Introduction to Analog and Digital System

Analog signal and Digital signals

Everything in real world is analog, the audio we hear, the scene we see in front of our eyes, the environment parameters like, humidity, temperature, atmospheric pressure and so on are all example of analog signals. In earlier time may be way back in 80’s and before, these analog quantity were measured and displayed in analog format. VCRs, tape players, and record players are analog devices. This is because they record data linearly from one point to another. Analog devices read the media, such as tapes or records, by scanning the physical data off the media. The data on the records are the exact and the real information.

Difference between analogue and digital systems

Therefore, due to enormous opportunities thrown to the engineers, the digital technology is now-a-days is felt in almost any electronic system.  The basic difference between analogue and digital systems are enumerated below:

Analog Systems
Digital Systems
Analog systems can include just passive components like resistors, capacitors, and inductors, transformers, and diodes, but more often include active components such as bipolar or field-effect transistors, operational amplifiers (op-amps), and power regulators.
Digital circuits use logic gates such as NOT, AND, OR, XOR, NAND, NOR, XNOR to perform operations defined by Boolean algebra
Analog systems provide such functions as filters, oscillators, amplifiers, modulation and demodulation, and power regulation operating on a variety of voltage levels, using continuously varying voltages.
. These gates can be combined into more complicated arrangements to form components such as flip-flops, counters, registers, multiplexers and demultiplexers, and arithmetic logic units (ALU’s).
Analog circuits are susceptible to electrical noise entering the system.
Digital circuits are nt (or very less) susceptible to electrical noise entering the system.
Most analog devices are packaged as discrete components, except for op-amps and power regulators which are integrated circuits.
All digital devices are packaged as Integrated circuit.

Opportunities in using Digital Mode

There are enormous opportunities offered by digital mode of operation:

  1. Resolution and accuracy: – A Quartz thermometer has a built-in quartz oscillator in the temperature probe, thus displaying temperature proportional to the frequency. It has an resolution of 0.0001oC and accuracy of 0.1 oC .
  2. Possibility of error control is another opportunity offered by digital technology.
    1. Data stored is in two state, take an example of the magnetic cores, the cores are either magnetized clockwise or anti-clockwise. We can represent the accuracy say fo 10 cores to 0.01% and with 20 cores to 0.0001%.
    2. Information in digital can be represented as sequence of 0’s and 1’s to any accuracy. For example a 1 is represented by a +ve pulse and a 0 by a –ve pulse. Any distortion is then irrelevant as long as the pulse are recognized at the reception as +ve or –ve .
    3. Unlike in analog systems, where data is stored as voltage on capacitor in an analog computer, the digital mode is not subject to slow deterioration of information as time passes.
  3. It provide automatic sequencing of operations is another powerful feature in digital mode
  4. Airplane cargo loading system, the system measures the weight exterted by the airplane n each wheel to provide gross weight and center of gravity of the airplane.
  5. Digital mode provide compatibility with desirable input and outputs
    1. Digital Display
    2. Stepper motor as an example of digital output. A motor having 800 steps can be given command every 833 micro-seconds to step in either direction
  6. Another reason of digital mode use is the availability of low cost, high-performance ICs. These ICs have the feature of mass production

Analog to Digital Conversion

Having, gone through the advantage and opportunities of digital mode of operation, the next thing to know is how the information is represented in analog and digital format.

Analog Signal:

In analog systems information is represented as a continuous waveform, i.e. information is present at each and every instant on the time line. There is no discontinuity of the signal as shown in figure-1.

Figure-2 : Analog Waveform

Digital Representation:

The analog waveform is converted to its digital equivalent by following the steps given below:

  1. Sampling:
  2. Quantization
  3. Coding


In 1933, Nyquist gave a sampling theory which state “if sampling is  done at a frequency greater than twice the signal frequency, then it is possible to  receive the original signal from the sampled signal by passing it through a low pass filter having a cut-off equal to the highest frequency in the original signal”

For a telephone system, frequency band used is 4 KHz i.e. between 300Hz to 3400Hz, Thus according t the sampling theorem, the sampling frequency must be   > 2 original signal frequency

                    Fs > 2 * 4KHz


And the sampling interval per frame = Fs/2   = 1/8K = 125 micro seconds


For ease of transmission this sampled signal is further processed i.e. converted into digital format in space. That is the sampled signal is quantized. In this process, each sample is compared to a standard scale of discrete values and is given a binary number representing its approximate amplitude. In general the quantization interval used is 256 (0 to +127 and -1 to -128), such a large quantization numbers reduces the signal distortion at the receiving side.

Figure-3: Quantising the continuous samples


Coding comes after quantization where we name the quanta formed during quantization. There could be number of ways of naming these quanta.

In digital systems, we represent all information in terms of binary or Boolean variable which can take on either of only two values. Physically these two values could be represented by :

  • Presence or the absence of a hole in punched paper tape
  • The clockwise or counterclockwise magnetization of a ferrite core in a computer memory
  • A voltage of +5V or 0V on a wire

In all these cases we shall represent the two values of a Boolean variable by the values 0 and 1. To code more quanta we use combinations of Boolean variable. For example two Boolean variables can represent four quanta; three Boolean variable represent up to 8 quanta

Two state devices

A transistor is an example of a two state operation when operated in fully ON and fully OFF state. When Vin is high, the transistor turns ON and the output V0 equals zero. When the ‘0’ is applied as Vin, the transistor does not turn ON and as result, the output Vo equal Vcc.


Figure-4: transistor as two state device


Thus we see that a transistor produces high output when a ‘0’ is applied at the input and produces a low output when a ‘1’ is applied at the input Vin. Thus we see that transistor is a good example of a device producing binary value. ‘0’ and ‘1’.


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