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COMUNICAÇÃO
DE ÁUDIO E VÍDEO
INSTITUTO
SUPERIOR TÉCNICO
Year 2011/2012 - 2º
Semester, Responsible: Prof. Fernando Pereira
2nd Exam – 29th
June 2012 (Friday)
MEEC: The marks should be out
before 2nd July (Monday) at
12am at the CAV Web page and the exam checking session will on the 2nd July (Monday) at 5pm in
room LT4.
MERC: The marks should be out
before 2nd July (Monday) at
12am at the CAV Web page and the exam checking session will on the 2nd July (Monday) at 2pm in
room 0.15.
The
exam is 3 hours long. Answer
all the questions in a detailed way, notably by including all the computations
made and justifying in detail your answers.
Don’t get ‘trapped’ by any
question; move forward to another question and return later. Good luck !
I (1 + 0.5 + 1 = 2.5 val.)
Consider an analogue TV system with the
following characteristics:
•
Number of ‘image
elements’ per line:760; Kell factor: 0.7; Form
factor: 16/9
•
Total duration of a
line: 64 μs; Frame rate: 25 Hz
•
Useful duration of a
line scanning: 0.92; Useful percentage of lines per image: 0.90
a) Using only the values
above, and without making any specific
further assumption, determine the minimum bandwidth (base band) needed for
the luminance signal. (R: 6.454 MHz)
b) Determine the total number of lines in an image for the system above (still without
further assumptions). (R: 625)
c) Provide and explain two main reasons justifying the
adoption of the Vestigial Side Band modulation for the luminance signal.
II (1 + 1 + 1 = 3 val.)
Consider the JPEG standard to code photographic
images.
a) Determine the compression factors that you would need
to achieve for the luminance and for the chrominances
to spend an average number of 0.64 bit/pixel (considering both the luminance
and the chrominances) when coding a 4:2:0 image with
8 bit/sample, knowing that the average luminance compression factor is twice
the average chrominances compression factor. (R: 15; 12.5)
b) Determine the total number of bits that have to be
spent to code a 720×576, 4:2:2, 8 bit/sample image if an average number of
3 DCT coefficients are coded per block and each coefficient costs, on average,
4 bits; additionally consider that the EOB (End of Block) word costs 2 bits and all blocks in the image spend bits. (R: 181440
bit/image)
c) Consider a 720×576,
4:2:2, 8 bit/sample image coded with the hierarchical mode. How many layers can we use to code the
image if the base layer global (luminance and chrominances)
compression factor is 20, the global compression factor doubles for each new
layer, each new layer has twice the resolution in both directions, and the
total number of bits spent should be less than 106 bits. (R: 2)
III (2 + 1.5 + 1 = 4.5 val.)
Suppose that you have been contacted by a
company to design a videoconference solution to work between the various
EURO’2012 stadiums using the lowest possible bitrate
while guaranteeing the necessary minimum video quality. The company also
requires that the initial visualization delay (measured as the maximum time
difference between corresponding acquisition and visualization instants) is
below 300 ms. The video resolution is CIF (352×288
luminance samples), 4:2:0 at 12.5 Hz with 8 bit/sample. Assume that you have
available and providing the necessary minimum video quality two systems:
1.
H.261 system with
average compression factors of 18 and 22 for the luminance and chrominances, respectively, and critical compression
factors (for the most difficult images) of 12 and 15 for the luminance and chrominances, respectively.
2.
MPEG-1 Video system
with N = M = 3 and average compression factors of
a.
18 and 22 for the
luminance and chrominances, respectively, in the I
frames
b.
25 and 35 for the
luminance and chrominances, respectively, in the B
and P frames
The critical compression factors are 75 % of the
average compression factors for all frame types.
a) Determine which of the solutions above you would
select to better satisfy the needs of your client assuming that the
transmission rate for each solution correspond to its coding rate. (R: H.261)
b) Determine for which transmission bitrates the
MPEG-1 solution would satisfy the initial visualization delay requirement,
assuming that for this solution the transmission rate may be regulated (meaning
that the coding and transmission rate would be different). (R: > 1.024 Mbit/s)
c) Explain 2 performance impacts of increasing the
value of M, this means increasing the number of B frames between two anchor
frames.
IV (1 + 1 + 0.5 + 1 = 3.5 val.)
Consider the MPEG-1 Audio standard to code
audio content with 22 kHz bandwidth; assume reasonable compression factors and
the most usual number of bits per sample.
a) How many complete stereo music pieces, with a duration of 4 minutes, can we store in a 900 MBytes disk using the Layer 3 of the MPEG-1 Audio standard
to code the music content with a transparent quality regarding CD music
content. (R: 255)
b) What is the maximum duration of each music piece that we can afford
if we want to store 1000 musics in the same disk as
above using a Layer 2 MPEG-1 Audio codec? (R: 40.91 s)
c) Explain how would the maximum
number of stored musics vary if we
increase the audio bandwidth three times but the audio becomes mono and not
anymore stereo.
d) Describe two main technical differences between
the MPEG-1 Audio Layer 2 and Layer 3 codecs and the
corresponding advantages.
V (1 + 1 + 0.5 + 0.5 + 0.5 = 3.5
val.)
Consider a DVB digital TV system.
a) Knowing that a DVB
solution may ‘insert’ 10 Mbit/s of total bitrate in a 8 MHz bandwidth channel, determine what would
be the source bitrate
that may be ‘inserted’ if all the system parameters stay the same with the
exception of the channel coding ratio that goes from ½ to 1/3 and the
modulation that goes from 8-PSK to 64-QAM. (R: 6.666 Mbit/s)
b) Why is it essential
in a Single Frequency Network that the transmitters send the same data and do
that well synchronized to transmit the same symbol at precisely the same time ? How do the transmitters obtain the necessary time reference ?
c) What are the two main components of the channel coding solution in DVB-x2 ?
d) What parameter can be used to tune the correction capability
of the channel coding solution and what does this parameter express
?
e) What is the main reason justifying the availability
of two channel coding block lengths in DVB-x2 ?
VI (1 + 0.5 + 0.5 + 0.5 + 0.5 = 3 val.)
Consider a 3D video system.
a) Explain what is a frame compatible stereo format. Also explain the
difference between a spatial multiplexing and a time multiplexing frame
compatible stereo format.
b) Explain why
frame-compatible stereo video tends to have higher spatial frequency content
characteristics.
c) What is the most
important new feature/tool of the Multiview Video
Coding (MVC) standard regarding the H.264/AVC standard ?
How does it work ?
d) What is the
implication of the ‘backward compatibility’ requirement for the MVC standard ?
e) If the backward
compatible view in a MVC stereo pair spends 2 Mbit/s,
what is the minimum rate that the second view has to spend if a perceptual
quality similar to stereo simulcasting with 2+2 Mbit/s had to be achieved ? Why?
(R: 500 kbit/s)