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(comp342)[2009](s)midterm~cs_tcxaa^_10204.pdf
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Hong Kong University of Science and Technology
COMP342: Computer Music

Spring 2009
Midterm Examination

Student Name: ____________________Key_______________________________

Student ID: __________________________________________________________

Instructions:
1.
This is a closed-book, closed-notes examination.


2.
Answer all questions in the space provided.


3.
Write your name and student ID on this page.






Question
Score

1
/4

2
/5

3
/2

4
/5

5
/8

6
/8

7
/4

8
/10

9
/4

10
/ 14

11
/ 20

12
/ 11

13
/ 5

Total
/100




1. Acoustics [4 marks]
a)
Plunking a string of length L produces a tone of 220 Hz. The string is then cut into 2 pieces. The shorter one is 0.25 L in length and the longer other is 0.75 L in length. What are the frequencies produced by plunking the strings individually? (2 marks)

Choose the BEST answer.

i.
Shorter: 440 Hz; Longer: 293 Hz


ii.
Shorter: 440 Hz; Longer: 330 Hz


iii.
Shorter: 880 Hz; Longer: 293 Hz


iv.
Shorter: 880 Hz; Longer: 660 Hz

Answer: __________


Answer: iii
b)
What is the period of a waveform with frequency 2000 Hz? (1 mark)

Answer: 0.0005 s


c)
What is the frequency of the 3rd harmonic given that the fundamental frequency is 262 Hz? (1 mark)

Answer: 786 Hz




2. Waveforms [5 marks]
a)
What is the side effect of digitization on a signal? (1 mark)

Answer:
It introduces noise and limits the frequency range.


b)
What is the advantage of a higher sampling rate? (2 marks)

Answer:
Waveforms can be represented more accurately.



c)
A tone at 16000 Hz is above the Nyquist frequency and below the sampling rate and it aliases to sound like 4000 Hz. What are the sampling rate and the Nyquist frequency? (2 marks)

Answer: sampling rate is 20000 Hz; Nyquist frequency is 10000 Hz







3. Additive Synthesis [2 marks]
a)
What is the principle behind additive synthesis? (2 marks)

Answer:
Any waveform can be expressed as a linear combination (weighted sum) of sine (periodic) waves.





4. Wavetable Synthesis [5 marks]
a)
What is the advantage of using piecewise linear approximation to simplify an amplitude envelope? Please explain briefly. (3 marks)

Answer:
It reduces the number of parameters and computation for evaluating the envelope because it interpolates the envelope by approximation points.


b)
What advantage does wavetable synthesis have over additive synthesis on computation time? Explain briefly. (2 marks)

Answer:
Since wavetable synthesis only does computation for one period and stores for later look up, it spends less computation time than additive synthesis. Also additive synthesis does computation for every period. (Or any meaningful and correct answers)




5.