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digfiltreview - ECE 421 Sum 2010 Notes Set 1 Digital...

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Unformatted text preview: ECE 421 - Sum 2010 Notes Set 1 : Digital Filtering 1 INTRODUCTION Many components of modern media systems and communication systems are implemented using digital signal processing (DSP). Among these systems are cell phones, audio CDs, video DVDs, digital television, digital imaging, etc. One of the common applications in many of these systems is digital filtering. Like analog filters, digital filters pass or reject specific frequencies, either to remove noise or to enhance information. There are many other DSP structures which are used in applications other than the tradi- tional filtering described above. Some examples are channel identification in high-speed internet access methods (like DSL and Wi-Fi), echo cancellation in long distance telephone networks, and low bit-rate speech coding for cell phones. We will study some of these ad- vanced applications in this course. However, digital filtering is perhaps easier to understand initially and we will continue here with this application. Digital filters operate on a set of numbers obtained from the voltage (or the current) of an underlying analog signal. The device which samples and digitizes the analog information signal is often called an Analog-to-Digital Converter (ADC). An actual ADC can only produce a finite number of bits representating the true analog amplitude. Additionally, the storage device can only store a finite number of bits for each sample. This is what is meant when we say the input is quantized to 8 bits or 16 bits. Quantization is an important topic in DSP and we will study this effect later in the course. To motivate our study of digital filters, we will study an application which is common in many digital audio, video, and instrumentation applications. Consider an original analog signal which has been corrupted with noise, and we desire to remove the noise component while retaining as much of the information content as we can. We will use a digital filter to remove this noise. At the output of the digital filter, a complementary device called a Digital-to-Analog Converter (DAC) then creates an analog signal corresponding to the digital output values. In this manner we have created a virtual analog filter using DSP techniques. We will discuss in class why this is often desirable. ECE 421 - Sum 2010 Notes Set 1 : Digital Filtering 2 BLOCK DIAGRAM To gain more understanding of digital filtering, lets consider in more detail the noisy acqui- sition process described on the previous page. Let the analog information signal be s t and let the additive noise be v t . This noise may be from physical processes unrelated to the signal, in which case we would like to filter out the noise before we store the information....
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This note was uploaded on 09/07/2010 for the course ECE 421 taught by Professor Hallen during the Summer '08 term at N.C. State.

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digfiltreview - ECE 421 Sum 2010 Notes Set 1 Digital...

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