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Variable Bit Rate Video Coding

Variable Bit Rate Video Coding. April 18, 2002 (Compressed Video over Networks: Chapter 9). Topics. Introduction Variable rate compression of video Delay constraints Impact of transmission modes Encoder rate constraints Rate control algorithms Conclusions. Introduction. VBR/CBR

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Variable Bit Rate Video Coding

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  1. Variable Bit Rate Video Coding April 18, 2002 (Compressed Video over Networks: Chapter 9)

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

  3. 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

  4. 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)

  5. 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)

  6. 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

  7. 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

  8. 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

  9. 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)

  10. 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

  11. 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

  12. 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

  13. 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)

  14. 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.

  15. 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

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