1、 CEA Bulletin A/V Synchronization Processing Recommended Practice CEA-CEB20 R-2013 July 2009 NOTICE Consumer Electronics Association (CEA) Standards, Bulletins and other technical publications are designed to serve the public interest through eliminating misunderstandings between manufacturers and p
2、urchasers, facilitating interchangeability and improvement of products, and assisting the purchaser in selecting and obtaining with minimum delay the proper product for his particular need. Existence of such Standards, Bulletins and other technical publications shall not in any respect preclude any
3、member or nonmember of CEA from manufacturing or selling products not conforming to such Standards, Bulletins or other technical publications, nor shall the existence of such Standards, Bulletins and other technical publications preclude their voluntary use by those other than CEA members, whether t
4、he standard is to be used either domestically or internationally. Standards, Bulletins and other technical publications are adopted by CEA in accordance with the American National Standards Institute (ANSI) patent policy. By such action, CEA does not assume any liability to any patent owner, nor doe
5、s it assume any obligation whatever to parties adopting the Standard, Bulletin or other technical publication. This document does not purport to address all safety problems associated with its use or all applicable regulatory requirements. It is the responsibility of the user of this document to est
6、ablish appropriate safety and health practices and to determine the applicability of regulatory limitations before its use. This document is copyrighted by the Consumer Electronics Association (CEA) and may not be reproduced, in whole or part, without written permission. Federal copyright law prohib
7、its unauthorized reproduction of this document by any means. Organizations may obtain permission to reproduce a limited number of copies by entering into a license agreement. Requests to reproduce text, data, charts, figures or other material should be made to CEA. (Formulated under the cognizance o
8、f the CEA R4 Video Systems Committee.) Published by CONSUMER ELECTRONICS ASSOCIATION 2014 Technology Measurement guidelines for DVB Systems 9. Blu-ray Disc, System Description, Blu-ray Disc Read-Only Format, Part 3: audio Visual Basic Specifications, Version 1.0 10. DLNA Networked Device Interoperab
9、ility Guidelines, Volume 1: Architectures and Protocols (expanded: October 2006) 11. OC-SP-CCIF2.0_I15-080620, OpenCable Specifications, CableCARD Interface 2.0 Specification 12. ISO/IEC 13818-9:1996 Information Technology Coding of audio visual objects Part 9: Extension for real time interface for
10、systems decoders 13. IEC 61937-3:2007 (E), Digital Audio Interface for non-linear PCM encoded audio bitstreams applying 14. IEC 60958 Part 3: Non-linear PCM bitstreams according to the AC-3 and enhanced AC-3 formats 15. ATIS-0800005, IPTV Packet Loss Issue Report, December 2006 2.1.2 Informative Ref
11、erence Acquisition ATIS Documents: Alliance for Telecommunications Industry Solutions, 1200 G Street NW, Suite 500, Washington, DC 20005, USA; Phone 202-628-6380; Fax 202-393-5453; Internet http:/www.atis.org ATSC Standards: The Advanced Television Systems Committee, 1776 K St N.W., Washington, DC 2
12、0006-2304, USA; Phone 202-872-9160; Fax 202-872-9161; Internet http:/www.atsc.org BluRay Standards: BluRay Disc Association, Internet http:/us.blu- CEA-CEB-20 2DLNA Guidelines: Digital Living Network Alliance, DLNA Administration, c/o VTM Attn: Member Services, 3855 SW 153rdDrive, Beaverton, OR 9700
13、6, USA; Phone 503-619-0422; Fax 503-644-6708; Internet http:/www.dlna.org OpenCable Specifications: Cable Television Laboratories, 858 Coal Creek Circle, Louisville, CO 80027-9750, USA; Phone 303-661-9100; Fax 303-661-9199; Internet http:/ ISO and/or IEC Standards: Global Engineering Documents, Worl
14、d Headquarters, 15 Inverness Way East, Englewood, CO USA 80112-5776; Phone 800-854-7179; Fax 303-397-2740; Internet http:/; Email IEC Central Office, 3, rue de Varembe, PO Box 131, CH-1211 Geneva 20, Switzerland; Phone +41 22 919 02 11; Fax +41 22 919 03 00; Internet http:/www.iec.ch; Email pubinfo
15、riec.ch ISO Central Secretariat, 1, rue de Varembe, Case postale 56, CH-1211 Geneve 20, Switzerland; Phone +41 22 749 01 11; Fax +41 22 749 01 55; Internet http:/www.iso.ch; Email mbinfoiso.ch SCTE Standards: Society of Cable Telecommunications Engineers, 140 Philips Rd., Exton, Pa 19341-1318, USA;
16、Phone 610-363-6888; Fax 610-363-5898; Internet http:/www.scte.org; Email sctescte.org 3 Acronyms ATIS Alliance for Telecommunications Industry Solutions ATSC Advanced Television Systems Committee DVB Digital Video Broadcasting DTS Decoding Time Stamp ES Elementary Stream GOP Group of Pictures IEC In
17、ternational Electrotechnical Commission IRQ Interrupt Request ISO International Standards Organization MPEG Moving Picture Experts Group MPTS Multi-Program Transport Stream PCR Program Clock Reference PES Packetized Elementary Stream PLL Phase-Locked Loop PMT Program Map table PTS Presentation Time
18、Stamp PU Presentation Unit STC System Time Clock DLNA Digital Living Network Alliance DMP Digital Media Player DVR Digital Video Recorder IP Internet Protocol RF Radio Frequency RTP Real-time Transport Protocol SPTS Single Program Transport Stream TS Transport Stream UDP User Datagram Protocol VCXO
19、Voltage-Controlled Crystal Oscillator CEA-CEB-20 3 4 Definitions Access Unit See definition in ISO/IEC 13818-1 1 Acquisition function A function that acquires a transport stream from a transmission network. This function may filter an acquired transport stream to produce a partial TS stream for stre
20、aming and/or recording. An acquisition function and a playback function may but need not reside within the same physical device. BSn The size of a specific buffer in the T-STD buffer model. See ISO/IEC 13818-1 1 Section 2.4.2. De-multiplexer A decoder operation that filters and separates the transpo
21、rt into constituent elementary streams. Decoding Time Stamp (DTS) Field which may be present in a PES header when it differs from the value of the PTS. See ISO/IEC 13818-1 1 Section 2.4.3.6. This indicates the precise instant when the Access Unit is to be decoded. This is sometimes called “decode ti
22、me.” Decoder The combination of operations which impose any delay from the instant when a transport stream is acquired to the instant when the constituent elementary streams within that transport stream are presented to the end users eyeballs and ears. Decoder operations generally include a de-multi
23、plexer (including System Time Clock (STC) recovery and TS packet filtering), buffers, an audio decoder and a video decoder. Decoder operations may also include processors that enhance presentation units after decode (e.g., de-interlacing) or prepare the transport stream for long term storage. Decode
24、r operations may be split across two devices whereby one device acquires the transport stream and sends it, or a portion of it, to another device for playback. Decoder Clock A physical instantiation of the STC in the decoder. Encoder Clock A physical instantiation of the STC in the encoder. Jitter T
25、he variance in arrival time of data (from perfectly uniform arrival) due to various factors, including the process of packetizing data (into transport stream packets, PES packets, etc), and variable network delivery rates (e.g., in IP networks). Also refers to variance (vs. ideal theoretical values)
26、 of PCR. PCR Discontinuity See System Time-Base Discontinuity. Presentation For video, it is the first instant when a video presentation unit begins to be displayed to the end users eyeballs. For audio, it is first instant when an audio presentation unit begins to be rendered to the end users ears.
27、Presentation Unit See definition in ISO/IEC 13818-11. Partial TS A SPTS or a partial SPTS. Playback function A function that plays back a transport stream. An acquisition function and a playback function may but need not reside within the same physical device. System Time-Base Time-Base An epoch of
28、system time encoded in a transport stream where audio and video presentation units and their respective sample clocks are synchronized to a single STC abstraction. In general, the beginning and end of an epoch are demarcated by a system time-base discontinuity. However, in a receiver, a time-base ch
29、ange may be initiated by an asynchronous channel change. Note that this term is used in ISO/IEC 13818-11, but not completely defined. System Time-Base Discontinuity A point within a transport stream where one (system) time-base ends and another (system) time-base begins. System Time Clock (STC) An a
30、bstract common system clock with an exactly specified ratio to the CEA-CEB-20 4audio and video sample clocks. The Decoder and Encoder Clocks are physical instantiations of the STC. Transport stream MPEG-2 transport stream 5 Overview MPEG Systems provides the mechanisms for presentation synchronizati
31、on, e.g., audio/video synchronization. There are several MPEG fields and concepts which are used in this endeavor, described below. Implementers should note that each of the details described in this Consumer Electronics Bulletin are vital to achieving robust synchronization. This bulletin is constr
32、ained to Transport Stream timing (i.e. not Program Stream timing). ISO/IEC 13818-1 describes the timing models in terms of the Transport Stream constructs. It carefully does not (normatively) discuss the Encoder Clock or the Decoder Clock. The System Time Clock (STC) is an abstract clock from which
33、various physical clocks are instantiated. It is important when reading ISO/IEC 13818-11 to remember that it defines only the Transport Stream (bitstream) properties. These obviously place practical constraints on both the Encoder Clock and Decoder Clock. But conformant encoder and decoders are not d
34、efined by ISO/IEC 13818-11. Digital television emission equipment produces audio and video elementary streams (compressed audio and video), and wraps them together into a MPEG-2 Transport Stream. The encoders maintain a nominal 27 MHz clock, the Encoder Clock. The transport stream includes several t
35、ime values: Program Clock Reference (PCR), Presentation Time Stamp (PTS), and Decoding Time Stamp (DTS), all of which are created by the encoder from the Encoder Clock. The PCR values are periodically inserted into the Transport Stream, and used by the decoder to reconstruct the STC in the form of t
36、he Decoder Clock. PTS and DTS indicate when presentation units (video pictures, audio frames) should be shown to the viewer and when to decode presentation units, respectively. PCRs are required by ISO/IEC 13818-11 to arrive at the receiver at intervals no larger than 100 ms. Successive PCR values a
37、nd their arrival times are used by the decoder to reconstruct the STC in the form of the Decoder Clock. The PTS and DTS refer to a specific presentation unit (see ISO/IEC 13818-1 2.4.3.61). In general, PTS (and DTS) are included in the bitstream at intervals not exceeding 700 ms. ATSC constrains PTS
38、 (or DTS) to be inserted with each coded picture. All video and audio streams included in a program make use of the PCR and maintain a clock so that synchronization of the video and audio decoders can be accomplished. Decoders should implement the procedures detailed in this Bulletin in order to pre
39、sent programming that is properly synchronized. This means decoders should continuously monitor and take action on all PCR and PTS timestamps received. CEA-CEB-20 5 6 Receiver Architecture Model Figure 1 Receiver Hardware Reference Model There are various components of a receiver which are necessary
40、 to constitute the Decoder Clock and adjust the display time of presentation units in order to achieve synchronized output. 6.1 Demultiplexer PCR Assist The demultiplexer (or similar hardware) should have the capability to capture and store in registers available to the CPU (1) the value of a PCR fo
41、und in an acquired transport stream, and (2) the value of the Decoder Clock counter at the time the PCR arrived. These values are used as input to the Decoder Clock. The demultiplexer (or similar hardware) should have the capability to immediately set the value of the local Decoder Clock counter wit
42、h the contents of a received PCR. 6.1.1 PCR Arrival Interrupt Generation The demultiplexer should have the capability to generate an Interrupt to the CPU when a PCR is found in the selected program of the acquired transport stream. 6.1.2 Last PCR register A register accessible to the CPU which conta
43、ins the value of the last PCR received. 6.1.3 Last Decoder Clock at last PCR register A register accessible to the CPU which contains the value of the Decoder Clock counter at the moment the last PCR was received. 6.2 Decoder Clock The Decoder Clock is embodied within this receiver model as a closed
44、 feedback loop consisting of three elements: (1) Decoder Clock counter, (2) Controllable Oscillator, and (3) Feedback Loop. The startup and maintenance of a Decoder Clock is discussed in section 7. CEA-CEB-20 66.2.1 Decoder Clock counter A register accessible to the CPU which contains a counter incr
45、emented by the 27 MHz (nominal) controllable oscillator. This counter supplies the current Decoder Clock time and is used to properly time the display of audio and video presentation units and to assist with the elimination of the frequency variance between the Decoder Clock and Encoder Clock. 6.2.2
46、 Controllable Oscillator A 27 MHz (nominal) oscillator, with minor frequency adjustability accessible by the CPU. This oscillator defines the Decoder Clock frequency and is used to as a clock source to increment the Decoder Clock counter. It may be realized in a practical receiver by a VCXO or equiv
47、alent. ISO/IEC 13818-11 explicitly defines a STC rate tolerance of +/-810 Hz; the Controllable Oscillator should have pull-in range larger in order to accommodate actual implementation variance. 6.2.3 Feedback Loop A low pass filter that uses the difference between the PCR values and the Decoder Clo
48、ck counter values to eliminate Encoder Clock and Decoder Clock frequency variance. The output of the Feedback Filter is used to adjust the Controllable Oscillator. 6.3 Hardware for buffer management In general there are two kinds of butters that need to be managed on the decoder the compressed bitst
49、ream Presentation Unit buffer, and the buffer for the uncompressed Presentation Unit after decoding. Overflow of either buffer causes a problem in the decoder and can result in discontinuities in the audio or video playback and in a failure to maintain lip-sync. In order to manage these buffers the decoder has to first extract those PTS and DTS values that are available in the bitstream for some of the Presentation Units, and then derive PTS and DTS values for those Pre
