Digital to Analogue & Analogue to Digital Converters
The distinctive interface encoders and decoders which are used between analogue and digital devices are known as digital-to-analogue (D/A) and analogue-to-digital (A/D) converters. Digital to analogue and analogue to digital conversion are two important aspects of digital data processing. Digital to analogue conversion (D/A) means the conversion of digital information to equivalent analogue information. As such, a digital to analogue converter is also sometimes known as a decoding device (i.e. decoder). On the contrary, operation for the conversion of an analogue signal into an equivalent digital signal is known as analogue to digital (A/D) conversion. And the device which is used for this purpose, is called an A/D converter. As an A/D converter performs the functions of encoding an analogue signal to a digital signal, so that this signal can penetrate into some digital system, therefore an encoding device is also often known as encoder.
We know that all information within a real-world is available in an analogue form. For example, time, speed, weight, pressure, light intensity, and position or measurement of any item etc. As all these quantities exiting in the real world are available in analogue form, therefore as long as these quantities or signals are not converted to digital quantities or signals, until then a digital system cannot be processed through these quantities or signals (i.e. a digital system cannot be processed directly through these naturally available analogue quantities). Therefore, an imminent need to compulsorily convert these analogue signals or quantities to digital signals or quantities always exists. However, it is also worth reminding that in order to make digital signals or quantities which are received through a processing unit or system, more comprehensible, a digital to analogue (D/A) converter is essentially mounted along its output. In short, in order to operate a digital system through real quantities, it is absolutely mandatory to convert quantities into some digital form through an A/D encoder. Similarly, in order to harmonize quantities received from a digital system with real quantities, it also necessary to bring them back to their original state by means of a D/A converter (i.e. decoder).
In diagram 10.1, block diagram of a digital system consisting of an analogue input and analogue output has been illustrated. An analogue input has been provided on the input of this digital system, which consists of continuously fluctuating 0 – 3 voltages. A special device A/D converter or encoder has been set between digital unit and analogue input, which converts analogue signals or data information to digital signals or data information.
Figure 10.1 – A digital system with analogue input and analogue output
After the digital processing unit, a decoder has also been mounted, as has illustrated by the figure. This decoder converts digital information received from the digital processing unit to analogue output (i.e. output changing consistently between 0 -3 volts). As this device decodes digital information in an analague form, that’s why it is a called a decoder. A decoder is also known as a digital to analogue converter or simply a D/A converter.
The complete system shown in the diagram is called a hybrid system, because it consists of both type of analogue as well as digital devices. The encoders and decoders existing in the system, which convert analogue to digital and digital to analogue, are known as interface devices in engineers and technicians’ jargon. The word interface is normally used for such devices or circuits which convert one mode operation to some other mode operation. Here, we have to convert analogue to digital and digital to analogue. It is obvious from the input block of block diagram of a digital system shown in figure 10.1 that it consists of an analogue voltage, which has a range between 0 volt to 3 volt. These voltages can also be produced through a transducer. Transducer is a device which converts one type of energy to some other type of energy. For example, a photo cell can also be used as an input transducer, because when light transmits on it, it produces voltages proportionate to the light intensity. In this example, light energy has been converting into electrical energy through a photo cell (remember that energy can never be exhausted or depleted, however it is possible to change it from one state to another). Similarly, a heater can also be used as a transducer. When electrical energy is provided on a heater, it generates heat energy proportionate to that heat. In this example, electrical energy converts to heat or thermal energy through a heater. Microphones, speakers, strain gauge, photo resistive cells, temperature sensors and potentiometers etc. are some examples of transducer.
For the purpose of interfacing between digital and analogue words, two basic processes like A/D conversion and D/A conversion, are being required. This point can be elaborated through a CD player’s example. In figure 10.2, block diagram of a CD (compact disk) player has been given. An audio signal in pit form is digitally recorded on CD. When this CD is inserted into a computer’s CD ROM or any CD player, layers pickup head senses this signal and transmits it to amplifier. This point has been illustrated in the block diagram. Amplifier tends to amplify this signal (i.e. boosts this signal). This amplified signal is transmitted on digital signal’s processor in the form of pulses, which converts it into an array or series of binary codes or digital codes. These binary codes have actually been reflecting this audio signal, which has digitally been recorded on the CD. After this, the signal consisting of digital codes is transmitted on digital to analogue converter (DAC) which amplifies it and provides on output speakers, which we can listen quite comfortably.
Figure 10.2 – Basic block diagram of a CD player
We can easily understand A/D and D/A conversion through a digital the function of an audio tape (DAT) player or recorder. In diagram 10.3, fundamental block diagram of a DAT system has been illustrated, according to which when audio signal (which is in an analogue form) is applied on a DAT system in a record mode, it is received on analogue to digital converter (ADC) via an amplifier set on the input side. ADC converts this analogue signal to a digital signal. The digital codes transmitted from ADC output have actually been reflecting these audio signals, which are received by a processor after conversion into a digital form. Processor tape records it after processing. During the play mode, these recorded signals, which have been digitally recorded, are read via a tape through this system (as has been exemplified via above – mentioned example). Then processor decodes or process these signals and transmit it on digital to analogue (DAC) converter. This converter again converts these signals to analogue form and transmit it on amplifiers, which amplifies it and transmits it on speaker system.
Figure 10.3 – Basic block diagram of a DAT system
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