1. Multimedia Communications EG 371 and EG 348
Dr Matthew Roach
Lecture 4
Compression &formats cont.
Sell yourself big time to the people.
Did my PhD in the speech and image group Swansea.
I also held a post here as a senior research assistant working on a BT sponsored project. Looking at classification of video some of the work is included in this course.
For the last 2 years I have worked for a small systems engineering consultancy full time for the last year. In that time I’ve co-delivered training courses to a diverse range of companies, including Motorola, MoD, IEE, BSI.
Get in contact with me via-
Multimedia communications EG-371 Dr Matt Roach

2. Lossless: Huffman compression
reduces average code length
to represent symbols of an alphabet
occur frequently
short length codes
constructing a binary tree
arranging the symbols
adding two lowest probabilities
Sum of last two symbols is 1.
Code words formed tracing tree path
assigning 0s and 1s to the branches
Multimedia communications EG-371 Dr Matt Roach

3. Multimedia communications EG-371 Dr Matt Roach
Huffman coding
Determine the Huffman code for the following set of symbols:
Step 1 – List symbols in order of decreasing probability
Symbol m0 m1 m2 m3 m4
Probability
0.10
0.36
0.15
0.20
0.19
Symbol m1 m3 m4 m2 m0
Probability
0.36
0.20
0.19
0.15
0.10
Multimedia communications EG-371 Dr Matt Roach

4. Multimedia communications EG-371 Dr Matt Roach
Step 2 – Get 2 symbols with lowest probability. Give the combined symbol a new name:
m2(0.15) + m0(0.1.0) A(0.25)
Step 3 – Create a new list and repeat the process:
Symbol m1 A m3 m4
Probability
0.36
0.25
0.20
0.19
Symbol B m1 A
Probability
0.39
0.36
0.25
Symbol B C
Probability
0.39
0.61
Symbol D
Probability
1.0
Multimedia communications EG-371 Dr Matt Roach

5. An alternative approach is to construct this treem m
A 0.25 m m m
B 0.39
C 0.61
D 1.0
Multimedia communications EG-371 Dr Matt Roach

6. Multimedia communications EG-371 Dr Matt Roach
Assign bits (0,1) to the tree branches
Codewords determined by tracing
the path from root node to
symbol leaf: 1 B
0.39 C
0.61 m1
0.36 A
0.25 m2
0.15 m4
0.19 m3
0.20
Root m0
0.10
Symbol
Probability
Codewords m0
0.10
011 m1
0.36 00 m2
0.15
010 m3
0.20 10 m4
0.19 11
Compression is achieved by allocating frequently occurring symbols
with shorter codewords.
Multimedia communications EG-371 Dr Matt Roach

7. Multimedia communications EG-371 Dr Matt Roach
How much compression?
5 symbols
3-bits for each symbol.
message [m0m1m2m3m4]
require 5x3=15 bits.
Huffman coding
require 12 bits
compression of 15:12
Multimedia communications EG-371 Dr Matt Roach

8. Multimedia communications EG-371 Dr Matt Roach
Example
Consider the message
babbage babble baggage label bagel
Construct a Huffman code and determine the compression ratio.
Multimedia communications EG-371 Dr Matt Roach

9. Multimedia communications EG-371 Dr Matt Roach
Solution
construct a probability table
counting occurrence of each letters
Letter
Occurrence
Probability b 9
0.3 a 7
0.233 g 5
0.166 e l 4
0.133
Total: 30 1
Multimedia communications EG-371 Dr Matt Roach

10. Multimedia communications EG-371 Dr Matt Roach
Solution
b 0.3
b 0.3
B 0.4
C 0.6
D 1.0 a
A 0.3
b 0.3
B 0.4 g a
A 0.3 e g
Multimedia communications EG-371 Dr Matt Roach

11. Multimedia communications EG-371 Dr Matt Roach
Solution
Root 1
Symbol
Probabilities
Codewords b
0.4 00 a
0.233 10 g
0.166 11 e
010 l
0.133
011 C
0.6 B
0.4 1 1 b
0.3 A
0.3 a
0.233 g
0.166 1 e
0.166 l
0.133
Multimedia communications EG-371 Dr Matt Roach

12. Solution babbage babble baggage label bagel 5 symbols (bagel)
30 characters
uncompressed
3 bits/symbol
90 bits.
Huffman gives a compression of approx. 9:7
Letter
Occurrence
Codewords
No. bits b 9 00 18 a 7 10 14 g 5 11 e
010 15 l 4
011 12
Total bits: 69
Multimedia communications EG-371 Dr Matt Roach

13. Multimedia Communications EG 371 and EG 348
Dr Matthew Roach
Lecture 4
Formats cont.
Sell yourself big time to the people.
Did my PhD in the speech and image group Swansea.
I also held a post here as a senior research assistant working on a BT sponsored project. Looking at classification of video some of the work is included in this course.
For the last 2 years I have worked for a small systems engineering consultancy full time for the last year. In that time I’ve co-delivered training courses to a diverse range of companies, including Motorola, MoD, IEE, BSI.
Get in contact with me via-
Multimedia communications EG-371 Dr Matt Roach

14. Common Container Formats
AVI (.avi)
M-JPEG, DivX, nearly any format (not Sorenson).
Quicktime
Locked Apple Sorenson codec, or for Cinepak (free), also mjpeg
WMV (.wmv)
MPEG4; nearly any codec, Microsoft spinoffs of MPEG-4
ASF ("Advanced Streaming Format", .asf)
a subset of wmv, intended primarily for streaming: an early Microsoft implementation of an MPEG4 codec.
Multimedia communications EG-371 Dr Matt Roach

15. Multimedia communications EG-371 Dr Matt Roach
Common Codecs
MPEG-1
Old, supported by everything (at least up to 352x240), reasonably efficient.
A good format for the web
Video quality is not as crisp as MPEG-2
Small file size
Good picture quality
Compressed format
Require special playback program
Cannot Edit
MPEG-2
A souped-up version of MPEG-1, with better compression. 720x480. Used in HDTV, DVD, and SVCD.
Good Quality, Can burn onto DVD disc
Large file size, 4.7GB for 2 hours of video
MPEG-4
A family of codecs, some of which are open, others Microsoft proprietary.
MJPEG ("Motion JPEG")
A codec consisting of a stream of JPEG images. Common in video from digital cameras, but it doesn't compress well, so it's not good for web distribution.
Multimedia communications EG-371 Dr Matt Roach

16. Multimedia communications EG-371 Dr Matt Roach
Common Codecs cont.
WMV ("Windows Media Video")
A collection of Microsoft proprietary video codecs.
Since version 7, it has used a special version of MPEG4.
Small file size
Good picture quality
Ideal for web transmission
Compressed format
Cannot Edit
RM ("Real Media")
a closed codec developed by Real Networks for streaming video and audio. Maybe also a container?
DivX
incomplete early MPEG-4 codec inside an AVI container; DivX 4 and later are a more full MPEG-4 codec.. No resolution limit. Requires more horsepower to play than mpeg1, but less than mpeg2.
Hard to find mac and windows players.
Good Quality with reasonably small file size
Not a standard video format
Cannot produce video onto DVD or CD
Multimedia communications EG-371 Dr Matt Roach

17. Multimedia communications EG-371 Dr Matt Roach
Common Codecs cont.
DV ("Digital Video")
Usually used for video grabbed via firewire off a video camera. Fixed at 29.97FPS, or 25 FPS. Not very highly compressed.
Superb quality, record back to DV tape
Large file size, 25GB for 60 min of video
Sorenson 3:
Apple's proprietary codec, commonly used for distributing movie trailers (inside a quicktime container).
Quicktime 6:
Apple's implementation of an MPEG4 codec.
Good picture quality
Ideal for web transmission
Larger file size (compare to other streamable formats)
Cannot Edit
Multimedia communications EG-371 Dr Matt Roach