Multimedia Compression

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Multimedia Compression

• John Hall
• SMDC Training
• June, 2006

2
Concepts

• Lossy vs. Lossless Compression
• Container Format vs. Compression Format
• Constant vs. Variable Bit Rate
• Exposure to different compression and container
  formats

3
Examples

• JPEG Image Compression
• MPEG-1 Video Compression
• MPEG-2 Video Compression
• MPEG-4 Video Compression
• Containers (MP4, MOV, AVI)
• Audio Compression (AAC, MP3)

4
Demos

• Exploring Compression Artifacts
• Exporting from QuickTime Pro

5
Lossless Compression

• Run Length Encoding (RLE)
• aaaaaaabbbb
• 7a4b
• abababababa
• 1a1b1a1b1a1b1a1b1a1b1a
• Lossless compression relies on input being
  non-random to achieve compression.

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Lossy Compression

• Removes information
• Does so intelligently
• For media files, remove what is least noticeable
  by the senses.

7
Things Image/Video Compression Has Problems With

• Edges
• High Motion
• Transparency (Smoke, Shadow)
• Subtle gradations of color
• Small details
• Quick Cuts
• Fading In ./ Out

8
JPEG Compression

• Lossy (JPEG-2000 can be lossless)
• JFIF file format and JPEG compression
• 8x8 blocks matched tocombinations of 64 basic
  patterns
• Quality determineshow hard it tries to match

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JPEG Compression
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Video Compression

• Series of compressed images (JPEG)
• But can make smaller by compressing whats common
  between frames
• Only save the information thats changed between
  frames
• Match macroblocks to previous and possibly next
  frames

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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
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MPEG-1

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

I
P
B
I
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Things Image/Video Compression Has Problems With

• Edges
• High Motion
• Transparency (Smoke, Shadow)
• Subtle gradations of color
• Small details
• Quick Cuts
• Fading In ./ Out

14
MPEG-2

• Designed for DVD and Satellite TV
• Generally higher quality and larger than MPEG-1
• No free players
• Commecial disks are encrypted

15
MPEG-4

• Video compression is Part 2 of standard
• Used in podcasts
• Scalable - great for web at low quality
• Many parts to standard arent used
• Container / File is Part 10 of standard

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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)

17
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

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Container Formats

• Comparison of Container Formats

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Example - Quicktime Pro
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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

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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

22
MP3 File Format
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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/w
  iki/H.264/MPEG-4_AVC
• File Formats
• General info http//en.wikipedia.org/wiki/File_fo
  rmat
• 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

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Sources and Attribution

• DCT Basis Function Image http//en.wikipedia.org/
  wiki/ImageDctjpeg.png, GNU licensed
• JPEG Example Image http//en.wikipedia.org/wiki/I
  magePhalaenopsis_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