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CS 414 – Multimedia Systems Design Lecture 34 – Synchronization (Part 2)

CS 414 – Multimedia Systems Design Lecture 34 – Synchronization (Part 2). Klara Nahrstedt Spring 2011. Administrative. MP3 ongoing mi-clicker experiment ongoing. Outline. Synchronization in Distributed Environments Location of Synchronization Clock Synchronization

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CS 414 – Multimedia Systems Design Lecture 34 – Synchronization (Part 2)

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  1. CS 414 – Multimedia Systems DesignLecture 34 – Synchronization (Part 2) Klara Nahrstedt Spring 2011 CS 414 - Spring 2011

  2. Administrative MP3 ongoing mi-clicker experiment ongoing CS 414 - Spring 2011

  3. Outline Synchronization in Distributed Environments Location of Synchronization Clock Synchronization Synchronization Reference Models CS 414 - Spring 2011

  4. Synchronization in Distributed Environments • Information of synchronization must be transmitted with audio and video streams, so that receiver(s) can synchronize streams • Sync information can be delivered before start of presentation (used by synthetic synchronization) • Advantage: simple implementation • Disadvantage: presentation delay • Sync information can be delivered using separate sync channel - out-band (used by live synchronization) • Advantage: no additional presentation delay • Disadvantage: additional channel needed CS 414 - Spring 2011

  5. Sync in Distributed Environments • Sync information can be delivered using multiplexed data streams - in-band sync • Advantage: related sync information is delivered together with media units • Disadvantage: difficult to use for multiple sources CS 414 - Spring 2011

  6. Location of Sync Operation • Sync media objects by combining objects into new media object • Sync operation placed at sink • Demand on bandwidth is larger because additional sync operations must be transported • Sync operation placed at source • Demand on bandwidth smaller because streams are multiplexed according to sync requirements CS 414 - Spring 2011

  7. Clock Synchronization • Sync accuracy depends on clocks at source and sink nodes • Ta = Tav – Nla – Oa • Tv = Tav – Nlv – Ov • End-to-end delay • Nla = EEDa = Tav-Ta-Oa • Nlv=EEDv = Tav-Tv-Ov • EEDa = (Ta1-Ta2)/2 • NTP (Network Time Protocol ) CS 414 - Spring 2011

  8. Network Time Protocol • Protocol to sync clocks of computer systems over packet-switched, variable –latency data networks • Uses UDP port 123 • Designed to resist effects of variable latency (jitter buffer) • Designed in 1985 by Dave Mills at U. Delaware • Can achieve accuracy of 200 µsec • Based on Marzullo Algorithm CS 414 - Spring 2011

  9. Marzullo’s algorithm (1984)(intersection algorithm) • Agreement protocol for estimating accurate time from a number of noisy time sources • If we have estimates • 10 2, 12 1, 11 1, then interval intersection is 11.5 0.5 • If some intervals don’t intersect, consider intersection of majority of intervals CS 414 - Spring 2011 Source: wikipedia

  10. Clock strata • NTP uses hierarchical system of “clock strata” • Stratum levels define distance from reference clock and exist to prevent cycles in hierarchy • Stratum 0 • devices are atomic clocks, GPS clocks, radio clocks • Stratum1 • computers attached to stratum0 devices • Act as servers for timing requests from Stratum 2 servers via NTP • Stratum2 • (similar to Stratum1, but they also have peering relation to other stratum2 servers CS 414 - Spring 2011

  11. Precision Time Protocol (PTP) • High Precision time protocol (original IEEE 1588-2002) for synchronization used in measurement and control systems residing on a local area network • Accuracy in the sub-microseconds • PTPv2 – IEEE 1588-2008 improves accuracy, precision and robustness • PTP serves a niche not well served by NTP and GPS • it achieves accuracy beyond those attainable using NTP; • It is cheaper than GPS • It is accessible to receivers for which GPS signals are inaccessible. CS 414 - Spring 2011

  12. PTP Details • IEEE 1588 standards describe a hierarchical master-slave architecture for clock distribution • Ordinary clock – source for root timing reference, called ‘grandmaster’ • Boundary clock – intermediate synchronization bridge from one network segment to another – used by synchronization master to sync clocks within a network segment • PTP messages use IP-multicast over UDP (IPv4 or IPv6) • Selection algorithm of best clock source uses properties such as quality, accuracy, variance, … CS 414 - Spring 2011

  13. PTP Synchronization • After selection of master source of time, clocks determine the offset between themselves and their master • Let t represent physical time, given slave device, the offset o(t)at time t is: o(t) = s(t) – m(t), • m(t) is measured time at master at time t, and • s(t) is measured time at local clock at time t. • Master periodically broadcasts the current time to other clocks (up to 10 measurements per second in 1588-2008) CS 414 - Spring 2011

  14. PTP Synchronization T1’ – T1 = o + dand T2’ – T2 = -o + d, then we find o = (T1’ = T1 – T2’ + T2)/2 The clock now knows offset o during the transactions and can correct itself by this agreement with the master clock. CS 414 - Spring 2011

  15. Other Sync Issues • Sync must be considered during object acquisition • Sync must be considered during retrieval • Sync access to frames of stored video • Sync must be considered during transport • If possible use isochronous protocols • Sync must be considered at sink • Sync delivery to output devices • Sync must consider support of functions such as pause, forward, rewind with different speeds, direct access, stop or repeat CS 414 - Spring 2011

  16. Reference Models • We need reference models to • Understand various requirements for multimedia sync • Identify and structure run-time mechanisms to support execution of sync • Identify interface between run-time mechanisms • Compare system solutions for multimedia sync CS 414 - Spring 2011

  17. Synchronization Reference Model • Sync model we will be evaluating in detail is according to Meyer, Effelsberg, Steinmetz: • Sync multimedia objects are classified according to • Media level • Stream level • Object level • Specification level CS 414 - Spring 2011

  18. Media Level (1) • Each application operates single continuous media streams composed of sequence of LDUs • Each media stream responsible for intra-stream synchronization • Assumption at this level: device independence • Supported operations at this level: • read(devicehandle, LDU) • write(devicehandle, LDU) CS 414 - Spring 2011

  19. Media Level (2) - Example window = open(“videodevice”); movie = open(“file”); while (not EOF (movie) ) { read(movie, &LDU); if (LDU.time == 20) printf(“Subtitle 1”); else if (LDU.time == 26) printf(“Subtitle2”); write(window, LDU); } close(window); close(movie); CS 414 - Spring 2011

  20. Stream Level (1) • Operates on continuous media streams and groups of streams • Models inter-stream synchronization for need of parallel presentation • Offers abstractions: • notion of streams, • timing parameters concerning QoS for intra-stream and inter-stream synchronization CS 414 - Spring 2011

  21. Stream Level (2) • Supports operations: • Start(stream), stop(stream), create-group(list-of-streams); • Start(group), stop(group); • Setcuepoint(stream/group, at, event); • Classifies implementation according to • Support for distribution (end-to-end, local) • Support of type of guarantees (best effort, deterministic) • Support of types of supported streams (analog, digital) CS 414 - Spring 2011

  22. Object Level (1) • Operates on all types of media and hides differences between discrete and continuous media • Offers abstractions: • Complete sync presentation • Computes and executes complete presentation schedules that include presentation of non-continuous media objects and calls to stream level • Does not handle intra-stream and inter-stream synchronization • (relies on media and stream levels) CS 414 - Spring 2011

  23. Object Level (2) - Example • MHEG – Multimedia Hypermedia Experts Group of ISO • Defines representation and encoding of multimedia and hypermedia objects (object-based declarative programming language) • Provides abstractions suited to real-time presentations • implemented via multimedia synchronization functionalities • Provides abstracts for real-time exchange • implemented with minimal buffering • Evaluates status of objects and performs actions (e.g., prepare, run, stop, destroy) • For time-dependent streams – access to stream level • For time-independent streams – direct access the object to present it • Classification of this level according to (a) distribution capabilities, (b) type of presentation schedule, (c) schedule calculation CS 414 - Spring 2011

  24. Specification Level (1) • Open layer included in tools which allow to create sync specifications • Examples: • Synchronization editors, document editors, authoring systems, conversion tools • Examples of such tools: multimedia document formatter that produces MHEG specifications • Classification: • Interval-based spec • Time-axes based spec • Control flow-based spec • Event-based spec CS 414 - Spring 2011

  25. Specification Level (2) - SMIL • SMIL – World Wide Web Consortium’s Synchronized Multimedia Integration Language • SMIL 2.0 and SMIL 3.0 (developed by CWI, PI: D. Bulterman) • Features: Animations, Time Manipulations, Timing and Synchronization, Content Control, Meta-information, Media objects, Linking, Layout, Transitions • http://www.w3.org/TR/SMIL3/ CS 414 - Spring 2011

  26. SMIL “Standards, SMIL 2.0, Examples and Comparisons”, D. Bulterman, Siemens Corporate research, Editor: P. Liu, 2002 CS 414 - Spring 2011

  27. Conclusion CS 414 - Spring 2011

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