1、ETSI TR 1 O1 891 1.1.1 (2001-02) Technical Report Digital Video Broadcasting (DVB); Professional Interfaces: Guidelines for the implementation and usage of the DVB Asynchronous Serial Interface (ASI) COPYRIGHT European Telecommunications Standards InstituteLicensed by Information Handling Services2
2、ETSI TR 101 891 V1.l.l (2001-02) Reference DTWJTC-DVB-114 Keywords DVB ETSI 650 Route des Lucioles F-O6921 Sophia Antipolis Cedex - FRANCE Tel.: +33 4 92 94 42 O0 Fax: +33 4 93 65 47 16 Siret No 348 623 562 00017 - NAF 742 C Association but non lucratif enregistre la Sous-prfecture de Grasse (06) No
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6、extend to reproduction in all media. 8 European Telecommunications Standards Institute 2001. O European Broadcasting Union 2001. All rights reserved. ETSI COPYRIGHT European Telecommunications Standards InstituteLicensed by Information Handling Services3 ETSI TR 1 O1 891 V1.l.l (2001-02) Contents In
7、tellectual Property Rights 4 Foreword 4 Introduction 4 1 Scope 5 2 References 5 3 Abbreviations . 5 AS1 transmission links 6 5 Recommendations 8 6 Clarifications 8 4 History . 9 ETSI COPYRIGHT European Telecommunications Standards InstituteLicensed by Information Handling Services4 ETSI TR 101 891 V
8、1.l.l (2001-02) Intellectual Property Rights IPRs essential or potentially essential to the present document may have been declared to ETSI. The information pertaining to these essential IPRs, if any, is publicly available for ETSI members and non-members, and can be found in ETSI SR O00 314: “Intel
9、lectual Property Rights (IPRs); Essential, or potentially Essential, IPRs notified to ETSI in respect of ETSI standards“, which is available from the ETSI Secretariat. Latest updates are available on the ETSI Web server ClattD:/www.etsP.c/iDr. Pursuant to the ETSI IPR Policy, no investigation, inclu
10、ding IPR searches, has been carried out by ETSI. No guarantee can be given as to the existence of other IPRs not referenced in ETSI SR O00 314 (or the updates on the ETSI Web server) which are, or may be, or may become, essential to the present document. Foreword This Technical Report (TR) has been
11、produced by Joint Technical Committee (JTC) Broadcast of the European Broadcasting Union (EBU, Comit Europen de Normalisation ELECtrotechnique (CENELEC) and the European Telecommunications Standards Institute (ETSI). NOTE: The EBUETSI JTC Broadcast was established in 1990 to Co-ordinate the drafting
12、 of standards in the specific field of broadcasting and related fields. Since 1995 the JTC Broadcast became a tripartite body by including in the Memorandum of Understanding also CENELEC, which is responsible for the standardization of radio and television receivers. The EBU is a professional associ
13、ation of broadcasting organizations whose work includes the Co-ordination of its members activities in the technical, legal, programme-making and programme-exchange domains. The EBU has active members in about 60 countries in the European broadcasting area; its headquarters is in Geneva. European Br
14、oadcasting Union CH-1218 GRAND SACONNEX (Geneva) Switzerland Tel: +41 22 717 21 11 Fax: +41 22 717 24 81 Founded in September 1993, the DVB Project is a market-led consortium of public and private sector organizations in the television industry. Its aim is to establish the framework for the introduc
15、tion of MPEG-2 based digital television services. Now comprising over 200 organizations from more than 25 countries around the world, DVB fosters market-led systems, which meet the real needs, and economic circumstances, of the consumer electronics and the broadcast industry. Introduction The DVB As
16、ynchronous Serial Interface (ASI) i is a very popular standard interface for conveying MPEG-2 transport streams between professional equipment. However, there are concerns over interoperability in the market place, based on system integrators experiences with available equipment from multiple suppli
17、ers. The present document is intended to explain some of the causes of problems and to offer guidelines to AS1 implementers that will encourage maximum interoperability. ETSI COPYRIGHT European Telecommunications Standards InstituteLicensed by Information Handling Services5 ETSI TR 101 891 V1.l.l (2
18、001-02) 1 Scope The present document addresses interoperability issues specific to AS1 data transmission links, and explicitly is not concerned with general MPEG-2 interoperability issues. An example of an AS1 interoperability problem is where equipment receiving an AS1 data stream occasionally drop
19、s out of lock, or never achieves lock at all. An example of a problem not addressed by these guidelines is where the video and audio on the output of a decoder have poor clock stability, because of PCR clock recovery problems at some point in the end-to-end equipment chain, for example resulting in
20、LF wander in a regenerated PALJNTSC subcarrier. The present document contains a clause providing a description of the design issues confronting AS1 equipment designers. The present document also contains a recommendation clause, which provides simple measures to improve interoperability between AS1
21、equipment. There may be situations where systems will work outside these recommendations, depending on precise system and equipment implementation. 2 References The following documents contain provisions which, through reference in this text, constitute provisions of the present documen t. Reference
22、s are either specific (identified by date of publication, edition number, version number, etc.) or non-specific. For a specific reference, subsequent revisions do not apply. For a non-specific reference, the latest version applies. u1 EN 50083-9: “Cabled distribution systems for television, sound an
23、d interactive multimedia signals; Part 9: Interfaces for CATVEMATV headends and similar professional equipment for DVBMPEG-2 transport streams“ (DVB Blue Book AOlO), Annex B, Asynchronous Serial Interface. 121 ETSI ETR 290: “Digital Video Broadcasting (DVB); Measurement guidelines for DVB systems“.
24、3 Abbreviations For the purposes of the present document, the following abbreviations apply: AS1 Asynchronous Serial Interface tx-clk transmission clock rx-clk receiver clock ETSI COPYRIGHT European Telecommunications Standards InstituteLicensed by Information Handling Services6 ETSI TR 101 891 V1.l
25、.l (2001-02) 4 AS1 transmission links The AS1 is a uni-directional transmission link to transfer data between professional digital video equipment. Figure 1 presents an abstract model of an AS1 transmission link. The model represents signals at the Layer lLayer0 interface of figure B.l of EN 50083-9
26、 i. tx-clk n-clk Figure 1 : Abstract AS1 transmission model The diagram contains an AS1 transmission node, where data are held in a transmission buffer T. Data are read from this buffer at a constant rate determined by the transmission clock (tx-clk). This generates an isochronous data stream. One s
27、hould keep in mind that the AS1 is asynchronous. This gives implementers the freedom to deviate ftom isochronous data delivery. The diagram models this explicitly by including a “Variable Delay“ function. The modified stream is transported over the AS1 link to arrive at the receiver buffer R. Data a
28、re removed from this buffer at a constant rate, determined by the receiver clock (rx-clk). The abstract AS1 delivery model is used to make sure that the isochronous output stream from the receiver buffer is similar to the isochronous input stream to the variable delay function. A design issue in thi
29、s transmission model is that bytes need to be removed from the receiver buffer at a high enough data rate. To achieve this, the receiver clock frequency needs to be equal to or greater than the transmission clock frequency. If this is not the case, the receiver buffer will overflow. The present docu
30、ment assumes that the receiver clock is linked to the transmission clock, but is silent about how to achieve this in practice. When the receiver clock is linked to the transmission clock, the remaining design issue is to remove any aperiodicity introduced in the isochronous data stream. On the AS1 l
31、ink, the bytes can be displaced in time with respect to their isochronous position. This displacement can occur for a variety of reasons, e.g. technical convenience at the generating end of the link. The AS1 specification allows for unlimited time displacement of data bytes. To improve interoperabil
32、ity, AS1 implementations need to be subject to certain restrictions on the data transmission. The variable delay block in the diagram models the asynchronous transmission characteristics of the ASI. The aperiodicity can be expressed as a short-term change in the transport rate. A good starting point
33、 is the definition of data rate from the DVB Measurement Group that is contained in ETR 290 2. In this DVB specification, the data rate is averaged over a fixed time gate (called “window“). Each window contains a fixed number of N “time slices“. For the time slice number k, the notation Dk represent
34、s the number of bytes transferred in that period of time. The average data rate (in bytes per second) is then given by the formula: n=N-1 Dk-n z-rate = n=o T Where: (bytedsecond) N T is the integer number of time slices in the time gate window. the duration of the time gate window After each measure
35、ment the time gate window is shifted by one time slice unit of time, denoted by z = T/N . The rate specification permits different values for Nand T. For this analysis, it is assumed that the transmission clock equals the average rate expressed by the formula. As the actual rate may deviate from the
36、 average, we introduce the peak rate as the highest rate found in the N time slices. ETSI COPYRIGHT European Telecommunications Standards InstituteLicensed by Information Handling Services7 ETSI TR 101 891 V1.l.l (2001-02) The highest rate then fills the receiver buffer more rapidly than the average
37、 rate at which data is removed from the buffer. A temporary higher rate is followed by a temporary lower rate. The combined effect is that the receiver buffer should be empty at the end of the time gate window. An example is shown in figure 2. 50,400 603 50,350 503 50,300 50,250 50,200 50,150 403 30
38、3 203 103 1 11 21 31 41 51 61 71 81 91 11019283746566473829l103 Figure 2: Random aperiodic transport stream rate and buffer utilization In this example, a simulated data transport is shown with an average data rate of 50 kbids. The time gate window consists of 100 time slices. The number of bytes tr
39、ansferred in each time slice is shown in the left curve of figure 2. The rate varies ftom 45 kbids to 55 kbids. As the bytes are removed from the AS1 receiver buffer at the average rate, the buffer occasionally fills with excess bytes. The number of bytes in the receiver buffer is shown in the right
40、-hand curve. One can see that the buffer occasionally Underflows. Both the buffer underflow and the relatively large buffer size should be avoided in practical situations. Another example uses a more well-behaved stream. Here each transport packet is played out with a constant delay between the Sync
41、 bytes. The packets are transmitted in a burst with a constant data rate of 8 Mbids. After every packet a small gap is present where no data bytes arrive at the receiver. The data rate is much more constant, and the required receiver buffer also is much lower. 1 11 21 31 41 51 61 71 81 91 101 1 10 1
42、9 28 37 46 56 64 73 82 91 103 Figure 3: Deterministic aperiodic transport stream rate and buffer utilization Figure 3 shows that a deterministic aperiodic transport stream has a lower and more predictable buffer utilization for approximately the same variations in transport rate. The worst case rece
43、iver buffer size occurs when all data in the peak rate time slice is transmitted in one burst at the maximum AS1 link rate of 27 Mbids. In such a worst case scenario, the peak time slice transports all data and all other time slices carry no data. Both examples shown above highlight the fact that in
44、 an AS1 design careful consideration needs to be given to the properties of the delay variation of the link. The receiver buffer size, which is required to properly receive the data stream when the transmit and receive rates are linked, should be specified. Hence this measure is used in the recommen
45、dations below. ETSI COPYRIGHT European Telecommunications Standards InstituteLicensed by Information Handling Services8 ETSI TR 101 891 V1.l.l (2001-02) 5 Recommendations It is recommended that equipment manufacturers include the following information on their product data sheets: For equipment with
46、 an AS1 input, the size of the AS1 receiver buffer in transport packets that is available to remove AS1 transmission aperiodicity should be stated; For equipment with an AS1 output, the minimum AS1 receiver buffer size in transport packets required to remove the AS1 transmission aperiodicity it crea
47、tes should be stated. To maximize equipment interoperability, it is suggested that equipment with an AS1 output should be designed to work with equipment with a small AS1 receiver buffer. Additionally, an AS1 input should be designed with a receiver buffer that accommodates as wide a variety of AS1
48、outputs as possible within appropriate commercial constraints. The aperiodicity of some AS1 output streams and the receiver buffer size they require for proper reception is illustrated in the following diagrams. The signals represented in these diagrams occur at the Layer lLayer0 interface of figure
49、 B.l in EN 50083-9 i. In the first two diagrams below, the AS1 receiver buffer size required would be one MPEG-2 transport packet: 1 transport packet period . I 1 transport packet period . I 1 transport packet period r r 1 transport packet period . I 1 transport packet period . I 1 transport packet period r r Key: MPEG sync byte MPEG transport byte(s) 0 AS1 stuffing byte(s): K28,5 characters. In the following example, an AS1 receiver buffer of three packets would be required to remove the AS1 aperiodicity: 3 transport packet periods 6 Clarif
