Variable Bit Rate Video Coding

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Variable Bit Rate Video Coding

• April 18, 2002
• (Compressed Video over Networks Chapter 9)

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Topics

• Introduction
• Variable rate compression of video
• Delay constraints
• Impact of transmission modes
• Encoder rate constraints
• Rate control algorithms
• Conclusions

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Introduction

• VBR/CBR
• Coding modes (no. of bits per frame)
• Transport modes (no. of bits that can be
  transmitted during certain periods of time)
• Video quality (end-to-end), distortion
• Depends on decisions at encoder (if lossy and
  online)
• Depends on transport (loss, bandwidth, delay)
• Tolerance depends on application

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Variable rate compression

• Variable rate nature of compressed video
• Input video characteristics (scene changes,
  redundancy varies)
• Coding parameters (quantizers, motion
  compensation)
• Constant quality requires variable bits per frame
• Offline encoding (two-pass approach for DVD)
• Distortion measurement (MSE)

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Variable rate compression

• Ways of creating variable rate video
• Quantizer selection (with each block/macroblock
  complicates for video since RD of future frames
  depend on motion vectors)
• Frame type selection (I, P, B I frames may be
  placed at fixed interval, eg. every 0.5 sec)
• Frame skipping (use more bits/frame but less
  frame/s use interpolation at receiver)

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Delay constraints

• Prevent decoder buffer underflow

Ri no. of bits assigned to frame i Ci no. of
bits recd. by decoder buffer during ith frame
interval Td I corresponds to ith frame
intervals having passed
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Delay constraints

• We assumed no frame skipping, and const. frame
  rate
• Encoding and decoding delay constant.
• Delays not considered
• Transmitter buffer delay ?tb
• Transmission channel delay ?ch
• ?tb ?ch should not violate decoder buffer
  underflow constraint
• Constraints vary on
• Pre-encoded video
• Live video
• Interactive video

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Impact of transmission modes

• CBR vs VBR transmission
• Better video quality, shorter delay, increased
  call capacity
• QoS, best-effort model of networks
• Constrained vs. unconstrained transmission
• Eg. Transmission rate subject to conditions (peak
  rate/sustainable rate in QoS) or congestion
  control in TCP/IP within best-effort transmission
• Feedback vs No Feedback
• Encoders can modulate data(quality/resolution/rate
  ) based on feedback
• Modes of operation U-VBR / U-SVBR/ C-VBR / F-VBR

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Encoder Rate constraints

• Derive rate constraints that encoder/transmitter
  must meet
• Avoid violating delay constraints
• Use channel information
• Consider type of application (PEV/LIV/LNIV)
• Memory restrictions (PDA)

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Encoder Rate constraints

• To prevent decoder buffer underflow, encoder must
  avoid the encoder buffer size exceeding
• CBR transmission mode
• VBR with known channel rate
• VBR with unpredictable channel rates

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Encoder Rate constraints

• Video caching
• Proxy caching of web objects
• Efficient sharing of resources
• Reduce initial latency
• Less likelihood of packet losses
• Replicate entire sequence
• store prefix (for ?td seconds)
• Store intermediate samples so that decoder does
  not starve
• Coarse layers cached

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Rate control algorithms

• Constraints placed so as to guarantee that
  decoder always has data to decode
• Video encoder should produce bitstream that meets
  these constraints
• To accomplish this, we need rate control
  algorithms
• H.263, MPEG-2 do not define operation of encoder,
  so free to use any rate control techniques

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Rate control algorithms

• Problem formulation
• Rate constraints for set of frames, not of
  individual frame
• Decision on how to allocate bits among the set,
  so that the average distortion is minimized RD
  optimization
• CBR algorithms
• Each coding parameter is associated with a rate
  and distortion pair. Find coding parameters
  within a finite set (for each frame) such that
  distortion is lowest
• First calculate RD at all points and then
  optimize
• Gets complicated when dependency exists among
  frames
• Allow change of frame rate (find optimum, no
  jerky motion, redundancy decreases among frames
  at low frame rate)

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Rate control algorithms

• VBR algorithms
• Many parameters (rate, delay, etc.)
• Algorithms monitor the long-term average
  transmission rate and keep it close to a value
• Real-time adaptation to channel conditions
• Encoder should react to channel changes (channel
  memory is long, channel feedback is fast)
• Layered/Scalable video
• Incorporated in MPEG-2 standard, high complexity.

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Conclusions

• VBR coding is the natural representation of video
• Constraints need to be placed at encoder for best
  quality
• Transmission medium and service plays a critical
  role
• Optimization algorithms used to find ideal rate
  at encoder
• Rate control gets even more challenging with
  wireless mediums