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School of computing science Simon Fraser University. CMPT 820 : Error Mitigation Schaar and Chou, Multimedia over IP and Wireless Networks: Compression, Networking, and Systems Presented by: Azin Dastpak. Outline. Motivation Operational Encoder Control Adaptive Intra Updates - PowerPoint PPT Presentation
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SCHOOL OF COMPUTING SCIENCESIMON FRASER UNIVERSITY
CMPT 820 : Error MitigationSchaar and Chou, Multimedia over IP and Wireless Networks: Compression, Networking, and Systems
Presented by: Azin Dastpak
2
Outline
Motivation Operational Encoder Control Adaptive Intra Updates Interactive Error Control
Mode 1:Acknowledged Reference Area Only Mode 2:Synchronized Reference Frames Mode 3:Regular Prediction with Limited Error
Propagation Mode 4:Unrestricted Reference Areas with Expected
Distortion Update Conclusion Questions
3
Motivation
Error propagation is the major problem when transmitting MCP-coded video over lossy channels.
Encoder shall change its encoding behavior if it’s aware that decoder has experienced loss, which may sacrifice some compression efficiency.
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Motivation
a)no error robustness b)adaptive intra updates c)interactive error control
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Operational Encoder Control
The encoder is responsible to select one of many encoding parameters Motion vectors MB modes Quantization parameters Reference frames Spatial and temporal resolution
considering constraints bit rate encoding and transmission delay complexity Buffer size
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Operational Encoder Control
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Operational Encoder Control
Bad decisions in parameter selection may lead to poor results in: Coding efficiency Error resilience Both..!
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Operational Encoder Control – Compression Efficiency
The distortion of MB b coded with option m:
The coding mode selected for MB b:
i
imbibmb ssd2
,,,, '
b )(minarg ,,*
mbOmbOmb rdm
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Adaptive Intra Updates
In the presence of errors, introducing more intra-coded MBs is important.
Sequence characteristics and bit rate influence the appropriate percentage of intra updates.
Modify the selection of the coding modes to take into account the influence of lossy channels. Replace the encoding distortion by decoder
distortion.
2
,,, ),(')(' mCssCd ttbtbtmb
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Adaptive Intra Updates
The channel behavior is not deterministic and channel realization observed by the decoder is unknown
The receiver can send reports on the experienced loss and delays statistics for the encoder to compute expected distortion:mbd , }),'('{)}'('{
2
,,',' mcssEcdE ttbtbCtmbC tt
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Interactive Error Control
In previous scenario only the statistics of the channel process C’ was known to the encoder.
In this scenario the -frame delayed version of the loss process experienced at the receiver is known at the encoder. Sending Acks for correctly received data units Sending Nacks for missing slices Both
The goal is to reduce, limit or even completely avoid error propagation by integrating the decoder state information in the encoding process
tC
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Interactive Error Control
At encoder side, each generated data unit Pi is assigned a decoder state.
This information will be integrated in a modified rate-distortion optimized operational encoder control
},,{, OAKNACKACKC ienc
b )'(minarg ,','*
mbOmbOmb rdm
13
Interactive Error Control
We have four different operation modes, differing only in set of coding option available at encoder, O’ and the applied distortion metric
Mode 1: Acknowledged Reference Area Only Mode 2: Synchronized Reference Frames Mode 3: Regular Prediction with Limited Error
Propagation Mode 4: Unrestricted Reference Areas with
Expected Distortion Update.
mbd ,'
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Mode 1: Acknowledged Reference Area Only
Only the decoded representations of data units that have positively acknowledged at the encoder are allowed to be referenced in the encoding process.
Encoding options are restricted to acknowledged areas only,
If no reference area is available or no satisfying match is found in the accessible reference area, intra coding is applied.
tACKOO ,'
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Mode 1: Acknowledged Reference Area Only
Error may be visible in the presentation of a single frame but error propagation and reference frame mismatch are completely avoided.
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Mode 1: Acknowledged Reference Area Only
Average PSNR versus bit rate for feedback mode 1
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Mode 2: Synchronized Reference Frames
In this mode not only positively acknowledged data units but also a concealed version data units with decoder state Nack are allowed to be referenced.
For the reference frames to be synchronized the encoder must apply exactly the same error concealment as the decoder.
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Mode 2: Synchronized Reference Frames
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Mode 2: Synchronized Reference Frames
The advantage of of mode 2 is for : Low bit rates Delays < Nref -1
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Mode 3: Regular Prediction with Limited Error Propagation
Mode 1 and 2 are mainly suitable in cases of higher loss rates, when the loss rates are low the performance is degraded by longer prediction chains due to feedback delay.
In this case we only alter the prediction in the encoder in case of the reception of a NAK. In this case the reference area and option set will be changed to acknowledged and concealed image parts.
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Mode 3: Regular Prediction with Limited Error Propagation
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Mode 3: Regular Prediction with Limited Error Propagation
For delay=0 mode 2 and 3 are the same. For increasing delays, mode 3 performs
significantly worse than mode 2.
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Mode 4: Unrestricted Reference Areas with Expected Distortion Updates
In this case the set of encoding options is not altered, that is O’=O.
Only for the data units with outstanding acknowledgement at the encoder, the randomness of the observed channel state is considered. For all other data units the observed channel state is no longer random.
mbd ,')}'('{ ,}'{ imbC CdE
i
)}('{ ,}{ imbC CdEi
OAKC ienc ,
OAKC ienc ,
24
Mode 4: Unrestricted Reference Areas with Expected Distortion Updates
Compared to mode 1 and 2 this method is beneficial if the feedback is significantly delayed.
Compared to mode 3, it reduces the unsatisfying performance in case of error propagation
For this mode turns to mode selection without any feedback at all
For this mode is identical to mode 2 and 3.
0
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Conclusion
Error propagation is the most important issue in error-prone video transmission
If no feedback is available, increased percentage of intra MBs, selected by channel-adapted optimization schemes, performs best
If feedback is available, interactive error control can be applied For short delays or low error rates: Modify the
prediction only in case of Nack message reception. In all other cases: Reference only those areas for
which the encoder is sure that the decoder has exactly the same are
26
Questions?