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(comp342)[2010](s)midterm~cs_tcxaa^_10205.pdf
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Hong Kong University of Science and Technology
COMP342: Computer Music

Spring 2010
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
/5

2
/4

3
/12

4
/15

5
/14

6
/12

7
/6

8
/ 8

9
/ 16

10
/ 4

11
/ 4

Total
/100




1. Acoustics [5 marks]
a)
We have a string of length 20 cm producing a tone of 440 Hz. If we want a tone of 2200 Hz, should the string be shorter or longer? (1 mark)




Answer: Shorter

b)
How long should it be? Show your steps. (2 marks)




Answer: 20cm / 5 = 4cm

c)
What is the period of the waveform produced by the new string? Show your steps. (2 marks)




Answer: 1 / 2200Hz = 0.000454 s
(wrong or missing unit -0.5)

2. Waveforms [4 marks]
a)
Given that the sampling rate is 44100 Hz, what is the Nyquist frequency? (2 marks)




Answer:
22050 Hz

b)
A tone at 12000 Hz is above the Nyquist frequency but below the sampling rate. It aliases to sound like 6000 Hz. What is the sampling rate? (2 marks)




Answer:
sample rate is 18000 Hz

3. Pitch & Tempo [12 marks]
a)
Given that the frequency of the note E4 is 329 Hz, what are the frequencies of E5 and E2? (4 marks)




Answer:
E5 = 659 Hz (within 2 Hz is acceptable)
E2 = 82 Hz

b)
Given a Csound score consisting of 80 beats, please write a tempo statement that delivers a constant tempo and the resulting audio signal should last for exactly 50 seconds. (2 marks)




Answer:
t0 96 (zero marks for non-constant tempo)

c)
Given the following tempo statement,




t0 60 8 120

what are the instantaneous tempos at the 5th and 10th beat? (4 marks)

Answer:
5th beat = 97.5 (60 + (120 - 60) * (5 / 8), error of 2 Hz is acceptable)
10th beat = 120

d)
Write down a tempo statement which sustains a tempo of 60 bpm for 5 seconds and suddenly changes to 96 bpm at the 5th second. (2 marks)




Answer:
t0 60 5 60 5 96 (deduce 1 mark for 1 mistake)





4. Additive Synthesis [15 marks]
a)
If the fundamental frequency is 300 Hz, what is the frequency of the 3rd harmonic? (2 marks)




Answer:
900 Hz

b)
For an instrument tone, we know that 2 harmonic frequencies are 300 Hz and 500 Hz, but we do not know their harmonic numbers. What is the greatest possible fundamental frequency? (2 marks)




Answer:
100 Hz (GCD of 300 and 500)

c)
Fill in the blanks in the following Csound orchestra file which generate a sine wave with simple attack-decay amplitude envelope