COS 116 The Computational Universe Laboratory 5: Digital Sound and Music In this lab you will learn about digital representations of sound and music, especially focusing on the role played by frequency (or pitch). You will learn about frequency spectra of sound and about digital sampling, which will give you a concrete insight into everyday technologies such as MP3 and music compression (crucial for Internet and the iPod). One experiment will give you an introduction to the challenges of automatic speech recognition. If you get stuck at any point, feel free to discuss the problem with another student or a TA. However, you are not allowed to copy another student’s answers. You should email your reports until Sunday, March 11th 11:59 pm. Include all the programs (Exp. 6 & 7) you wrote (use “File -> Save As…” in the Scribbler Control Panel to save a program, and “File -> Print” to print it). Briefly analyze how well each program worked, and write about any problems you needed to overcome. Include responses to questions printed in bold and your comments as appropriate for all the experiments (number them by Experiment and Step), as well as the Additional Questions at the end. Submission: Email the report as one pdf file per group to [email protected] There are many easy ways to create a pdf file, including “Save As” in MS Word. Subject of the email: Lab Report 5 Filename format: netId_Lab5_Report.pdf (put your Princeton netId in the filename, e.g. “schaudhu_Lab5_Report.pdf”) Before you begin: 1) You’ll need to download the following files: beethoven.wav flutesolo.wav grunge.wav octaves.sgp from http://www.cs.princeton.edu/courses/archive/spring08/cos116/lab4_files/. Save the files to any convenient location on your computer.COS 116 – Lab 4 2 2) Your TA will give you a headset/microphone at the beginning of the lab. This is not yours to keep; please return it when you are done with the lab. Plug the headset into the computer’s audio jacks, and turn up the volume on the computer. (The laptops in Friend 007 have two volume controls – the usual Windows icon on the taskbar, and also physical buttons on the laptops themselves. You may need to fiddle with both to get the volume right.) 3) Most experiments below will use an audio recording and manipulation program called Goldwave. Start Goldwave from the Start menu (the ‘Goldwave’ program in the ‘Goldwave’ group). - To record and replay sounds, use the buttons at the top of the Control window (on the right side of the screen). Click the record button (a red circle) to begin recording; when you are finished, click the stop button (a red square). To play the sound, click the green arrow button. - Whenever you want to create or record a new sound file, start by clicking the “New” button on the toolbar. When asked, use the following settings: 1 channel, 44100 Hz sampling rate, 10 seconds length. When you save a sound file, accept the default options for “Save as type” and “Attributes”, unless instructed otherwise. Experiment 1: Recording and Pitch — Meet your “twin” of the opposite sex If you had a twin of the opposite sex what would he or she sound like? 1. Click the New button to start a new song file. Click the record button to begin recording. Sing “Row, Row, Row Your Boat” for 10 seconds. Save the file as “song.wav”, and then listen to the result. Repeat if needed. 2. Click the Effect menu and click Pitch. If you are a man, increase the pitch by setting Scale to 150%; if you are a woman, decrease it by setting Scale to 70%. Be sure to check the box labeled “Preserve tempo.” 3. Play the altered sound and comment in your notes about what you hear.COS 116 – Lab 4 3 Experiment 2: Pure tones You learned in class that Sine waves are perceived by our ears as “pure tones.” Let us listen to them. 1. Start a new sound in GoldWave. (Click the New button on the toolbar.) 2. Open the Expression Evaluator (Tools menu, Expression Evaluator). The Expression Evaluator creates a sound from a mathematical function you enter. For instance, to create a sound at a single frequency, use the sine function. Enter sin(2*pi*f*t+pi/2), replacing f with the desired frequency. 3. Create a sine wave with a frequency of 1000 Hz, and save as “sine1000.wav”. Enter the expression sin(2*pi*1000*t+pi/2) and click OK. 4. Repeat 1-3 to create a sine wave with a frequency of 500 Hz and one with a frequency of 2000 Hz. Save these as “sine500.wav” and “sine2000.wav”. 5. Listen to all three sine waves, and comment on what you hear (or don't). What is the relationship between frequency and the sound of a tone? Open each in a separate window for easier comparison. 6. Try zooming in to the waves and compare them visually. GoldWave displays your recording as a graph. The x-axis represents time, and the y-axis represents the air pressure of the sound at each instant. This is called a “time-domain representation” of the sound. Press Shift+1 to view the waveform at full size (1 sample per screen pixel). Can you see distinct peaks and valleys in the graph? A sound at a frequency of 1000 Hz will have 1000 evenly spaced peaks (and 1000 valleys between them) over 1 second. 7. Synthesize a sound with two frequencies instead of just one. Use the expression (sin(2*pi*1000*t) + sin(2*pi*2000*t))/2 (the average of the expressions for the two individual sine waves). Such a wave is called a “superposition.” 8. Listen to your two-tone sound and predict what the two-tone wave will look like. Examine it by zooming in (Shift+1), and sketch a picture in your notes. 9. Use the Expression Evaluator to create a square wave with a frequency of 1000 Hz. Use the expression int(2*t*1000)%2*2-1. Save as “square1000.wav”. While a sine wave smoothly transitions between peaks and valleys, a square wave shifts instantaneously from maximum to minimum value. 10. View both “sine1000.wav” and “square1000.wav” at full size (Shift+1) and compare them visually. Listen to them and compare what you hear. Which sounds more like a “pure” tone to you?COS 116 – Lab 4 4 Experiment 3: Sampling Rates and Quantization Noise 1. Reopen the file “sine1000.wav” that you created in Experiment 2. 2.
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