Network Requirements of audio and motion video

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Network Requirements of audio and motion video
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Problems for live usage

• Error rate
• Network may not deliver bits reliably
• Shouldnt happen that often in a digital network
  due to error control
• Transit delay
• Data is not delivered at a constant speed, or at
  the right speed

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Audio

• Ear is a differentiator
• Has high powers of differentiation
• Means that small audio problems are easily
  recognized

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Video

• Eye is an integrator
• It puts signals together
• Hard to recognize multiple intermixed drawings
  and interpret them
• Missing information is less noticeable
• Means video can tolerate more errors and higher
  delays

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Implications for multimedia

• In a transmission of mixed audio and video, audio
  should have a higher priority
• Audio more sensitive to delays
• Note - this is a problem on the internet
• Internet currently has no priority mechanism

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Compression

• Can we reduce the amount of information we need
  to send?
• We can compress it
• two types of compression
• Lossy - Cant exactly recreate original
• Lossless - Can exactly recreate original file,
  bit for bit.

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Compressing different types of info

• Images/ Graphics
• Gif (lossless)
• JPEG - Can be lossy
• Sound (first digitize at some sampling rate)
• Then compress
• Video (usually lossy)
• Intraframe compression
• interframe compression

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Video compression

• Intraframe compression
• Compress the pixels within one frame
• Interframe compression
• Since video frames often change very little
  between frames, we can increase compression by
  only recording the changed bits from one frame to
  the next.
• This works extremely well for many situations
• Video - 1 frame 6404808 bits 300K bytes (
  10MB/sec 30 fps)

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Compression Problems

• Sending compressed data means that problems in
  one block (packet) of data may impact prior and
  subsequent blocks that have related information

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Partial solution to network delay

• Add an offset delay to reception of message
• The receiving side will collect N seconds of
  information before it starts playing the audio or
  video clip
• How big should N be?
• Depends on effective speed of communications
  channel
• higher variance means larger N

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Other Problems

• Intermedia Synchronization
• Need to synchronize audio and video streams
• Compression and burstiness
• Compression can introduce burstiness into a
  non-bursty transmission
• Example. Audio uncompressed has a fixed number of
  bits per second, compressed it can go down to
  zero (periods of silence)

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Conclusion

• Network multimedia demands many tradeoffs between
  bandwidth, errors, delays and cost
• Compression makes the problem even more complex
• Error rates have to be lower to use compression
• What are some reasonable levels for audio and
  video?

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Digital Audio - ISDN

• To digitize speech, the standard is to take 8000
  samples a second, each sample having a value
  (0-255, I.e. 8bits)
• Hence 1/2 of an ISDN line is 64kb (64000 bits per
  second)
• Note, this is uncompressed. We wil talk about
  compression later)

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Bit rates for different audio streams

• Telephone
• G.711 PCM
• G.721 ADCMP
• G.722
• G.728
• CD
• CD - DA
• MPEG consumer
• MPEG studio

• Bit rate Kbps
• 64
• 32
• 48,56,64
• 16
• 1411
• 192
• 384

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Motion Video Bit Rates (Mbps)

• HDTV
• Compressed
• Digital TV (studio)
• MPEG2
• Broadcast TV (mpeg2)
• VCR (MPEG-1)
• Videoconferencing

• 2,000
• 34
• 166
• 3-6
• 2-4
• 1.2
• .1

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Some typical network rates

• 56k modem - 56K bits per second
• ISDN - 64kbps (2 channels 128)
• Wireless - 2MB - 8MB
• Cable Modem - 10 MBS
• ethernet/token ring 10MBS
• T3 - 45 MBS
• OC3 - 154MBS

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Alternatives for Networked Video

• Real Networks (www.real.com)
• Real Player G2
• Supports multiple speeds with a single streaming
  file. (28.8 up to 400kps)
• Supports live video streams
• Higher bit rates need faster processors
• Microsoft
• Netmeeting
• Netshow , Netshow Theater
• Cisco IP/TV

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Real Networks Tools Beta versions available for
free

• Real Player G2
• Helper App to play Real player audio and video
• Real Publisher
• Set of tools to encode audio/video to support a
  variety of speeds (network and computer)
• Supports SURESTREAM
• Automatically switches speeds
• REAL SERVER
• Host content

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Other Features

• REAL TOOLS
• REAL PIX - Stream images
• REAL TEXT - Stream Text
• REAL FLASH - Stream Macromedia Flash
• SMIL - Synchronize all of the above

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Final Project Ideas

• Stern TV Station - Multiple Channels
• Live events
• Archived events
• News?
• Video classes with power point slides coordinated
  with talk.