Transcript Tutorial # 4 - WordPress.com

Tutorial # 4
IT 342 : Fundamentals of Multimedia
Q: My old SoundBlaster card is an 8–bit card.
(a)
(b)
What is it 8 bits of?
What is the best SQNR (Signal to Quantization Noise
Ratio) it can achieve?
Answer :
Quantization levels (not sampling frequency)
Best SQNR is 1 level out of 256 possible levels.
Calculate SQNR using largest value in dynamic range:
SQNR= 20 log_10 (255/2ˆ0 )
˜= 20 log 2ˆ8
= 20*8*log 2
˜= 20*8* 0.3
= 48 db (actually, 48.16 db)
(a)
(b)
Tutorial # 5
IT 342 : Fundamentals of Multimedia

Q 1: Thinking about my large collection of JPEG images
(of my family taken in various locales), I decide to unify
them and make them more accessible by simply combining
them into a big H.261-compressed file. My reasoning is that
I can simply use a viewer to step through the file, making a
cohesive whole out of my collection.
Comment on the utility of this idea, in terms of the
compression ratio achievable for the set of images.
Answer :
This will not achieve a good compression, since no temporal
redundancy is available. And it may be worse, since extra
header information is required.
Q 2: In block-based video coding, what takes more effort:
compression or decompression? Briefly explain Why .
Answer :
Compression. The encoder needs to do Motion Compensation
(generate the motion vectors) which is time-consuming.
Q 3: As we know, MPEG video compression uses I-, P-, and
B-frames. However, the earlier H.261 standard
does not use B-frames. Describe a situation in which video
compression would not be as effective without B-frames.
(Your answer should be different from the one in Fig. 11.1.)
Answer :
Besides occlusion, the following could also call for bi-directional
search: lighting (color and/or intensity) changes, changing
views of 3D shape and/or texture, etc.
Q 4: Suggest an explanation for the reason the default
quantization table Q2 for inter-frames is all constant,
as opposed to the default quantization table Q1 of intraframes.
Answer :
 Intra-frames (I-frames) are basically coded as images. As in
JPEG, larger values are used in the lower right part of Q1. As
a result, a better compression can be achieved by introducing
more losses to higher spatial frequency components.
 For Inter-frames, it is the difference image (not the video
frame itself) that is transform coded and quantized. So the
reason for using non-constants in Q2 is not as compelling.
Indeed, it can be argued that the high-frequency changes in the
difference image are as important as the low-frequency ones.