Hossam FadeelNational Telecommunication Institute

2010-2011

Introduction to Digital Signal Processing

Objectives List the essential elements in a digital signal

processing system Explain how analog signals are converted to digital

form Discuss the purpose of filtering Describe the sampling process State the purpose of analog-to-digital conversion Explain how several types of ADCs operate Explain the basic concepts of a digital signal

processor (DSP) Describe the basic architecture of a DSP Name some of the functions that a DSP performs State the purpose of digital-to-analog conversion Explain how DACs operate

Lecture Sessions

3

Digital Signal Processing Basics

Converting Analog Signals to Digital

Analog-to-Digital Conversion Methods

The Digital Signal Processor (DSP)

Digital-to-Analog Conversion Methods

Introduction Digital signal processing is a powerful technology that is widely

used in many applications, such as automotive, consumer, graphics/imaging, industrial, instrumentation, medical, military, telecommunications, and voice/speech applications.

Digital signal processing incorporates mathematics, software programming, and processing hardware to manipulate analog signals.

This presentation provide a brief look at digital signal processing. To completely cover the topic in depth a list of references is available at the last slide.

Much information, including datasheets about DSPs can be found on Manufacturer websites like: Texas Instruments: www.ti.com Motorola: www.motorola.com Analog Devices: www.analogdevices.com

Section 1:DIGITAL SIGNAL PROCESSING BASICS

DIGITAL SIGNAL PROCESSING BASICS Digital signal processing converts signals that

naturally occur in analog form, such as sound, video, and information from sensors, to digital form and uses digital techniques to enhance and modify analog signal data for various applications.

A digital signal processing system first translates a continuously varying analog signal into a series of discrete levels. This series of levels follows the variations of the analog signal and resembles a staircase, as illustrated for the case of a sine wave in Figure-1. The process of changing the original analog signal to a "stairstep" approximation is accomplished by a sample-and-hold circuit.

DIGITAL SIGNAL PROCESSING BASICS

Next, the "stairstep" approximation is quantized into binary codes that represent each discrete step on the "stairsteps" by a process called analog-to-digital (A/D) conversion. Once the analog signal has been converted to a binary coded form, it is applied to a DSP (digital signal processor).

DIGITAL SIGNAL PROCESSING BASICS The DSP can perform various operations on the

incoming data, such as: removing unwanted interference increasing the amplitude of some signal frequencies

and reducing others encoding the data for secure transmissions, and

detecting and correcting errors in transmitted codes. After a DSP processes a signal, the signal can be

converted back to a much improved version of the original analog signal.

This is accomplished by a digital-to-analog converter (DAC).

DIGITAL SIGNAL PROCESSING BASICS

Anti-aliasing

filter

Sample and Hold

circuitADC

DSP

DACReconstruction filter

Figure-2 Basic block diagram of a typical digital signal processing system.

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DIGITAL SIGNAL PROCESSING BASICS DSPs are actually a specialized type of

microprocessor but are different from general-purpose microprocessors in a couple of significant ways. Typically, microprocessors are designed for

general-purpose functions and operate with large software packages.

DSPs are used for special-purpose applications; they are very fast number crunchers that must work in real time by processing information as it happens using specialized algorithms (programs).

Review What does DSP stand for? What does ADC stand for? What does DAC stand for?

Section 2:CONVERTING ANALOG SIGNALS TO DIGITAL

CONVERTING ANALOG SIGNALS TO DIGITAL In order to process signals using digital

techniques, the incoming analog signal must be converted into digital form. Sampling and Filtering

Sampling is the process of taking a sufficient number of discrete values at points on a waveform that will define the shape of waveform. The more samples you take, the more accurately you can define a waveform. Sampling converts an analog signal into a series of impulses, each representing the amplitude of the signal at a given instant in time. Before a signal can be sampled, it must be passed through a low-pass filter (anti-aliasing filter) to eliminate harmonic frequencies above a certain value as determined by the Nyquist frequency.

CONVERTING ANALOG SIGNALS TO DIGITAL

The Sampling Theorem The sampling theorem states that, in order to represent an analog signal, the sampling frequency, fsample, must be at least twice the highest frequency component fa(max) of the analog signal. The frequency fa(max) is known as the Nyquist frequency and is expressed in

Equation -1.

In practice, the sampling frequency should be more than twice the highest analog frequency.

CONVERTING ANALOG SIGNALS TO DIGITAL

Figure -3 illustrates the process of sampling.

CONVERTING ANALOG SIGNALS TO DIGITAL The Need for Filtering Low-pass filtering is

necessary. An alias is a signal produced when the

sampling frequency is not at least twice the signal frequency.

If the analog signal contains frequencies above the Nyquist frequency, these frequencies overlap into the spectrum of the sample waveform as shown and interference occurs.

CONVERTING ANALOG SIGNALS TO DIGITAL To avoid an aliasing error, the filter must at least eliminate

all analog frequencies above the minimum frequency in the sampling spectrum, as illustrated in Figure -5.

Aliasing can also be avoided by sufficiently increasing the sampling frequency. However, the maximum sampling frequency is usually limited by the performance of the analog-to-digital converter (ADC) that follows it.

CONVERTING ANALOG SIGNALS TO DIGITAL

Holding the Sampled Value The holding operation is part of the sample-and-hold block shown in Figure -2. After filtering and sampling, the sampled level must be held constant until the next sample occurs. This is necessary for the ADC to have time to process the sampled value.

CONVERTING ANALOG SIGNALS TO DIGITAL

Analog-to-digital conversion is the process of converting the output of the sample-and- hold circuit to a series of binary codes that represent the amplitude of the analog input at each of the sample times.

Analog-to-Digital Conversion

CONVERTING ANALOG SIGNALS TO DIGITAL The process of converting an analog value to a code is called

quantization. During the quantization process, the ADC converts each sampled

value of the analog signal to a binary code. The more bits that are used to represent a sampled value, the more

accurate is the representation.

Quantization

CONVERTING ANALOG SIGNALS TO DIGITAL If the resulting 2-bit digital codes are used to reconstruct the

original waveform, which is done by digital-to-analog converters (DAC), you would get the waveform shown in Figure -9. As you can see, quite a bit of accuracy is lost using only two bits to represent the sampled values.

CONVERTING ANALOG SIGNALS TO DIGITAL

CONVERTING ANALOG SIGNALS TO DIGITAL

Review What does sampling mean? Why must you hold a sampled

value? If the highest frequency

component in an analog signal is 20 kHz, what is the minimum sample frequency?

What does quantization mean? What determines the accuracy

of the quantization process?

Section 3:ANALOG-TO-DIGITAL CONVERSION METHODS

ANALOG-TO-DIGITAL CONVERSION METHODS Analog-to-digital conversion is the process by

which an analog quantity is converted to digital form.

It is necessary when measured quantities must be in digital form for processing or for display or storage.

Two important ADC parameters are resolution, and throughput.Resolution:

which is the number of bits

Throughput:is the sampling rate an ADC can handle in units of samples per second (sps).

ANALOG-TO-DIGITAL CONVERSION METHODS

Flash (Simultaneous) Analog-to-Digital Converter The flash method utilizes comparators that compare reference

voltages with the analog in-put voltage. When the input voltage exceeds the reference voltage for a given

comparator, a HIGH is generated. In general, 2n - 1 comparators are required for conversion to an n-bit

binary code. The number of bits used in an ADC is its resolution. The large number of comparators necessary for a reasonable-sized

binary number is one of the disadvantages of the Dash ADC. Its chief advantage is that it provides a fast conversion time

because of a high throughput. The reference voltage for each comparator is set by the resistive

voltage-divider circuit. The output of each comparator is connected to an input of the priority encoder.

The encoder is enabled by a pulse on the EN input. and a 3-bit code representing the value of the input appears on the encoder's outputs. The binary code is determined by the highest-order input having a HIGH level.

Section 4: THE DIGITAL SIGNAL PROCESSOR (DSP)

THE DIGITAL SIGNAL PROCESSOR (DSP) Essentially, a digital signal processor (DSP) is a special

type of microprocessor that processes data in real time.

Its applications focus on the processing of digital data that represents analog signals.

A DSP, like a microprocessor, has a central processing unit (CPU) and memory units in addition to many interfacing functions.

The digital signal processor (DSP) is the heart of a digital signal processing system.

THE DIGITAL SIGNAL PROCESSOR (DSP)

Sound cards used in computers use an ADC to convert sound from a microphone, audio CD player, or other source into a digital signal.

The ADC sends the digital signal to a digital signal processor (DSP). Based on instructions from a ROM, one function of the DSP is to

compress the digital signal so it uses less storage space. The DSP then sends the compressed data to the computer's

processor which, in turn, sends the data to a hard drive or CD ROM for storage.

To playa recorded sound, the stored data is retrieved by the processor and sent to the DSP where it is decompressed and sent to a DAC.

The output of the DAC I which is a reproduction of the original sound signal, is applied to the speakers.

COMPUTER NOTE

THE DIGITAL SIGNAL PROCESSOR (DSP) DSP Programming

DSPs are typically programmed in either assembly language or in C. assembly language is used much more in DSPs than in general-purpose microprocessors. DSP programs are usually much shorter than traditional microprocessor programs because of their very specialized applicationsDSP Applications The DSP, unlike the general-purpose microprocessor, must typically process data in real time; that is, as it happens. Many applications in which DSPs are used cannot tolerate any noticeable delays, requiring the DSP to be extremely fast. In addition to cell phones, digital signal processors (DSPs) are used in multimedia computers, video recorders, CD players, and disk drivers, digital radio modems, and other applications to improve the signal quality. Also, DSPs are becoming more common in television applications.

THE DIGITAL SIGNAL PROCESSOR (DSP)

Telecommunications Speech Generation and Recognition Radar Image Processing Filtering

DSP Applications

References Dahnoun, Nairn. Digital Signal Processing Implementation Using the

TMS320C6000 DSP Hayes, Monson. Schaum's Outline of Digital Signal Processing. Kuo, Sen, and Bob Lee. Real-Time Digital Signal Processing:

Implementations. Applications. and Experiments with the TMS320C55x. Lyons, Richard. Understanding Digital Signal Processing. Marven, Craig, and Gillian Ewers. A Simple Approach to Digital Signal

Processing. New York: John Wiley & Sons. 1996. Oppenheim, Alan, and Ronald Schafer. Digital Signal Processing. Orfanidis. Sophocles. introduction to Signal Processing. Proakis, John, and Dimitris Manolakis. Digital Signal Processing:

Principles, Algorithms, and Applications, 3d ed. Steiglitz, Ken. Digital Signal Processing Primer: With Applications to

Digital Audio and Computer Music. Williams, Douglas, and Vijay Madisetti. Digital Signal Processing

Handbook.

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