1. Chap 3: Encoding Video Content1

2. Learning Objectives
After completing this learning object, you should be able to:
Explain compression goals and characteristics.
Describe the technical architecture of the MPEG-2, H.264/AVC, and VC-1 compression standards. 2

3. Compression Goal
The goal of video compression is to reduce the quantity of data used to represent video content without substantially reducing the quality of the picture.
Digitization
Compression
Decode
NTSC/PAL
Encode
/ /
Transport
Compressed
Digital Bitstream
Uncompressed
Analog Video
Sequence
Film or
Video
Camera
Analog TV
Digital

4. About MPEG An acronym for Moving Pictures Experts Group
Widely used by satellite, cable, and terrestrial TV systems
The group has produced a family of major compression standard
MPEG Format
Description
MPEG-1
The MPEG-1 file format was originally developed in 1988 and was primarily used to compress video data at bit rates of 1.5 Mbps. MPEG-1 content is used for such services as DAB (Digital Audio Broadcasting) and is the standard format on the Internet for quality video. MPEG-1 is also the basis of the MP3 standard, which is widely used for music on the Internet.
MPEG-2
MPEG-2 builds on the powerful video compression capabilities of the MPEG-1 standard. MPEG-2 is widely used in the delivery of broadcast-quality television and storing video content on DVDs. A number of international television standards are based on this compression format.
MPEG-4 (Part 2)
MPEG-4, whose formal ISO/IEC designation is ISO/IEC 14496, was finalized in October 1998 and became an international standard in Part 2 of the standard is divided into a number of profiles that address the requirements of various video applications ranging from mobile phones to surveillance cameras.
MPEG-4 Part 10
MPEG-4 Part 10 also called H.264/AVC has been designed to deliver broadcast and DVD-quality video at minimum data rates.
Also produced specifications for describing A/V content, delivery, and consumption:
MPEG-7
MPEG-21 4

5. Compression Key Concept – Remove Redundancy
Compression Algorithms are able to reduce the size of a video bit stream significantly because video typically contains duplicate or redundant information both within and between frames.
Image and video compression algorithms can take advantage of two primary types of redundancy to reduce the size of the resulting bit-stream.
Spatial Redundancy
Temporal Redundancy
Spatial compression works on 1 image – Temporal compression works on several images 5

6. Spatial Compression - Overview
Spatial Compression (intra-frame compression):
Spatial compression is applied to a single picture
Spatially compressed pictures are called I Frames or I Slices
Spatial method is performed by taking advantage of the fact that the human eye is unable to distinguish small differences in color.
Neighboring pixels in an image often have similar values: color or brightness of an object typically does not vary significantly over small areas.
Instead of encoding each pixel individually, a compression algorithm could save bits by encoding only the difference between neighboring pixels.
That difference is typically a smaller value than the full range of possible pixel values and therefore can be encoded with fewer bits. 6

7. Temporal Compression:Intro
Temporal compression refers to the bit reduction between successive frames
Typical frame rate: 25 or 30 frames/sec
Change between two adjacent frames: 1/25 or 1/30 seconds
In most case most of the picture is unchanged, while only parts of the picture are moving. There is an obvious redundancy between frames.
Temporal compression only encodes the small changes or the direction that a part of the image moved between frames.
These changes typically require fewer bits then representing the whole image again.
Temporal compression 7

8. MPEG-2 Frame Types: I Frames
I-Frame Characteristics:
Starting point for a sequence
Undergo minimal compression
Independently encoded as a single image
No reference to any past or future frames
Encoding scheme used is similar to JPEG compression (.25 bits represents a single pixel, whereas 2.5 bits per pixel for higher quality)
Self contained and used as a foundation to build other types of frames
Effectively a JPEG image
I frames are typically large ( 00’s to 000’s of IP packets per frame)
I frame is aproximately 64,000 bytes
One I-frame will occur approximately every 0.4 seconds of video runtime.
More I-Frames make an MPEG stream ‘more editable’. I B B P B 8

9. MPEG-2 Frame Types: P Frames
Forward predicted frames (P-frames) Characteristics:
Based on past “I” or “P” frames
Moderately compressed
Not actually an encoded image
Contains motion information (vectors) that allows the IPTVCD to rebuild the frame
P-Frames require less bandwidth than I-Frames (Important for IPTV Deployments)
P-frames are typically much smaller than I-Frames (0’s to 00’s of packets per frame) . 9

10. MPEG-2 Frame Types: B-Frames
Bi-directional predicted frames (B-frames) Characteristics:
Made up from information from both past and future “I” frames and “P” frames
Encoding for B-frames is similar to P-frames, except that B-frames can specify two motion vectors (one to past and one to future)
Extensive compression - B-frames occupy less space than I-Frames or P-Frames - (0’s to 00’s of packets per frame)
A stream containing a high-density of B-frames requires less bandwidth compared to a digital stream built with a high density of I & P frames.
When frame dropping occurs, B-frames get discarded first because they have the lowest impact on video quality, compared to I & P frames. 10

11. Group of Pictures (GOPs)
GOP Characteristics:
I, P & B images are combined to form a sequence of picture frames called a GOP.
Each GOP must begin with a full reference I-Frame - mitigates propagation of errors to one GOP.
All frames depend on the contents of the I-Frame
GOPs vary in size – Average GOP for IPTV deployments is between 12 and 15 frames in length. Some GOP configurations can however include 250 frames.
There are typically between 10 and 12 P- and B-frames occurring between each I-frame.
[I B B B P B B B P B B B P B B B P] 2 4 6 8 10 I B P
Relative amounts of data for each
frame type in a typical MPEG GOP 11

12. MPEG-2 Shortcomings
Although MPEG-2 has served the cable and satellite industries well for the past decade it has shortcomings when deployed on networks that have limited bandwidth capacities:
A telephone network was not designed to carry MPEG coded video.
New advanced compression schemes with better capabilities have been developed in recent years for the purpose of delivering video content over bandwidth constrained networks:
MPEG-4 Part 10 AVC/H.264
Microsoft’s Windows Media Video (also known as VC-1) 12

13. Example MPEG-4 Compression
A farmer herding cattle.
The scene can be decomposed into a number of objects:
The field and houses in the background
The sky
The farmer, son, and cattle walking along the road.
The farmer’s voice
Noises emanating from the cows
H.264/AVC treats each one of these objects separately.
Compression is applied to each object. 13

14. Others
VC-1
Standardized by the Society of Motion Picture and Television Engineers (SMPTE)
Most high profile implementations has been its adoption by Microsoft’s Windows Media Video (WMV) 9 multimedia coding platform
A number of other international standards including the high-definition DVD formats HD-DVD and Blu-ray have also adopted VC-1.
AVS
China has developed a standard called AVS.
Efficiency levels achieved by this standard are quite similar to the performance levels achieved by MPEG-4 Part 10.
Also covers areas such as digital copyright and content management 14