MULTIMEDIA COMMUNICATION
INSTITUTO SUPERIOR TÉCNICO
Year 2012/2013 – 2nd
Semester, Responsible:
Prof. Fernando Pereira
1st Exam – 8th
June 2013 (Saturday), 9am
The marks should be out before 10th
June (Monday), 9am at the CMul Web page and the
exam checking session will on the 11th
June (Tuesday), 10am in room 0.73.
The exam is 3 hours long.
Answer all the questions in a detailed way, including all the computations
performed and justifying well your answers.
Don’t get ‘trapped’
by any question; move forward to another question and return later. Good luck !
Consider a facsimile transmission using the READ
coding method at 3200 bit/s for pages with 1000 lines, each line with 1728
samples. Consider also that, on average, 80% of the samples in each line are
white.
a) Assuming that
1. the unidimensionally coded lines have an average compression
factor of 10 for the back runs and 20 for the white runs
2. the bidimensionally coded lines have an average compression
factor of 20 for the back runs and 25 for the white runs
compute the global compression factor when a value of k equal to 3 is used to limit the
propagation of channel errors. (R: 20.83)
b) Assume now that there
is the need to increase the MSLT (Minimum Scan Line Time) due to receiver
limitations and this implies a reduction in 20% of the compression factors
stated above (this means the values are now 80% of the values above). For this
case, determine the maximum value of k
that may be used if one needs to obtain decoding resynchronization, on
average, at least once every 500 bits.
(R: 5)
c) In general, identify two advantages and one drawback of
using lossless source coding regarding lossy source
coding.
II (0.5 + 0.5 + 0.5 +
1.0 + 1.0 val. = 3.5 val.)
Consider the JPEG standard to code photographic
images with a 576×720 luminance resolution, 4:2:2 colour
subsampling and 8 bit/sample.
a) How many more luminance blocks than (total) chrominance
blocks exist in this type of images. (R: the same number)
b) Determine the average number of bits per pixel
(considering both the luminance and the chrominances)
that are spent when coding this type of image with a global compression factor
(for the luminance and the chrominances together) of
20. (R: 0.8 bit/pel)
c) Determine the total number of bits that have to
be spent to code an image if an average number of 4 DCT coefficients are coded per
block and each coefficient costs, on average, 3 bits for the luminance and 2
bits for the chrominance; additionally consider that the EOB (End of Block)
word costs 2 bits. (R:155520 bit)
d) Why is it reasonable
to say that the DCT representation involves a frequency interpretation of the image content?
e) Explain a mechanism allowing to exploit some
redundancy between neighboring blocks in JPEG coding ?
III (1.0 + 1.5 + 0.5
+ 0.5 = 3.5 val.)
Consider a videotelephony
communication using Recommendation ITU-T H.261. The video sequence is coded
with a CIF spatial resolution and a frame rate of 12.5 Hz at a rate of 128 kbit/s.
The encoder processes sequentially the macroblocks in the GOBs, and the bits are uniformly
generated in the time period that the encoder usually dedicates to encode each
image. At the encoder, the bits wait for transmission at the encoder output
buffer.
Knowing that the first image has used 15360
bits, the second image 20480 bit, and the third image 10240 bits, determine:
a) The time instants at which the encoder has
for sure generated all (coding) bits for the first, second and third images
considering the first image is acquired at time instant 0 s. (R: 80, 160 and 240
ms)
b) The minimum size of the encoder output buffer
in order all bits above are transmitted without problems. (R: 15360 bit)
c) The initial visualization delay associated
to the system defined in b). (R: 200 ms)
d) The maximum number of bits that the 5th image
may spent. (R: 20480 bit)
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. (R: number would become 2/3)
d) Describe two main technical differences between
the MPEG-1 Audio Layer 2 and Layer 3 codecs and the
corresponding advantages.
V (3 val.)
Consider using the MPEG-1 Video codec to code
CIF (396 macroblocks) video information at 25 Hz to
be stored in a CD. Assume that M=3 is used and the I get 3 times more bits than
the P frames while the P frames get 4 times more bits than the B frames (always
on average). The average number of bits per macroblock
in a B frame is 50.
For the conditions above, determine the acceptable
set of N values if it is requested that the video bitrate
does not exceed 1.8 Mbit/s and the maximum access
time does not exceed 400 ms.
Assume that the reading rate is the same as the coding rate. (R: N=6 and M=3)
VI (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 all transmitters send precisely 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 main 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 ?