IT 342 : Fundamentals of Multimedia Tutorial # 4 IT 342 : Fundamentals of Multimedia

Q: My old SoundBlaster card is an 8–bit card. What is it 8 bits of? What is the best SQNR (Signal to Quantization Noise Ratio) it can achieve?

Quantization levels (not sampling frequency) 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)

IT 342 : Fundamentals of Multimedia 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 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 intra- frames.

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.