Analog and Digital Communications

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.

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In digital communications , a digital signal is a continuous-time physical signal, alternating between a discrete number of waveforms, [3] representing a bitstream. The shape of the waveform depends the transmission scheme, which may be either a line coding scheme allowing baseband transmission ; or a digital modulation scheme, allowing passband transmission over long wires or over a limited radio frequency band.

Such a carrier-modulated sine wave is considered a digital signal in literature on digital communications and data transmission, [9] but considered as a bitstream converted to an analog signal in electronics and computer networking. 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.

Analog & Digital Communications | Voice, Data, Prof. Services

The main advantages of digital signals for communications are often considered to be the noise immunity to noise capability, 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. A waveform that switches 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 two states 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. 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 rising edge is the transition from a low voltage level 1 in the diagram to a high voltage level 2.

The falling edge is 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.

Analog and Digital Communications

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. To create a digital signal, an analog signal must be modulated with a control signal to produce it. The simplest modulation, a type of unipolar encoding , is simply to switch on and off a DC signal, so that high voltages represent a '1' and low voltages are '0'.

In digital radio schemes one or more carrier waves are amplitude , frequency or phase modulated by the control signal to produce a digital signal suitable for transmission. Asymmetric Digital Subscriber Line ADSL over telephone wires , 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. Digital signals may be sampled by a clock signal at regular intervals by passing the signal through a flip-flop.

When this is done, the input is measured at the clock edge, and the signal from that time. The signal is then held steady until the next clock. This process is the basis of synchronous logic. 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. This is a transducer which takes a physical input and converts it to an electrical signal Example: This block also consists of an analog to digital converter where a digital signal is needed for further processes.

The source encoder compresses the data into minimum number of bits. This process helps in effective utilization of the bandwidth. It removes the redundant bits unnecessary excess bits, i.

The Necessity of Digitization

The channel encoder, does the coding for error correction. During the transmission of the signal, due to the noise in the channel, the signal may get altered and hence to avoid this, the channel encoder adds some redundant bits to the transmitted data. These are the error correcting bits. The signal to be transmitted is modulated here by a carrier. The signal is also converted to analog from the digital sequence, in order to make it travel through the channel or medium. The channel or a medium, allows the analog signal to transmit from the transmitter end to the receiver end. This is the first step at the receiver end.

The received signal is demodulated as well as converted again from analog to digital. The signal gets reconstructed here.

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The channel decoder, after detecting the sequence, does some error corrections. The distortions which might occur during the transmission, are corrected by adding some redundant bits. This addition of bits helps in the complete recovery of the original signal. The resultant signal is once again digitized by sampling and quantizing so that the pure digital output is obtained without the loss of information.

The source decoder recreates the source output.

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