. Synchronization in multimedia systems referes to the temporal 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
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1. Prof Pallapa Venkataram,
Electrical Communication Engineering,
Indian Institute of Science,
Bangalore – 560012, India Multimedia Streams Synchronization
2. Synchronization in multimedia systems referes to the temporal 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.
3. Classification of Media Use
4. 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
5. 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.
6. 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.
7. 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
8. 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.
9. Intra-Stream Synchronization Audio and video streams are isochronous in nature.
processing and network delay jitter (i.e., the variance in delay);
variations in rates of recording and playback; and
unreliable transmission of stream data units.
10. Inter-Stream Synchronization A temporal relationship may exist at the source between multiple continuous streams
multiplexing of streams
aggregation in one data structure
11. Spatial Synchronization all participants in the conference receive the audio and video data at the same time, to maintain 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
12. Synchronization Handlers roles : to identify the stakeholders involved, as well as the prohibition and policy concepts.
Roles of these components in handling synchronization
13. Service Provider Intra-stream synchronization: SP manipulates incoming audio/video streams so that outgoing streams are within the 10 ms jitter boundary;
Inter-stream synchronization: SP will manipulate incoming audio/video streams so that related outgoing audio and video streams are within the -20 ms and +40 ms range;
Spatial synchronization: SP is responsible for ensuring that outgoing audio/video streams are played out simultaneously at the multiple users within the 0.25 s boundary.
14. 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.
15. 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.
16. 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.
17. 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 Ioth 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.
18. Synchronization Methods Startup Synchronization
Beginning of first phase Max of the round trip delay for all n substreams
Difference between the arrival times of arbitrary media units
19. Example of Startup Synchronization
20. Example of Startup Synchronization
21. Intra-Stream Synchronization
Jitter based Intra-Stream Synchronization Worst case estimate for error and buffer req.
22. Inter-Stream Synchronization Inter-stream synchronization example that shows temporal relations in a multimedia presentation including audio, video, animation, and picture frames
23. Inter-Stream Synchronizations Live Synchronization: Maintains the temporal relations as they existed during the capturing process
Synthetic Synchronization: Temporal relations are artificially specified.
24. Synchronization Reference Model
27. Synchronization Models The Temporal Synchronization Model
29. Synchronization Models Unix Synchronization Model
32. 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:
33. Deadline based Scheduling for Sync n MDUs in a multimedia document, set of m channels in network
Effective bandwidth rate cj 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
34. 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).
35. 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.
36. Different Playout Strategies
38. Synchronized multicast media streaming framework