Error control in video Streaming

Introduction

Development of different types of n/ws such as internet, wireless and mobile networks has created new applications and communications.

Multimedia and video communication Eg) youtube(a video sharing website)

Compression is an essential component of digital video.

Uncompressed digital video requires - huge storage

- more bandwidth

Video Communication Systems

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End-to-End Video Transmission

The figure is about the packet based video transmission system.

Packet losses may happen in the lossy channel.

Compressed video is very sensitive to packet loss.

Video Conferencing over Packet-Switching Networks.

Packet Loss

•¡¡ Compressed video frame is packetized and transmitted.•¡¡ Packet loss can occur quite frequently due to congestion •and user mobility.

To solve the problem of packet errors or packet losses the application can use

Forward error correction (FEC)

Automatic repeat request (ARQ)

Conventional downloading application use ARQ for robust file transfer(FTP).

Two problems

* receivers may need to large buffers.

* users need to wait for a long time before starting playback.

Error control Techniques in video

Transport level error control– Error detection and correction through FEC– Retransmission of lost packets

Error resilient encoding– Adding redundancy in the bit stream to

help the decoder recover from transmission error Error concealment

– Recover lost/damaged regions based on image/video characteristics and human visual system properties at the decoder

Encoder-decoder-network interactive error control

– Joint source/channel coding Ex: layered coding with unequal error propagation

– Feedback-based adaptive encoding Ex. Reference picture selection, Selective intra update

– Path diversity Different bit streams sent through separate paths

Channel coding for video multicast

Providing a service such as video delivery for multiple users.

Four different approaches for this. Virtual multicast Multicast Broadcast Peer to peer

Virtual multicast

Group of users Server starts a separate unicast for

each client. Requires more resources Lack of efficiency So max amount of client is restricted.

Multicast

Clients can share the same channel. Server put the data or stream on

channel once , the clients can receive it. If feedback channel is available, the

users can send feedback, therefore the server can monitor the transmission quality.

Broadcast

In contrast to video multicasting, video broadcasting server is responsible for large no. of users, up to millions in tv stations.

So server cannot monitor the receivers quality or channel conditions

Peer to Peer

Difference between general peer to peer and latest peer to peer media streaming system is that in the data sharing mode among peers, the former uses the open-after-downloading mode while the later uses the play- while-downloading.

Conventional error control techniques for multicasting.

FEC ARQ Hybrid ARQ RPS Rate Distortion optimization and

Collision Distortion optimization

Forward error correction

. Forward Error Correction

send parity packets over a PTDD period.

Periodic frame Periodic frame

Parity Packet for periodic frame

FEC

FEC is a technique used for controlling errors in data transmission over unreliable or noisy communication channels.

A number of FEC algorithms Hamming code, Reed-Solomon code Bose-Chandhuri-Hocquenghem code.

Error detecting

assume the list of numbers that you wish to transmit might be 7, 3, 8, 10, 12, and 21.

6, 7, 3, 8, 10, 12, 21, and 61. The person at the receive end would look at the first

number and then ensure that the correct number of additional numbers is received and then verify that the number at the end of the sequence that is the sum of the transmitted numbers.

They allow you to determine if the transmission was accurate, but they don’t allow you to correct the errors.

Error correction

two terms binary –base 2 numbers Parity

Parity is a term that is used in binary communication systems to indicate whether the numbers of 1's in a transmission is even or odd. If the number of 1's is an even number, then parity is said to be even and conversely for odd parity.

Hamming code

2^k>=n+k+1

n – no. of bits

k – parity bits

Error-resilient Video Transmission

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System Overview

Features

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@Sender MBs are grouped in data units and entropy

coding used Error Control before transmission over lossy

channel Forward Error Correction(FEC) Backward Error Correction (BEC) Prioritization Methods Combinations of above

Features

@Receiver Erroneous and missing data

detected and localized Decoder gets correct data units or

error indication Error concealment applied at

positions where no data received Report loss of data units to encoder

Design Principles

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error-resilience tools decrease compression efficiency Main goal:

Shannon’s separation principle: compression separated with transport

In low delay situations, error-free transport is impossible System Design Principles

1. Loss correction below codec layer 2. Error detection 3. Prioritization methods 4. Error recovery and concealment 5. Encoder-decoder mismatch avoidance

Error Concealment Basic Idea

Decoder should generate a representation for lost area

Match as close as possible to the lost info Within manageable complexity

Techniques Spatial Error Concealment Temporal Error Concealment Hybrid Concealment Other Techniques

Spatial Error Concealment

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Based on assumption of continuity of natural scene content in space

Use pixel values of surrounding

available MBs Estimate of lost pixel:

αβγ are weighing factors Determine relative impact of vertical,

Horizontal, upper, lower… Disadvantage

Blurred reconstruction

Temporal Error Concealment

Rely on the continuity of a video sequence in time

Use temporally neighboring areas to conceal lost regions

Previous Frame Concealment (PFC) Use previous corresponding data to copy to

current frame Only good when little motion Widely used due to simplicity

Hybrid Concealment

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When only apply spatial concealment Concealed regions are significantly blurred

When only use temporal error concealment Significantly discontinuities in the concealed

regions Hybrid temporal-spatial technique applied

MB mode info of reliable and concealed neighbors decide which concealment method to use

Hybrid (cont.)

For intra-coded images Only use spatial concealment

For inter-coded images Use temporal concealment when more than half

of the available neighbor MBs are inter-coded Otherwise, use spatial concealment

Referred to as Adaptive temporal and spatial Error Concealment (AEC)

Selected Results

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Performance of different error concealment strategies