Massachusetts Institute of Technology Department of Electrical Engineering and Computer Science 6 002 Electronic Circuits Fall 2004 Homework 11 Handout F04 053 Due 12 3 04 Introduction This homework assignment focuses on the analysis and design of a system for playing back a digitally stored audio signal Additionally this assignment serves as the pre lab exercises for Lab 4 which will involve the construction testing and demonstration of the audio playback system Consequently you should save a copy of your results for use during Lab 4 A block diagram of the audio playback system is shown in Figure 1 At the center of the system is a digital memory in which 131 072 samples of the audio signal are stored Each sample in the memory has a unique numerical address between 0 and 131 071 inclusive Consecutive samples are stored at consecutive addresses To obtain 131 072 consecutive samples of the audio signal 16 384 seconds of continuous analog audio signal are first sampled at an 8 kHz rate The analog audio samples are then digitized by an 8 bit analog to digital converter That is the samples are quantized to take on one of 256 possible discrete digital values between 0 and 255 inclusive Here the digital value of 0 corresponds to the most positive signal voltage and the digital value of 255 corresponds to the most negative signal voltage The resulting digital data is then written into the memory To retrieve the stored audio signal samples in sequence at the proper rate the memory is addressed by a counter which counts from 0 to 131 071 at an 8 kHz rate established by an external clock After counting to 131 071 the counter returns to 0 and the retrieval process repeats itself As the memory address increments the corresponding data appears at the memory output This data is converted back to an analog voltage in a piecewise constant manner by a digital to analog converter During the course of recording and playing back the analog audio signal the signal is sampled Headphone Clock Counter Memory D A Converter LowPass Filter Volume Control Figure 1 block diagram of the audio playback system in time quantized in amplitude and reconstructed in a piecewise constant manner As you will learn in 6 003 this process introduces undesirable high frequency components into the signal To minimize the perceived impact of these components the signal is filtered by a low pass filter after it is reconstructed by the digital to analog converter Finally the signal is fed into a volume control stage which in turn drives a headphone In the course of this homework assignment you will analyze and design four of the functional blocks shown in Figure 1 These blocks are the clock the digital to analog converter the low pass filter and the volume control In Lab 4 you will construct these blocks and verify that they perform as desired Then you will combine them with the counter the read only memory and the speaker to construct and demonstrate the entire audio play back system Since you will construct the system from the components in your 6 002 lab kit your design of the blocks must account for the fact that the available components are limited Problem 1 The Clock The circuit shown in Figure 2 is the system clock which is a square wave oscillator followed by a CMOS inverter the inverter functions only as a buffer The oscillator is constructed from another CMOS inverter a resistor and a capacitor Both inverters are powered between the positive supply voltage VS and ground and both exhibit the hysteretic input output characteristic defined in the figure The inverters are otherwise ideal A Assume that vCAP has just charged up to VH so that vOSC has just switched to 0 V How much time elapses before vCAP decays to VL which in turn causes vOSC to switch to VS B Assume that vCAP has just decayed to VL so that that vOSC has just switched to VS How much time elapses before vCAP charges up to VH which in turn causes vOSC to switch to 0 V C Determine the frequency of the oscillator in terms of R C VL VH and VS D Assume that VL 1 8 V VH 3 0 V and VS 5 0 V Choose values for R and C so that the oscillator oscillates at or very near 8 kHz Since oscillator frequency alone under specifies R R IN vCAP OUT C vOSC vCLK vOUT VS vIN VL Figure 2 the system clock VH VS and C there is no single correct choice Therefore choose values for R and C that are easily implemented with the components in the 6 002 lab kit E For the choice of R and C from Part D sketch and clearly label a single graph that displays vCAP vOSC and vCLK as a function of time over one period of oscillation Problem 2 The Digital To Analog Converter The circuit shown in Figure 3 is the digital to analog converter The voltage sources vDB0 through vDB7 represent the voltages supplied by the eight data bits of the digital memory DB0 through DB7 These voltages will be approximately 5 V when the corresponding data bit is a logical high and approximately 0 V when the corresponding data bit is a logical low The voltage vOFF which is set by a potentiometer is an offset voltage that is used to center the output of the converter around 0 V Assume that the op amp in the converter is ideal A Determine vDAC as a function of vDB0 through vDB7 and vOFF B With vOFF 0 V the output of the digital to analog converter should span the range of 0 V to 2 5 V Thus the output of the converter should be given by vDAC 2 5 V 7 X 2i i 0 255 DBi where each data bit DBi takes on the numerical value of 1 when high and 0 when low In this manner each successive data bit from DB0 to DB7 is given a voltage weighting twice that of the preceding data bit making it possible for the converter to output voltages from 0 V to 2 5 V in steps of 2 5 255 V Given this determine R2 in terms of R1 The voltage rating of the headphone is approximately 1 25 V Since the low pass filter and buffer between the converter and the speaker both have unity voltage gain over the frequency R1 2R1 2R1 2R1 vDB0 R1 R1 vDB1 2R1 2R1 vDB6 vDB7 2R1 R2 5V vOFF Figure 3 the digital to analog converter vDAC range of interest the output range of the analog to digital converter must be designed to match the headphone rating This is why the range is chosen to be 0 V to 2 5 V with vOFF 0 Note further that the output range of the converter is negative This is because the converter is based upon the inverting amplifier configuration C The role of vOFF is to offset the output of the digital to analog converter so that …
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