Everything you ever wanted to know about a Codec

1
Everything you ever wanted to know about a Codec

• But were afraid to ask????

2
Its software

• In simplest form, a codec is a software algorithm
  that transforms data from one format to another
• Stands for coder-decoder or compressor-de-compress
  or
• Codec used to compress file must be the same used
  to decompress file

3
Two main types

• Lossless codecs the original can be recreated
  exactly for the compressed file
• Lossy codecs decompressed file is an
  approximation of the original file. How
  approximate depends on amount of compression used

4
Many file formats use codecs

• JPEG images - lossy
• GIF images - lossy
• ZIP files - lossless
• StuffIt archives - lossless

5
How lossless codecs work

• Original files can be recreated exactly
• Takes advantage of redundant information
  reprenting more efficiently
• AAAAAAAA becomes 8A reducing storage by 75
• Huffman coding replaces frequently occurring
  sequences with shorter code
• Hyper Text Markup Language becomes HTML.

6

• When de-compressed HTML becomes Hyper Text Markup
  Language
• Great for quality concerns but minimal
  compression ratios.
• Would not work well for Streaming Media, for
  example.

7
Lossy codecs

• GIF
• JPEG
• They are only approximations of the original image

8

• Achieve much higher rates of file size reduction
  by discarding data that is deemed unnecessary or
  redundant.
• Since information is discarded, recreating the
  original file exactly is not possible when file
  is de-compressed.
• Ideally, some data can be discarded and not
  missed.

9
Poor, poor Codecs

• The codecs job is difficult and underappreciated
  they must try to keep the most important
  information in the file and discard the rest.
• In cases of extreme compression, codecs MUST be
  ruthless in discarding data
• They try to retain original quality or to
  retain at least enough information to keep you
  interested.

10

• Try to take some time to tell codecs you love and
  appreciate them.
• Theirs is a tough and thankless job.

11
Streaming Media Codecs

• Its a tough job but somebodys gotta do it.
• Uncompressed audio 1.4Mbits per second
• Raw video gt25Mbytes per second for full screen
  NTSC (National Television Standards Committee
  decided on 30 frames per second as the standard)
• 56K modem downloading at the speed of wait

12
Extreme Compression

• Staggering divide between raw bit rates of audio
  and video and bit rates available to Internet
  users bridged by codecs
• If sustained 34K data rate (of 56K modem)
  CD-quality audio must be compressed approximately
  401.
• With video, compression numbers are ludicrous

13
Perceptual Coding

• What is a conscientious codec to do?
• Use models about how humans perceive audio and
  video
• Allow for reasonably intelligent decisions about
  what to keep and what to throw out

14

• When digitizing audio and video, goal is to
  accurately represent input signal
• Audio sample input 44,100 times per second,
  using 16-bit words to represent each sample.
  Assumes our ears give equal attention to each and
  frequency
• Video assumes that 24 bits of information needed
  for every pixel on the screen, all 640 X 480 of
  them, 30 times a second - NOT

15

• Human eyes and ears register extremely wide range
  of audio and video inputs, though both are more
  sensitive to particular ranges of audio and video
  spectrum.
• And, what is perceived is determined by the
  brain, which filters raw information.
• Perceptual coding makes decisions about
  compressing audio and video based on how humans
  perceive sight and sound.

16

• For example loud sounds take precedence over
  quieter sounds in the brain bright objects and
  movement take visual precedence
• Evolved from primitive survival characteristics
  it was more important to hear the saber tooth
  tiger roaring at you than your wife telling you
  to take out the trash and, you want to be able
  to see that bright tiger running through the
  grass toward you.

17

• Humans retain these perceptual characteristics
• perceptual coding is effective in keeping what is
  important and discarding what is not
  important.

18
Audio Codecs

• Balance between fidelity, frequency response,
  dynamics number of channels.
• Collapsing from stereo to mono cuts amount of
  data in half
• Sampling rate further reduces data to be
  processed
• frequency response and
• sample bit depth

19

• FM radio frequency - 12KhZ drop frequency
  response and maintain quality
• For voice 8kHz or even 5kHz
• Thus dont have to use full 16 bits to represent
  dynamic range
• Technology making it even better need less to
  sound good

20
What about music?

• Must be prepared for wide range of sounds from
  screaming bass guitar to cymbals to soft flute in
  the background.
• Thus, special AND different codecs for audio,
  speech and video

21

• Voice codecs and music codecs are completely
  different
• Not interchangeable voice codecs make music
  sound awful
• and mostly visa versa.

22
Video Coding

• Video content with motion smooth at about 20
  fps
• Little motion can go below that
• But that is still not enough
• Economize by using inter-frame compression

23

• Theoretically, much of frame will remain
  unchanged between frames
• Digitize or encode only the section of the frame
  that changes

24

• Video codec digitizes one complete frame to
  establish all elements in the frame
• After background established subsequent frames
  digitize only changing elements or differences
• Establishing frames are keyframes
• Rest are called difference frames

25

• Video keyframes allow you to change an effect
  over time, or move the position and size of the
  frame over time.
• Video keyframes are used during custom special
  effects, but are not needed for most common
  operations.

26

• Thats all I want to confuse you with
• Just remember the jist