Multimedia Compression

John Hall

SMDC Training

June, 2006

Concepts

• Lossy vs. Lossless Compression

• Container Format vs. Compression Format

• Constant vs. Variable Bit Rate

• Exposure to different compression and container formats

Examples

• JPEG Image Compression

• MPEG-1 Video Compression

• MPEG-2 Video Compression

• MPEG-4 Video Compression

• Containers: (MP4, MOV, AVI)

• Audio Compression (AAC, MP3)

Demos

• Exploring Compression Artifacts

• Exporting from QuickTime Pro

Lossless Compression

• Run Length Encoding (RLE):– aaaaaaabbbb

• 7a4b

– abababababa• 1a1b1a1b1a1b1a1b1a1b1a

– Lossless compression relies on input being non-random to achieve compression.

Lossy Compression

• Removes information

• Does so “intelligently”

• For media files, remove what is least noticeable by the senses.

Things Image/Video Compression Has Problems

With• Edges

• High Motion

• Transparency (Smoke, Shadow)

• Subtle gradations of color

• Small details

• Quick Cuts

• Fading In ./ Out

JPEG Compression

• Lossy (JPEG-2000 can be lossless)• JFIF file format and JPEG compression• 8x8 blocks matched to

combinations of 64 basic patterns

• ‘Quality’ determineshow hard it tries to match

JPEG Compression

Video Compression

• Series of compressed images (JPEG)

• But can make smaller by compressing what’s common between frames

• Only save the information that’s changed between frames

• Match macroblocks to previous and possibly next frames

MPEG-1

• 3 frame types:– I frame - Independent, full image frame– P frame - depends on previous frame– B frame - depends on last and next frame

I P B I

MPEG-1

• Group of Pictures (GOP)– Pattern of I,P and B frames– Implications for editing– Implications for errors

I P B I

Things Image/Video Compression Has Problems

With• Edges

• High Motion

• Transparency (Smoke, Shadow)

• Subtle gradations of color

• Small details

• Quick Cuts

• Fading In ./ Out

MPEG-2

• Designed for DVD and Satellite TV

• Generally higher quality and larger than MPEG-1

• No free players

• Commecial disks are encrypted

MPEG-4

• Video compression is Part 2 of standard

• Used in podcasts

• Scalable - great for web at low quality

• Many parts to standard aren’t used

• Container / File is Part 10 of standard

When to use Which?

• MPEG2 - Must use for Video-DVD

• MPEG1 - If distributing on CD for older computers

• MPEG4 - For web downloading

• Real - For streaming (contact UMS)

Containers / Files• MP4, AVI and MOV are container & file

formats• They can contain a variety of audio and video

and specify the codecs to decode them• For instance:

– A MOV file can contain MPEG-4 video with AAC audio

• OR

– A MOV file can contain Sorenson video with MPEG-3 audio

• OR…

– MP4 (M4V) is MPEG-4 Part 14

Container Formats

• Comparison of Container Formats

Example - Quicktime Pro

Audio Compression

• Uncompressed audio is usually contained in a .wav or .aiff file

• MP3 and AAC are compressed

• MP3 is MPEG-1 Layer 3 audio, not MPEG-3

• AAC is MPEG-4 audio, better sounding for same size but not yet as universal

Bit Rate

• How much information is decoded per second

• MP3 typically 128Kbps - 192Kbps

• MPEG-4 Video: 64Kbps, 256Kbps…

• Quality depends on bit rate AND encoder quality

• Variable bit rate

MP3 File Format

For More Information• Image Compression

– JPEG: http://www.faqs.org/faqs/jpeg-faq/part1/– JPEG: http://en.wikipedia.org/wiki/JPEG

• Video Compression– MPEG-4 pt. 2: http://en.wikipedia.org/wiki/MPEG-4_Part_2– MPEG-4 pt. 10/H.264/AVC: http://en.wikipedia.org/wiki/H.264/MPEG-4_AVC

• File Formats– General info: http://en.wikipedia.org/wiki/File_format– Containers: http://en.wikipedia.org/wiki/Container_format– MPEG-4 pt. 14: http://en.wikipedia.org/wiki/.m4a– Codec list: http://www.fourcc.org/

• Audio– MP3: http://en.wikipedia.org/wiki/MP3

Sources and Attribution• DCT Basis Function Image: http://en.wikipedia.org/wiki/Image:Dctjpeg.png, GNU licensed• JPEG Example Image: http://en.wikipedia.org/wiki/Image:Phalaenopsis_JPEG.jpg , by Ilmari Karonon at

Wikipedia, Creative Commons Attribution-ShareAlike 2.5 License• MP3 File Image: http://en.wikipedia.org/wiki/Image:Mp3filestructure.jpg