Chpt.8 A-D and D-A Converters

Chpt.8 A-D and D-A Converters - Chapter 8 Analog-to-Digital...

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1 Chapter 8 Analog-to-Digital and Digital-to-Analog Conversion
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2 Introduction So far, we have studied both digital and analogues transmission and detection principles. Nowadays, communication systems are gradually moving towards digital. However, many digital systems, such as phones and TV, are actually used to deliver analogue signals. (After all, we human beings can only see and listen analogue signals. Can you read a “digital book” written in a digital language such as “0001000111010101000010001000”?) Analogue-to-digital (or A-D) conversion is an important subject in modern digital communication systems. However, A-D conversion has now been broadly categorized as a topic in “signal processing” that will be covered in much depth in an independent module. Therefore, in this course, our discussion on A-D conversion will be very brief. Our focus is mainly on the impact of A- D conversion on system bandwidth requirement. We will make the following two main observations. High precision (i.e., high quality) A-D conversion generally leads to high bandwidth requirement. Some techniques can be applied to improve the trade-off between quality and bandwidth cost.
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3 A-D Conversion and PCM A-D conversion is usually the first operation of a digital system. It involves the process of transforming analogue signals into digital ones. A typical digital signal is of the form of binary sequences. It is normally represented by electronic pulses. Hence it is sometimes called Pulse Coded Modulation (PCM). If the source signal is already digitized, such as the signal from a computer, then this operation is unnecessary. Key difficulty : The analogue signal on the left can have an infinite number of possibilities. The digital signal on the right has only a finite number of combinations. (How many possibilities can be represented by K bits? ) This indicates that we have to deign the mapping between them with great care. analog signal digital signal 0001010…. A/D conversion t t
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4 Two Main Steps in PCM The following is an illustration of the operations related to PCM at the transmitter and receiver sides. (1) Sampling: In this step, a continuous time analogue signal is transformed into a discrete time signal. This step prepares a good flat top stable signal for the quantization step below. (2) Quantization: In this step, a discrete time signal is transformed into a serious of binary bits. Normally distortion is introduced in this step referred to as “quantization noise”. The cause of “quantization noise” is due to the fact that it is impossible to represent a signal with infinite accuracy using a finite number of bits. Reconstruction and filtering are, respectively, the corresponding “opposite” operations of quantization and sampling. t t 00010101001111010110 … t Step 1 Step 2 sampling quantization reconstruction filtering transmitter side receiver side
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5 The Sampling Process In a digital system, we can only transmit a finite number of bits. However, we may need to use these bits to represent a continues signal, which has an infinite number of possibilities. A natural way to handle this is to describe the
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This note was uploaded on 01/11/2011 for the course EE 3008 taught by Professor Pingli during the Fall '08 term at City University of Hong Kong.

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Chpt.8 A-D and D-A Converters - Chapter 8 Analog-to-Digital...

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