Video Compression Standards

1
Video Compression Standards

• Video Compression Standards extend the transform
  based, still image compression techniques to
  include methods for reducing temporal or frame to
  frame redundancy
• Depending on intended application, the standards
  can be grouped into two broad categories
• Video teleconferencing standards
• Multimedia standards

2
Multimedia Video Compression

• Developed for Video on Demand, HDTV, CD image
  video database
• MPEG-1 for high quality video as on CDROM
  supporting bit rate of 1.5MBits/s VCD (MP3 is MP1
  layer 3)
• MPEG2 addresses applications involving video
  quality between NTSC/PAL and CCIR a range that is
  suitable for cable TV broadcast, narrow channel
  satellite broadcasting, 2 to 10Mbits/s (MP3 is
  MP1/2 layer 3)
• MPEG1 and MPEG2 are meant for storage and
  transmission efficiency
• MPEG4 provides improved video compression
  efficiency. Targets bit rates of 5 to 64 Kbps
  for mobile devices and 4Mbits/s for TV and film
• http//en.wikipedia.org/wiki/Moving_Picture_Expert
  s_Group

3
Problems with Multimedia over networks

• LAN conferencing deploys information by dividing
  it into packets
• Each packet has an ID and destination address
• Packets are sent individually through a network
• Depending on conditions packets may take
  different routes and may arrive at different
  times or out of order
• Can be buffered at receiver end and reordered,
  but delay detrimental to interactive applications
• Lost data can cause jerky video/sound

4
MPEG STANDARD

• Compression ratios 501 to 2001
• MPEG frames in video sequence
• I frame intra frame
• P frame predicted frame
• B frame bidirectional interpolated frame
• I frames are intracoded. They can be
  re-constructed without any reference to previous
  frames
• The P-frames are forward predicted from the last
  I-frame or P-frame, i.e. it is impossible to
  reconstruct them without the data of another
  frame (I or P).
• The B-frames are both, forward predicted and
  backward predicted from the last/next I-frame or
  P-frame, i.e. there are two other frames
  necessary to reconstruct them.
• P-frames and B-frames are referred to as inter
  coded frames.

5
I,P B frames

• Depending on the type of scene the encoder will
  decide which types of frames to use
• Newscast more P and B frames little change
• Action movie more I frames lots of changes

6
A ReminderStandard Multimedia and The Web

• Files must be downloaded first
• Will not display until all downloaded
• Usually launched in a separate application
• Will not be linked to the information on the web
  page
• May not be compatible
• May need converting
• May need plugins
• Format specific to operating system
• 3 Types Of Viewing
• Download
• Progressive Download
• Streaming

7
Download vs Progressive Download

• Download
• Largest files
• Must be downloaded before playing
• Progressive Download
• The file is served from a normal web server
  through an HTTP request just like a normal web
  page or any other downloadable document.
• Unlike traditional download-and-play methods of
  video delivery, however, the file starts playing
  before it has completely downloaded when you use
  progressive download.

8
Streaming

• Streaming does not download the file
• Click on a button and seconds later
• Listen to radio
• Watch a live show
• Music thousands of radio stations available on
  line
• Video different formats available - should be
  able to choose format
• Needs a media server to do properly
• Live broadcast
• sent direct from event pop concert, sports event
• http//www.bbc.co.uk/
• can get news items streamed to your desktop
• http//www.apple.com/trailers/
• movie trailers

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How does streaming media work?

• Down-loading is like pouring a drink from a
  bottle into a glass,
• ....then drinking it
• Streaming is like drinking it straight from the
  bottle

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Download vs Streaming

• Downloaded files are stored, reassembled and
  displayed
• Streaming files are played back as they arrive
  and then discarded

11
How does it work?

• Break file into small packets and send them one
  after the other
• When the packets reach their destination, they
  are decompressed and resembled into a form that
  can be played
• Copy of file is not stored on the users
  computer.
• Streaming media files maintain an open connection
  which is negotiated between media server and
  media client.
• File is sent over that connection until the
  entire file has been received or the player is
  stopped

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How does it work?

• To maintain the seamless play the packets are
  buffered
• Process by which the media player downloads a few
  seconds of the media file prior to actual
  playback.
• Allows for uninterrupted playback as when buffer
  plays more packets are downloaded and queued for
  playback
• Streaming can be opened before a download is
  complete
• Organised to be rendered ASAP
• Play as data is received
• Streams can be encoded and downloaded at
  different bit rates.
• Users bandwidth determines the maximum bit rate
  that can be played.
• User can select the usual connection speed from
  the different options given

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Streaming is a way of delivering digital media to
an audience also enabling speed and interactivity
Client continuously receives data while
simultaneously displaying what has already been
received
Media is captured and stored on the server
machine
Media stream payloads are broken into packets
Client receiver
Data accessed and viewed via receivers i.e.
Internet, Digital TV, Cell phone, Cable,
Satellite
Buffer resends and recovers lost data but packet
loss may cause poor quality video/sound
Protocols attempt to maintain the integrity of
the stream
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Two types of streaming

• Live

• On-demand

client
client
1 request
2 request
3 stream
3 stream
Server
Server
1 Upstream
2 access
No files stored server identifies data stream,
encoder digitises encodes and sends
Clips stored as files
15
Rates for MPEG Type standards

• 28.8K modem - 20kbps FM mono low motion
  content small screen 160x120 1-10 fps
• 56.6Kmodem - 32kbps FM stereo low motion
  content small screen 160x120 5-12 fps
• ISDN64 (128kbps) - 96kbps CD good low motion
  content, ok high motion screen size determines
  quality 160x120 good 240x180 and 320x240
  compromised image quality 10-30 fps
• Cable Modem (512kbps-10mbs) - 128-300 kbps CD
  transparent very good quality (VHS and a better)
  at 300kbps 320x340 screen good
• LAN - 160-800 kbps CD transparent very good
  quality (VHS and better), 320x240 and higher
  15-30 fps

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Streaming Media in Context

• Could in the near future be the medium of choice,
  potentially therefore a huge industry
• Driving demand for wireless mobile services
  i.e.3G
• Potentially very democratisingallows greater
  access and participation
• Streaming media servers are optimised for
  streaming making better use of network bandwidth,
  hard drive bandwidth and server resources than
  regular servers
• Intelligent streaming monitors stream and
  re-buffers automatically if rate of stream drops

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Encoding

• The process of digitising and compressing a piece
  of traditional media into a format capable of
  being broadcast over the internet and played by a
  computer
• Raw data is typically uncompressed
• Big files - Contains all data essential to proper
  play
• Raw data is then encoded (compressed) to stream
• Encoders use audio and video codecs to compress
  data
• Compression is different for audio and video

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Codecs

• A codec is software that is used to compress or
  decompress a digital media file, such as music or
  video.
• When you rip a song from an CD to your computer,
  a codec is used to compress the song into an
  audio file.
• When you play the song that you ripped to your
  computer, the Player uses a codec to decompress
  the audio file and output the music.
• Compresses multimedia content prior to
  transmission, and decompresses upon playback.
• This uses less bandwidth and increases playback
  speed.
• Users must have the proper media player to stream
  each format. (Real now plays most codecs)

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MPEG4

• The standard for multimedia for the web and
  mobility.
• MPEG-4 is able to throw away a lot more
  information and to save files 8 to 12 times
  smaller than those of MPEG-2.
• Real power of MPEG-4 is the interactivity that
  can built into the video file or stream.
• Multimedia producers and software and game
  developers can isolate parts of the video for
  particular special effects.
• Pocket PC devices, and Windows CE already play
  back MPEG-4 and related codecs.
• Proprietary codecs still used because of
  licensing/market share

20
SMS vs MMS

• SMS
• Text messaging only
• Low band width required
• More real time than email
• MMS
• Can include media files images, sound and video
• Higher bandwidth required
• Poor quality video/sound depending on bandwidth
• Body language, gestures
• Multi-party conference
• Needs extra hardware webcam, microphone
• Anxiety on camera

21
Streaming Servers

• In order to stream, a web site must install media
  servers.
• Capacity is typically measured by the maximum
  number of concurrent streams that the server can
  serve.
• From a normal web server
• Content has been encoded for one bit rate only
• Not really streaming, just being progressively
  downloaded
• Transmission Control Protocol (TCP) resends lost
  packets and could cause playback pauses
• Dedicated media server
• Content can be encoded at multiple bit rates
• Server uses proprietary protocol to control
  content can choose best transport
• Server is tuned to provide optimum performance
  under heavy use big beefy machine, multiple
  CPUs, high memory and disk drive space.

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Transmission Models

• Unicast
• Streaming between sender and a single recipient
• Multicast
• Streaming between sender and multiple specific
  recipients
• Broadcast
• Streaming between a sender and any receiver
• Live and prescheduled content
• Some radio and television stations over WWW

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Good Use Of Streaming

• Streaming radio
• Promotional videos
• Short News Articles
• Longer video broken up into chapters
• Education and entertainment
• Problems With Streaming
• Compression
• Discard raw data until delivery requirements are
  met
• Audio is poor quality
• Video becomes jumpy and poor quality
• Low speed series of still images
• Display size for video is often reduced

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Problems With Streaming

• User Datagram Protocol (UDP)
• Bad packets are removed
• Audio will drop and pop
• Video will drop or lose frames
• Blank frames may be displayed
• Streaming Connection
• Can lose its connection
• If connection is lost playback will end and must
  be restarted
• If network is congested network will pause or
  delay
• Popular sites can become unavailable (Big
  Brother)
• Need server that can cope with it