs and their associated outputs. For example, the "NOT" gate shown above has one input and one output. Therefore, there is one column of inputs and one column of outputs. For single input, there are exactly two possible states: Therefore, there will be two rows of data for the NOT truth table.
That table is shown below. Otherwise the output is always a logic 0. With two inputs and one output, the AND gate's truth table will have two columns for inputs and one column for outputs. It is shown below. Note that if input A is a logic 0, it doesn't matter what input B is, the output is always 0. Similarly, if input B is 0, A's state does not matter. These situations are called "don't cares" and are represented by the symbol X.
Using don't cares, we can redo the truth table for the AND gate. The OR gate's output is set to logic 1 if either of the inputs are 1. It is 0 ONLY when both inputs are 0. Its truth table is shown below. The XOR gate's output is set to logic 1 if the inputs are different and 0 when the inputs are the same. In a digital signal, the physical quantity representing the information may be a variable electric current or voltage, the intensity, phase or polarization of an optical or other electromagnetic field , acoustic pressure, the magnetization of a magnetic storage media, etcetera.
Digital signals are used in all digital electronics , notably computing equipment and data transmission. In digital electronics a digital signal is a pulse train a pulse amplitude modulated signal , i.
In digital signal processing , a digital signal is a representation of a physical signal that is a sampled and quantized. A digital signal is an abstraction which is discrete in time and amplitude. The signal's value only exists at regular time intervals, since only the values of the corresponding physical signal at those sampled moments are significant for further digital processing.
The digital signal is a sequence of codes drawn from a finite set of values. In digital communications , a digital signal is a continuous-time physical signal, alternating between a discrete number of waveforms,  representing a bit stream message.
The shape of the waveform depends the transmission scheme, which may be either:. In communications, sources of interference are usually present, and noise is frequently a significant problem. The effects of interference are typically minimized by filtering off interfering signals as much as possible and by using data redundancy. The main advantages of digital signals for communications are often considered to be the immunity to noise that it may be possible to provide, and the ability, in many cases such as with audio and video data, to use data compression to greatly decrease the bandwidth that is required on the communication media.
In computer architecture and other digital systems, a waveform that switches between two voltage levels or less commonly, other waveforms representing the two states of a Boolean value 0 and 1, or Low and High, or false and true is referred to as a digital signal or logic signal or binary signal when it is interpreted in terms of only two possible digits. The clock signal is a special digital signal that is used to synchronize many 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:. 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 will not correspond to either a logically high or low voltage.
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 signal is low , when above high. 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 unipolar 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.
In Asymmetric Digital Subscriber Line over telephone wires , ADSL does not primarily use binary logic; the digital signals for individual carriers are modulated with different valued logics, depending on the Shannon capacity of the individual channel.
Often digital signals are "sampled" by a clock signal at regular intervals by passing the signal through an "edge sensitive" flip-flop. When this is done the input is measured at those points in time, and the signal from that time is passed through to the output and the output is then held steady till the next clock. This process is the basis of synchronous logic , and the system is also used in digital signal processing.
However, asynchronous logic also exists, which uses no single clock, and generally operates more quickly, and may use less power, but is significantly harder to design. From Wikipedia, the free encyclopedia.