CM3106
CARDIFF UNIVERSITY
EXAMINATION PAPER
Academic Year:
2013/2014
Examination Period:
Autumn
Examination Paper Number: CM3106
Examination Paper Title:
Multimedia
Duration:
2 hours
Do not turn this page over until instructed to do so by the Senior Invigilator.
Structure of Examination Paper:
There are 4 pages.
There are 4 questions in total.
There are no appendices.
The maximum mark for the examination paper is 75 and the mark obtainable for a question
or part of a question is shown in brackets alongside the question.
Students to be provided with:
The following items of stationery are to be provided:
ONE answer book.
Instructions to Students:
Answer 3 questions.
The use of calculators is permitted in this examination.
The use of translation dictionaries between English or Welsh and a foreign language bearing
an appropriate departmental stamp is permitted in this examination.
1
PLEASE TURN OVER
CM3106
Q1.
(a) State Nyquist’s Sampling Theorem.
[2]
(b) What general considerations affect the selection of the sampling rate in multimedia data?
[3]
(c) For each of the following media types: audio, graphics, images and video, briefly
discuss how sampling affects the quality of the data, the cause of sampling artefacts, and the form in which they manifest themselves in each data modality. [20]
Q2.
(a) What is the difference between reverb and echo?
[2]
(b) Describe two filter based approaches to simulating the reverb effect in digital
audio, explaining how one approach builds on the other and how filters are used
to achieve the desired effect.
[8]
(c) Describe, briefly how Convolution Reverb is implemented. What the is fundamental theorem that underpins this approach?
[5]
(d) A new audio application requires that reverb be simulated as would be heard at
a precise location within an acoustic space. This location must be allowed to
vary and will be user-defined. Describe how this may efficiently be implemented
via Convolution Reverb. Clearly state what challenges this approach presents for
standard convolution reverb approaches and outline how your solution addresses
such problems.
[6]
(e) An audio engineer has been presented with a rare audio recording at an established concert hall. The concert hall is still in existence but the performer is not.
The audio engineer has been tasked to create a new piece of music incorporating this recording with a new studio recorded backing. The problem is that the
reverberation of the concert hall recording does not match the reverb of the new
recording. How may the audio engineer achieve a better match to the reverb
audio characteristics of the two recordings?
[4]
2
CM3106
Q3.
(a) Explain briefly what motion compensation is used for in MPEG video
compression.
[2]
(b) Assume 2 × 2 macroblock is used for motion compensation. For the following
macroblock
# # # #
# 5 7 #
# 4 5 #
# # # #
the corresponding intensities in the reference frame are given as follows:
1 4 6 7
2 5 3 7
1 2 4 8
5 2 4 4
Calculate the motion vector, with complete search within a ±1 pixel search window. List the steps to obtain the result. Having computed the motion vector,
determine the the macroblock to be coded after motion compensation.
[7]
(c) What is the key difference between I-Frames, P-Frames and B-Frames?
[3]
(d) Explain briefly why JPEG compression is not always suitable for compression
of images that contain sharp edges or abrupt changes of intensity (such as black
text on a white background).
[4]
(e) Consider the following block of frequency domain values from a video frame
arising during MPEG compression:
196
1
11
75
207
7
73
78
1
129
73
139
129
199
194
135
Apply successively to this block: (1) MPEG quantisation using a constant quantisation value of 64. (2) Zig-zag scanning. (3) Run length encoding.
[9]
3
PLEASE TURN OVER
CM3106
Q4.
(a) Explain why lossy data compression is sometimes preferred over lossless.
[2]
(b) Consider the following DNA fragment:
...GTACCCGACACTTCCGTCCCCTTC...
Assume that the frequencies of symbols in the rest of the sequence are the same
as in this fragment. Estimate the probabilities of each symbol {A, G, T, C} and
hence derive the Huffman code for each. Estimate the average number of bits
per symbol required to encode the sequence using Huffman code under these
circumstances.
[8]
(c) What advantage does arithmetic coding offer over Huffman coding for data compression?
[3]
(d) Given the following string as input:
ABRACADABRA
with the initial dictionary below, encode the sequence with the LZW algorithm,
showing the intermediate steps.
[12]
Index
1
2
3
4
5
Entry
A
B
C
D
R
4X
END OF EXAMINATION