CSC 101: Lab #7 Digital Audio Due Date: 5:00pm, day after lab session

Transcription

1 CSC 101: Lab #7 Digital Audio Due Date: 5:00pm, day after lab session Purpose: The purpose of this lab is to provide you with hands-on experience in digital audio manipulation techniques using the Audacity editing tool. The techniques should reinforce some of the ideas previously seen in lecture, as well as add to your portfolio of digital experience (audio manipulation). RECORD YOUR ANSWERS TO THE QUESTIONS ON THE Lab 7 REPORT document handed out in class. Examining digital audio files You will first analyze a clip of the song Solace by Scott Joplin. This audio file was digitized from an analog recording (a vinyl record) and thus includes various faults and imperfections ( noise ) that are natural to playing records. Start Audacity (found under Start, All Programs, _WFU Digital Media Tools, Audio Editors) and load from your CSC101Lab/lab7 folder the file solace.wav. A sonogram should appear, similar to the one below, with sampling rate and bit depth information located in an information panel on the left hand side of the sonogram. Using your headphones (if you have them), turn up the volume on your laptop so you can hear the song played and press the Play (green arrow) button in Audacity. You should be able to hear various faults in the recording, which should sound familiar if you have experienced listening to vinyl records. Locate the Zoom In tool and click it repeatedly until you see individual dots (cross-hairs) on the waveform s sonogram. These dots are the individual quantized samples in the digital audio. The blue connecting lines are actually just for visualization, as the digital audio file only contains numbers that correspond to each sample. Question 1 (3 points): If we could count them, how many of these dots appear in Audacity s representation of ½ second of this file s sound? (You should not actually count how many dots exist within a window of half-a-second, but instead should come up with a different way of answering this question!) Locate the Zoom Out tool and click it repeatedly until you see the entire waveform again.

2 Perform a frequency analysis of the solace.wav file by using Edit, Select All to select the entire waveform, and then executing the Analyze, Plot Spectrum menu option to create a Frequency Analysis graph. This graph shows the relative amplitude (on the Y axis) of every frequency in the waveform (on the X axis). Question 2 (3 points): At what point do the frequencies disappear in the Frequency Analysis for this WAV file? A. Below 13 KHz B. Between 13 and 16 KHz C. Between 16 and 19 KHz D. Between 19 and 22KHz E. Above 22 KHz Now save this file in MP3 format by using the File, Export as MP3 menu option. Save the file as solace.mp3 in your lab7 folder. When a dialog box asks you to Edit the ID3 tags, just click OK to skip that step. If Audacity says you need to install the LAME Encoder for MP3s, please talk to one of the instructors to help you in that installation. In Windows (not in Audacity), open your lab7 folder and compare the file sizes of the two files: solace.wav and solace.mp3. Question 3 (3 points total): The file size of the mp3 file is what percentage of the file size of the original wav file? Now open the solace.mp3 file in Audacity, meaning you have two versions of the sound open the original WAV file and the MP3 file you just created. Listen to both files and see if you can tell any differences between the audio. Perform a new frequency analysis on the MP3 version of the file, using the same technique you used earlier (Select All of waveform; Analyze, Plot Spectrum). Compare this frequency analysis against that for the WAV file. Audacity won t let you have both Frequency Spectrum windows open at the same time, so you may have to alternate between windows and repeatedly apply the Analyze, Plot Spectrum menu option. Question 4 (3 points): There are very few frequencies above which point in this MP3 file? A. Above 2 KHz B. Above 7 KHz C. Above 10K D. Above 14 KHz E. Above 18 KHZ

3 Question 5 (6 points [2 each]): Consider the two frequency analyses you just performed. A. What part of the frequency spectrum shows the biggest difference between the two analyses? B. Suggest, given what you have learned in class, why the biggest difference is in the region of the frequency spectrum you highlighted in part A. C. Other than the differences you picked out in Part A, do the frequency analyses appear similar? Is your answer to be expected, or are you surprised by it? Removing noise The MP3 file has retained all of the noise that was in the original audio file, with noise defined as unwanted additional sounds. Question 6 (8 points [2 each]): There are two main kinds of noise in this recording. Describe briefly the two kinds of noise that you hear and any visual evidence in the sonogram that appears to relate to this noise. Make sure you understand the definition of noise provided above. Noise type 1: Visual evidence for noise type 1: Noise type 2: Visual evidence for noise type 2: Choose either the solace.wav or solace.mp3 window and then, to clean up noise, apply the following Audacity steps 1. Use your cursor to select just the first 0.5 seconds of the waveform (the flat part of the wave before the beginning of the music). 2. Execute Effect, Noise Removal and then press the Get Noise Profile button. This will sample information from the background of the audio file using your selected short region. 3. Select the entire waveform, and then execute the menu option Effect, Noise Removal and press the Remove Noise button. Keep the slider where it is (in the middle between Less and More). Keep your eye on the waveform while you execute the command you should see some visible changes.

4 4. Play the waveform and listen for differences from the original recording these should be easy to hear. Question 7 (6 points [2 each]): Consider the noise reduction you just performed. A. From listening to the song, was Audacity successful in removing all of the noise in the recording? If not, what did it not remove? B. What 2 visible changes in the waveform did you see when executing the Remove Noise command? C. Apply the Remove Noise command (Step 3 in the process on the previous page) again, but use the maximum setting on the filtering slider to increase the amount of noise removed. What affect does this have on the overall sound quality? In Audacity, use the File, Export as MP3 menu option to save this modified (noise-removed-twice) file as solace_nonoise.mp3 in your lab7 folder. Recording and editing an audio track You will now use Audacity to record and manipulate a sample of your own voice (or if you prefer, manipulate my voice). There is a built-in microphone on your laptop that looks like two small dots. On a T400, it is on the screen part of the computer, just above the blue ThinkVantage button. If you want to use my voice, download the edudotwfu.wav file from the class webpage into your lab7 folder and open it in Audacity. If you want to use your voice, record a sample of your voice as follow:. When ready, in Audacity press the red-circle Record button and then speak slowly and clearly, with a short pause between each letter, the following: e d u dot w f u. Make sure to leave space between each letter. Press the yellow square Stop button when done recording. Be sure to speak at a comfortable volume about a foot from your microphone. Play the file and make sure you said the letters in the appropriate order. Use Select (dragging with the mouse over the part of the waveform you want to manipulate), Cut, and Paste to rearrange the audio so that it says, when played, w f u dot e d u. (Hint: your final recording will sound more natural if you just rearrange the w f and the e d instead of w f u and e d u ). Play the file and make sure the letters are now spoken in the correct order. Feel free to cut out any periods of silence at the start or end of the audio. Finally, perform a Frequency Analysis of your voice recording using the same method as before. Question 8 (3 points): Based on the Frequency Analysis graph you just created, what is the approximate range of the most predominate (most commonly occurring) frequencies found in the human voice? A. Below approximately 9,000 Hz B. Between 10,000 and 15,000 Hz. C. Above 16,000 Hz.

5 Question 9 (3 points): Based on the Frequency Analysis graph you just created, do you believe the telephone system (which samples at 8000Hz) is capable of preserving the nuances of the human voice? Choose - Yes or No - and then indicate why or why not Use the same techniques as in the previous part of the lab (process at bottom of page 3) to remove noise from this voice recording (with the slider set between low-to-medium filtering). Now, for fun, you will change the pitch of the voice through audio manipulation. Select the entire waveform and then choose the Effect, Change Pitch menu item. In the dialog box that appears, change the pitch from C up to A and click OK. This requires choosing C and A in the select boxes and clicking the up radiobox. Play the sound again it should sound funny (similar to the Chipmunks ). Question 10 (3 points): The Change Pitch command caused the pitch of your voice to be higher, but without increasing the speed/tempo of the recording. From looking at the waveform (zoom in if necessary on one part of the sound), suggest what modification the software made to the waveform to make this pitch change without changing the tempo. You might want to alternate between Undo (Ctrl-Z on the keyboard) and Redo (Ctrl-Y) of the Change Pitch command multiple times to see what changes are being made. To modify your voice even further, select the entire waveform (zoom back out if necessary) and use the Effect, GVerb menu option to add reverb to your voice (accept the default settings and choose OK). Play this sound to see what it sounds like. Save your voice recording, completely modified (pitch-changed, noise-removed, GVerb added) in WAV format (not MP3) by choosing File, Export as WAV Save the file under the name voice.wav in your lab7 folder. Submission Submit your answers to these questions by 5:00pm, the day after your lab session. In today s lab, you created three audio files in your lab7 folder: solace.mp3 solace_nonoise.mp3 voice.wav Upload all three sound files to Sakai using the Assignments, Lab 7 link by the deadline listed. A grade-sheet specifying how the 80 points for this lab document are distributed is included here. The remaining 20 points for this lab come from the pre-lab.

6 CSC 101 Fall 2010 Lab #7 Report Gradesheet Topic Points Notes Lab-report questions 41 total solace.mp3: Correctly saved in mp3 format solace_nonoise.mp3: Noise removal has been performed (clear baseline in first ½ second) and performed again solace_nonoise.mp3: Correctly saved in mp3 format 5 pts 5 pts voice.wav: Says w f u dot e d u in correct order voice.wav: Pitch has been reduced without tempo change voice.wav: Reverb added voice.wav: Correctly saved in wav format 5 pts Total: of 80 for lab report