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ERROR RESILIENT VIDEO COMMUNICATION

Under the guidance of

DR. ATHAR ALI MOINUDDIN

EL-400SEMINAR PRESENTATION ON

Presented byBILAL ARIF10LEB148A4LE-32

Outline

INTRODUCTION1

ERROR RESILIENCE TECHNIQUES2

ERROR RESILIENT ENCODING3

DECODER ERROR CONCEALMENT4

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CODEC INTERACTIVE ERROR CONTROL5

INTRODUCTION

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One inherent problem with any communication system is that information may be altered or lost during transmission, due to channel noise.

The effect of such information loss can be devastating for the transport of compressed video because any damage to the compressed bit stream may lead to objectionable visual distortion at the decoder.

Issues such as audio-visual synchronization and multipoint communications further complicate the problem of error recovery.

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Fig.1 A typical video communication system

Figure 1 shows steps involved in video communication.

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Fig 2 shows an example of effect transmission errors to a compressed video stream.

Coded,No loss

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3%

Fig.2 Example of reconstructed video frames from a H.263 coded sequence, subject to packet losses .

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Why Error Resilience Techniques?

Unless a dedicated link, that can provide a guaranteed quality of service is available between the source and destination, data packets may be lost or corrupted.

Error-free delivery can be achieved through retransmission of lost/damaged packets using mechanisms such as Automatic Repeat Request (ARQ).

But this introduces delays that are unacceptable for certain real-time applications. The use of retransmission algorithm is also prevented due to network flooding considerations.

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It is important to devise video encoding/decoding schemes that make the compressed bit stream resilient to transmission errors.

Designing proper interfacing mechanisms between codec (encoder and decoder) and the network, so that the codec can adjust its operations based on the network conditions, is also important.

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Why Error Resilience Techniques?

ERROR RESILIENCE TECHNIQUES

The Error Resilience (ER) techniques can be classified into three groups: (I) Those introduced at the source and channel encoder, to

make the bit-stream more resilient to potential errors;

(II) Those invoked at the decoder upon detection of errors, to conceal the effect of errors; and

(III) Those which require interactions between the source encoder and decoder, so that the encoder can adapt its operations based on the loss conditions detected at the decoder.

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ERROR RESILIENT ENCODING

The encoder operates in such a way so that transmission errors on the coded bit stream will not adversely affect the decoder operation.

ER encoders are less efficient compared to other coders, as they use more bits to obtain the same video quality in the absence of any transmission error.

The extra bits are called redundancy bits, introduced to enhance video quality when bit stream is subjected to transmission errors.

Achieving maximum gain in error resiliency with smallest amount of redundancy is the design goal.

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ERROR RESILIENT ENCODING

There are various techniques to introduce redundancy in the bit stream some of which are :

Layered coding (LC) with unequal error protection;

Multiple Description Coding (MDC);

Error Resilient Prediction

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ERROR RESILIENT ENCODING

ERROR RESILIENT ENCODINGLayered coding with unequal error protection(UEP)

Video is coded into a base layer and one or more enhancement layers,

Base layer can provide low but acceptable level of quality,

Enhancement layers incrementally improve the quality,

Users with different bandwidth capacity can access the same video at different quality levels, (illustrated in Fig 3), hence it is also called scalable coding.

Paired with UEP in transport system giving base layer more protection by assigning a more reliable sub-channel or allowing more retransmissions.

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ERROR RESILIENT ENCODING

Fig.3 Illustration of scalable coding

Multiple Description Coding (MDC) Independently-decodable and mutually-refinable streams of a

video source are generated.

These streams are called descriptions.

They are transmitted separately, possibly through different

network paths.

These multiple descriptions together enable the decoder to successfully decode the video ( depicted in Fig.4)

Advantage of MDC over LC is that it does not require special provisions in the network to provide a reliable sub-channel.

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ERROR RESILIENT ENCODING

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ERROR RESILIENT ENCODING

Fig.4 Depiction of MDC (Black boxes indicate lost information)

Error Resilient Prediction Temporal prediction is a major cause of incurrence of

transmission errors.

Once an error occurs so that a reconstructed frame at the decoder differs from that assumed at the encoder, the reference frames used in the decoder from there onwards will also differ in a similar fashion and consequently all subsequent reconstructed frames will be in error.

Insertion of Intra-Blocks or I-Frames is one of the solutions to stop temporal error propagation as prediction for an I-frame depends only on the current frame, there is no error propagation from the previous frames.

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ERROR RESILIENT ENCODING

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DECODER ERROR CONCEALMENT

Recovery or estimation of lost information due to transmission errors.

Packet losses typically lead to the loss of an isolated segment of a frame.

The lost region can be “recovered” based on the received regions by spatial/temporal interpolation.

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DECODER ERROR CONCEALMENT

Fig.5 Illustration of Decoder Error Concealment

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CODEC INTERACTIVE ERROR CONTROL

Feedback is provided from decoder to the encoder.

Decoder can inform the encoder about the information corrupted by errors and encoder operation can adjusted to suppress or eliminate the effect of such errors.

If the network protocol supports ARQ, retransmission of lost packets could be done but it introduces delay.

If online correction of errors cannot be afforded then it is important to limit the propagation scope of such errors.

This approach can reduce the losses, at the cost of increased complexity.

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CODEC INTERACTIVE ERROR CONTROL

Reference Picture Selection (RPS)

If the encoder learns through a feedback channel about damaged parts of a previously coded frame, it can decide to code the next frame not relative to the most recent, but to an older reference picture, which is known to be available in the decoder.

The penalty for using the older reference picture is significantly lower, if the reference picture is not too far away.

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CODEC INTERACTIVE ERROR CONTROL

We came across various techniques which help us to combat with transmission errors that are incurred in video communication.

For real-time applications techniques employing retransmissions cannot be put into use, instead ER techniques are employed.

ER encoders provide resilience at the cost of decrease in efficiency.

Even when a damaged bit stream is received, we can improve the quality of service using a well-designed decoder through the Concealment Technique.

Feedback between the codec can help to provide error resilience

Error Resilient Video Communication is very much possible, provided these techniques are exploited in a proper way.

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CONCLUSION

REFERENCES

Y. Wang, S. Wengar, J. Wen and A. K. Katsagellos, “Error Resilient Video Coding Techniques, Real Time Video Communication Over Unreliable Networks”: IEEE Signal processing Magazine, pp. 61-82. July 2000.

Mohammad Kazemi, Shervin Shirmohammadi, Khosrow Haj Sadeghi, “A Review of Multiple Description Coding Techniques for Error-Resilient Video Delivery”. Springer-Verlag Berlin Heidelberg 2013, April 2013.

Ramdas Satyan , Sunday Nyamweno, Fabrice Labeau, “Novel prediction schemes for error resilient video coding”, Signal Processing: Image Communication, Elsevier, pp.648-659. May 2010.

Y. Wang and Q. Zhu, “Error control and concealment for video communication: a review,” Proceedings of the IEEE, vol. 86, pp. 974-997. May 1998.

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THANK YOU