Chapter 6-Multimedia Streams Synchronization

8/9/2019 Chapter 6-Multimedia Streams Synchronization

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●

Synchronization in multimedia systems referes to thetemporal relations between media stream data units in

multimedia system.●Time dependent media data units: Temporal relations

between consecutive units of media stream exist.●Time independent media data unit is any kind of

traditional media like text and image.



Classification of Media Use



Multimedia Synchronization● The process of maintaining the temporal order of one or several media

streams

● Relations relevant for audio and video exchange:

– Intra-stream synchronization: Voice samples and video frames need

to arrive in time at the receiver before display or playout time to

maintain the continuity of playback.

– Inter-stream synchronization is needed to present voice and video

samples in a certain relation.

– Spatial synchronization: Participants in a multimedia conference

should receive audio-visual data at the same time although

geographically distributed.

● Life & Synthetic synchronization



Basic Synchronization Issues

● Content Relations: define a dependencies of media data

units on some data

● Spatial Relations: usually as layout relationships define

the space used for the presentation of a media data

units on an output device at a certain point of time in a

multimedia presentation.

● Temporal Relations: define the temporal dependencies

between the more than one media stream data.



Sources of Asynchrony● Different delays: Assumption of independent network

connections imposes different delays.

● Network jitter: asynchronous data transfer destroys synchrony.

● End-system jitter: packetizing and depacketizing of media data

units with different size due to encoding introduces jitter as well

as passing media units through the lower protocol layers.

● Clock drift between the clocks in the servers and in the client is

present because we do not assume global clocks.

● Change of the average delay: the synchronization scheme has to

be adaptive with respect to a change of the average delay.● Server drop outs: due to process scheduling are a realistic

assumption when using non-real-time operating systems.



Types of Synchronization● Time based synchronization

● To synchronize and to disseminate multimedia data using time

as a major parameter

● Synchronization is achieved by mean of electronic time signals

generated ad hoc by devices directly connected to a primary

time standard and transmitted by radio or by cable.

● Time signals classes: video or sound signals (called right time

signals) for low accuracy manual synchronization, digital

signals for automatic synchronization with medium-high

accuracy

● Passive synchronization techniques: Do not use a time signal

generated ad hoc, instead it is used a signal generated for other

purpose



Types of Synchronization● Delay based synchronization

– Schemes compensate for these delays by computing well-defined starting times for each stream server of the application.

– Allow to initiate the synchronized playback of a media stream

that is composed of several multimedia streams.

● Jitter based synchronization

– jitter is experienced by media units traveling from the source to

the destination.

–

To smoothen out jitter, elastic buffers are required. – These schemes guarantee a smooth playback of the stream

– very low buffer requirements.



Intra-Stream Synchronization● Audio and video streams are isochronous in nature.

● processing and network delay jitter (i.e., the variance indelay);

● variations in rates of recording and playback; and

●

unreliable transmission of stream data units.● Solutions:

– buffer monitoring

–

feedback technique – global clock



Inter-Stream Synchronization● A temporal relationship may exist at the source

between multiple continuous streams● multiplexing of streams

● aggregation in one data structure

●

global clocks● synchronization marker

● synchronization channel

●

feedback technique



Spatial Synchronization

● all participants in the conference receive the

audio and video data at the same time, tomaintain a fair conference

● Based on global clocks synchronization channel

or feedback techniques as proposed for inter-

stream synchronization.● When global clocks are available, mechanisms

based on these clocks can achieve the most

accurate spatial synchronization



Synchronization Handlers● roles : to identify the stakeholders involved, as well as the

prohibition and policy concepts.

● Roles of these components in handling synchronization

–

Service provider

– Network provider

– End-user roles



Network Provider● Compulsory network service: a transportation service with

a deterministic service is provided

● Statistical reliable network service: a transportation

service with a certain percentage of QoS violations is

provided.

● Best effort network service: the request from a client for a

certain transport service is evaluated against the current

network traffic.



End-User● With respect to the display of audio and video it is

important that the following requirements to be met:

● lip-synchronization is a well known requirement and

should be in the -20 ms to +40 ms range;

● audio or video jitter should be within the range of 10 ms;

● loss of video frames or audio samples is tolerable when

less than 1% of the total sent;

● spatial synchronization should be in the range of -0.25 s

to +0.25 seconds.



End-User● compulsory end-user service: in this case the

synchronization requirements must be met.● statistical reliable end-user service: a certain percentage of

violations of the synchronization requirements is allowed.

● best effort end-user service: possibilities to full the

synchronization requirements are based on current

processing and storage activities.

● User policies are often application dependent.



Synchronization Methods● Startup Synchronization

●

Constant delay and zero jitter first n media units

where ti

indicates expected arrival time of the MDU i at

the client-site and t0

is the playout start time of the Io

th

stream

● Evaluation phase: round trip delays for each of the stream

of the application are calculated, while

● Synchronization phase: the starting time for each server

is calculated and transmitted back to the servers.



Synchronization Methods● Startup Synchronization

●

Beginning of first phase

Max of the round trip delay

for all n substreams

● Second phase

● Difference between

the arrival timesof arbitrary media units



Example of Startup Synchronization



Intra-Stream Synchronization●

● Jitter based Intra-Stream Synchronization

Worst case estimate for

error and buffer req.

● Interdependencies:



Inter-Stream Synchronization● Inter-stream synchronization example that shows

temporal relations in a multimedia presentation including

audio, video, animation, and picture frames



Inter-Stream Synchronizations● Live Synchronization: Maintains the temporal relations as

they existed during the capturing process

●

Synthetic Synchronization: Temporal relations areartificially specified.



Synchronization Reference Model



h d l



Synchronization Models● The Temporal Synchronization Model



S h i ti M d l



Synchronization Models● Unix Synchronization Model



Object Data Stream S nchroni ation Technique



Object Data Stream Synchronization Technique● Synchronization point is a point held by all participating media

streams needing to be synchronized

● Synchronization points defined on the composite stream specify the

places that synchronous presentation must be checked and

maintained.

● Let ∆ si

denote the maximum time interval that media mi

can skip

and di

denote the current delay time interval with the presentation

of media stream mi. We then have the following situations:

Deadline based Scheduling for S nc



Deadline based Scheduling for Sync● n MDUs in a multimedia document, set of m channels in

network

● Effective bandwidth rate c j per channel and transit delay

j, aggregate capacity is not sufficient to provide inter-

stream and intra-stream synchronization without pre-

fetching.

● If the ith MDU and playout deadline i is scheduled for

transmission on the jth channel at time j, according to some

scheduling policy , then its arrival time, ai, at the user site

becomes

Feedback Techniques for Sync



Feedback Techniques for Sync● The resynchronization policies are:

●

Conservative: This scheme reacts only when playout of different streams is guaranteed to be asynchronous,

● Aggressive: In this approach the server reacts as soon as

there is even a slight chance that playback is

asynchronous● Probabilistic: The server reacts on the average (assuming

the network delay distributions and playout rate

variations are known).

Virtual Local Time



Virtual Local Time

● Virtual local time

● Does not rely on a synchronized network clock, which is

not always feasible to procure on the Internet,

● Can be implemented with a minimum overheads, and

● Can produce a reasonably good quality even when the

network delay jitter is very high.

Different Playout Strategies



Different Playout Strategies



Synchronized multicast media streaming framework

Chapter 6-Multimedia Streams Synchronization

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