This file is raw output from pdftotext and may not be ideal for distribution. If you are a maintainer for Hackipedia, please sit down when you have time and clean this text version up. Source PDF: /mnt/main/jmc-storage/docs/ATSC/A-78a Transport Stream Verification (May 9, 2007).pdf Like all conversions the text below should be fully readable as UTF-8 unicode text. --------------------------------------------------------------- ATSC Recommended Practice: Transport Stream Verification Document A/78A, 9 May 2007 Advanced Television Systems Committee, Inc. 1750 K Street, N.W., Suite 1200 Washington, D.C. 20006 Advanced Television Systems Committee Document A/78A The Advanced Television Systems Committee, Inc., is an international, non-profit organization developing voluntary standards for digital television. The ATSC member organizations represent the broadcast, broadcast equipment, motion picture, consumer electronics, computer, cable, satellite, and semiconductor industries. Specifically, ATSC is working to coordinate television standards among different communications media focusing on digital television, interactive systems, and broadband multimedia communications. ATSC is also developing digital television implementation strategies and presenting educational seminars on the ATSC standards. ATSC was formed in 1982 by the member organizations of the Joint Committee on InterSociety Coordination (JCIC): the Electronic Industries Association (EIA), the Institute of Electrical and Electronic Engineers (IEEE), the National Association of Broadcasters (NAB), the National Cable Television Association (NCTA), and the Society of Motion Picture and Television Engineers (SMPTE). Currently, there are approximately 140 members representing the broadcast, broadcast equipment, motion picture, consumer electronics, computer, cable, satellite, and semiconductor industries. ATSC Digital TV Standards include digital high definition television (HDTV), standard definition television (SDTV), data broadcasting, multichannel surround-sound audio, and satellite direct-to-home broadcasting. Contact information is given below. Mailing address Advanced Television Systems Commmittee, Inc. 1750 K Street, N.W., Suite 1200 Washington, D.C. 20006 Telephone 202-872-9160 (voice) 202-872-9161 (fax) Web site http://www.atsc.org E-mail standards@atsc.org The revision history of this document is given below. A/78 Revision History A/78 approved 20 September 2006 Spelling correction made to Table 8.1 2 November 2006 Corrigendum No. 1 approved 18 January 2007 A/78A approved 9 May 2007 2 Transport Stream Verification, Table of Contents 9 May 2007 Table of Contents 1. SCOPE 5 2. INTRODUCTION 5 3. INFORMATIVE REFERENCES 7 4. ERROR CHARACTERIZATION 7 4.1 Discussion of Error Classification 8 5. PSI ERRORS 9 5.1 PAT 9 5.2 PMT 9 6. PSIP ERRORS 10 6.1 MGT 11 6.2 TVCT 11 6.3 CVCT 12 6.4 RRT 12 6.5 EIT and ETT 13 6.6 STT 13 7. TIMING AND BUFFER ERRORS 14 7.1 PCR 15 7.2 PTS 16 7.3 Buffer Errors 17 8. CONSISTENCY ERRORS 17 9. GENERAL ERRORS 19 3 Advanced Television Systems Committee Document A/78A Index of Tables and Figures Table 5.1 PAT Error Conditions 9 Table 5.2 PMT Error Conditions 10 Table 6.1 MGT Error Conditions 11 Table 6.2 TVCT Error Conditions 11 Table 6.3 CVCT Error Conditions 12 Table 6.4 RRT Error Conditions 12 Table 6.5 EIT and ETT Error Conditions 13 Table 6.6 STT Error Conditions 14 Table 7.1 PCR Error Conditions 16 Table 7.3 Buffer Error Conditions 17 Table 7.2 PTS Error Conditions 17 Table 8.1 Consistency Errors 19 Table 9.1 General Errors 21 Figure 2.1 Reference analysis point in the DTV system. 6 4 ATSC Recommended Practice: Transport Stream Verification 1. SCOPE This Recommended Practice provides a common methodology for describing Transport Stream conformance criteria. This RP explicitly describes the elements and parameters of A/53 and A/65 that should be verified in an ATSC Transport Stream for it to be considered a proper emission. It does not cover RF, captioning or elementary streams. While the ATSC standards define strict limits for each parameter, in practice the severity of the error may depend upon the magnitude of the deviation. This document recommends severity levels associated with the ranges of deviation from the standard in these parameters. 2. INTRODUCTION The ATSC standards define the contents and characteristics of the emission Transport Stream1. There may be a large number of interactions and interrelationships amongst various components. Successful tuning and display of programs can be enabled if this Transport Stream adheres to the ATSC standards. The connection between the emission remultiplexer and the 8-VSB modulator is the reference analysis point assumed in this document, as shown in Figure 2.1. Please note that this diagram shows only a conceptual, functional block view; actual implementations will usually share MPEG data with other devices using the MPEG-2 Transport Stream (TS), as Elementary Streams (ES) or Packetized Elementary Streams (PES) cannot tolerate the introduction of any transmission errors and once synchronization is lost the system crashes. Furthermore, real systems may group the functions in different ways and will likely appear different on an electrical or physical block diagram. This RP uses terms and acronyms defined in ISO 13818-1, A/53 and A/65, and assumes a fair degree of familiarity with MPEG-2 systems as implemented per the ATSC standards. Readers that do not recognize the terms should read A/54 and A/69. 1. Note that the initial release of this document does not address Enhanced VSB. Page 5 Advanced Television Systems Committee Document A/78A Analysis Point * Figure 2.1 Reference analysis point in the DTV system. This Recommended Practice identifies transport stream issues by type, dividing errors into the following categories: • Section 5: PSI tables (PAT and PMT) • Section 6: PSIP tables (MGT, VCT, etc.) • Section 7: Timing Model and Buffering • Section 8: Consistency • Section 9: General Errors Each error type is also provided with an error severity, as listed below: • Transport Stream Off Air (TOA) • Program Off Air (POA) • Component Missing (CM) • Quality Of Service (QOS) • Technically Non-Conformant (TNC) 6 Transport Stream Verification Recommended Practice 9 May 2007 3. INFORMATIVE REFERENCES [1] ATSC: “Digital Audio Compression (AC-3, E-AC-3) Standard,” Doc. A/52B, Advanced Television Systems Committee, Washington, D.C., 14 June 2005. [2] ATSC: “ATSC Digital Television Standard, Parts 1, 3, 4, 5, and 6” Doc. A/53:2007, Advanced Television Systems Committee, Washington, D.C., 3 January 2007. [3] ATSC: “Program and System Information Protocol for Terrestrial Broadcast and Cable, with Amendment No. 1,” Doc. A/65C, Advanced Television Systems Committee, Washington, D.C., 2 January 2006 (Amendment No. 1 dated 9 May 2006). [4] ATSC: “Recommended Practice: Program and System Information Protocol Implementation Guidelines for Broadcasters,” Doc. A/69, Advanced Television Systems Committee, Washington, D.C., 25 June 2002. [5] ATSC: “Synchronization Standard for Distributed Transmission, Revision A,” Doc. A/110A, Advanced Television Systems Committee, Washington, D.C., 19 July 2005. [6] ETSI: “TR 101 290 V1.2.1, Digital Video Broadcasting (DVB): Measurement guidelines for DVB systems,” May 2001. [7] ISO: “ISO/IEC IS 13818-1:2000 (E), International Standard, Information technology – Generic coding of moving pictures and associated audio information: systems”. [8] ISO: “ISO/IEC 13818-4: 2004 (E), International Standard, Information technology – Generic Coding of Moving Pictures and associated audio information: Conformance Testing.” 4. ERROR CHARACTERIZATION As noted previously, in this document, errors are categorized as: • Transport Stream Off Air (TOA) • Program Off Air (POA) • Component Missing (CM) • Quality Of Service (QOS) • Technically Non-Conformant (TNC) The distinctions between these are important and should be taken into account both by equipment manufacturers of ATSC TS monitoring equipment and the users of that equipment. Considering these classes of errors, particular attention needs to be paid to the “TNC” or “Technically Non-Conformant” class. In Standards as complex as MPEG-2 and those from the ATSC, situations arise where two “shall” statements collide. At such junctures, the Transport Stream (TS) may be momentarily non-conformant. An example of this might be “PAT repetition error” (see Section 5). If the multiplexer is faced with a choice of outputting a video PID carrying a PTS on the schedule required by Section 7.2 or a repetition of the PAT within the requirements of Section 5, an implementer may choose to output the PTS sample rather than the PAT. Either way, the TS is for a moment non-conformant. Neither choice of non-conformance will affect any real-world receivers. Neither will result in disruption of service. Either will be non-conformant. There is no avoiding these conflicts, and they arise periodically in real-world equipment. 7 Advanced Television Systems Committee Document A/78A Should a manufacturer of MPEG analysis equipment choose to make either of these errors the cause of an operator alarm, after multiple false alarms the operator may ignore all alarms. That is not desirable. A number of the possible errors within this Section fall into these categories. While a continuous occurrence of any should constitute an operator alarm, a single occurrence of any should (as indicated by the table) be treated merely as a minor problem, which, unless re- occurring, is not significant. 4.1 Discussion of Error Classification An explanation of the error classification scheme is as follows: 1) Transport Stream Off Air (TOA): The station is effectively off-air as the Transport Stream errors are severe enough that transport level logical constructs are damaged beyond utility. Receivers will not be able to tune and decode anything within the broadcast. The complete or repeated absence of sync bytes1 would be an example of this level of error. 2) Program Off Air (POA): A main service (virtual channel) is flawed to the point that that service is effectively off air for conformant/reasonable receiver designs. This could involve all of the program elements being improperly constructed or incorrect/missing signaling about elements. The absence of an entry in the VCT for a service would be an example of this type of error. 3) Component Missing (CM): One of the program components that is signaled by PSIP or the PMT as present is either not present or cannot be found and decoded. One example would be a mismatch between the video PID signaled in the SLD and the actual PID used for the video elementary stream. 4) Quality of Service (QOS): Parameters are out of specification by such a margin that a significant fraction of the receivers can be expected to produce flawed outputs. In many cases, the broadcast is viewable, but may exhibit some form of degradation to the viewer. An example might be the MGT cycle time being somewhat longer than the specification, which would cause slower than normal tuning. 5) Technically Non-Conformant (TNC): Violates the letter of the standard, but in practice will have little effect on the viewing experience. Errors of this type should be corrected, but do not have the urgency of higher severity errors. An example might be a single instance of a 152 ms MGT cycle time (with the remainder of the MGTs coming at less than 150 ms intervals). In most cases the error threshold for what may appear to be escalating categorization is based on: 1) the official metric to twice the metric, 2) twice the official metric to five times the metric, and 3) over five times the metric. The nominal mathematical expression of this is shown below, where Tc is the metric for the cycle time and t is the time since the last arrival (note that for clarity of expression of the time intervals, this document ignores time advances during each millisecond increment): 1. Note that A/110A requires inversion of the sync byte value on a periodic basis. Such is compliant with ATSC standards, but is technically MPEG non-compliant. 8 Transport Stream Verification Recommended Practice 9 May 2007 1) Tc < t ≤ 2Tc 2) 2Tc < t ≤ 5Tc 3) 5Tc < t This scale can prevent “shall-statement collisions” from producing meaningless error alarms, yet provides guidance to equipment makers and users regarding severities. 5. PSI ERRORS An ATSC transport stream is also required to be MPEG-2 conformant (see Section 5 in reference [8]). Therefore, an ATSC transport stream must include the two mandatory Program Specific Information (PSI) tables. These two tables are known as the Program Association Table (PAT) and the Program Map Table (PMT). The syntax is defined within ISO/IEC 13818-1. The maximum interval for the PAT is specified in the ATSC standards as 100 ms. The maximum interval for the PMT is specified in the ATSC standards as 400 ms. Exceeding the interval on each of these tables by a small amount should not have a major impact on a receiver, especially since each ATSC conformant receiver should be able to fully tune to any ATSC channel through the use of the ATSC PSIP tables. 5.1 PAT Error conditions for the Program Association Table are classified in Table 5.1 Table 5.1 PAT Error Conditions Error Condition Error Qualifier TOA POA CM QOS TNC PAT repetition error PAT repetition interval error (100ms < cycle time ≤ 200ms) X PAT repetition error PAT repetition interval error (200ms < cycle time ≤ 500ms) X X PAT absence error PAT not found (cycle time > 500ms) X X X X X PAT syntax error Packet with PID 0x0000 doesn’t have table_id 0x00 X X X X X PAT syntax error CRC is incorrect for table_id 0x00 within PID 0x0000 1 X PAT syntax error scrambling_control_field is not ‘00’ for packet within PID X X X X X 0x0000 2 Legend: TOA: TS Off Air, POA: Program Off Air, CM: Component Missing, QOS: Quality of Service, TNC: Technically Non- Conformant Notes: 1) Each instance of an incorrect CRC should be interpreted as the table not being present. This occurrence should be considered as part of a repetition or absence error determination. 2) An error in the setting of the scrambling control field is most likely an indicator of a hard failure or incorrect setting and will persist until corrected. 5.2 PMT Error conditions for the Program Map Table are classified in Table 5.2. 9 Advanced Television Systems Committee Document A/78A Table 5.2 PMT Error Conditions Error Condition Error Qualifier TOA POA CM QOS TNC PMT repetition PMT repetition interval error (400ms < cycle time ≤ 800ms) x error PMT repetition PMT repetition interval error (800ms < cycle time ≤ x x error 2000ms) PMT absence error PMT not found (cycle time > 2000ms) x x x x PMT syntax error Packet with “PMT_PID” doesn’t have table_id 0x02 x x x x PMT syntax error CRC is incorrect for table_id 1 x PMT syntax error scrambling_control_field is not ‘00’ for packets containing x x x x PMT 2 PMT syntax error “PMT_PID” referenced by PAT not found x x x x Legend: TOA: TS Off Air, POA: Program Off Air, CM: Component Missing, QOS: Quality of Service, TNC: Technically Non- Conformant Notes: 1) Each instance of an incorrect CRC should be interpreted as the table not being present. This occurrence should be considered as part of a repetition or absence error determination. 2) An error in the setting of the scrambling control field is most likely an indicator of a hard failure or incorrect setting and will persist until corrected. 6. PSIP ERRORS PSIP (A/65) [3] is the glue that holds the digital television (DTV) signal together. PSIP is a voluntary standard of the ATSC and version C has been fully and completely adopted into the regulations of the Federal Communications Commission (FCC), so it is, in fact, a requirement in terms of actual real-world operation1. In most locations, multiple DTV stations can be received— and in some cases, from multiple markets. The purpose of PSIP is to describe the information at the system and event levels and to enable an abstract of the collection of programs called a virtual channel. There are two main categories of information in the ATSC PSIP Standard, system information and schedule data. System information allows navigation among and access to the channels currently within the DTV transport stream. It is similar to the PSI data discussed elsewhere in this document. The schedule data provides necessary information for efficient browsing and selection of current and future events. There are two forms of virtual channel tables defined by the ATSC standards: Terrestrial (TVCT) and Cable (CVCT). A terrestrial broadcast must carry a TVCT and may optionally carry a CVCT. The ATSC standards require the carriage of up to 12 hours of EIT information (EIT0-3). Carriage of more EITs is recommended, but not required. Because EIT-0 carries signaling information about the current event, a more rapid cycle time has been set for it. EIT1-3, while required to be present, may arrive less frequently. For purposes of this document, the recommendations in the PSIP Recommended Practice (A/69) [4] have been utilized. The ATSC standards set limits on the cycle times and allowed inaccuracy of the time carried in the System Time Table (STT). While a receiver is tuned to a given station, the STT gives the receiver that channel’s current time. If there are differences between the STT times carried by 1. The current version of A/65 contains additional features and capabilities that are not covered by this RP. 10 Transport Stream Verification Recommended Practice 9 May 2007 different stations, then viewers can experience problems such as missing the beginning of programs or incorrect PVR switching. 6.1 MGT Error conditions for the Master Guide Table are classified in Table 6.1. Table 6.1 MGT Error Conditions Error Condition Error Qualifier TOA POA CM QOS TNC MGT repetition MGT repetition interval error (150ms < cycle time ≤ 300ms) X error MGT repetition MGT repetition interval error (300ms < cycle time ≤ 750ms) X X error MGT absence error MGT not found (cycle time > 750ms) X X X X X MGT syntax error CRC is incorrect for table_id 0xC7 1 X MGT syntax error scrambling_control_field is not ‘00’ for packets containing X X X X X MGT 2 Legend: TOA: TS Off Air, POA: Program Off Air, CM: Component Missing, QOS: Quality of Service, TNC: Technically Non- Conformant Notes: 1) Each instance of an incorrect CRC should be interpreted as the table not being present. This occurrence should be considered as part of a repetition or absence error determination. 2) An error in the setting of the scrambling control field is most likely an indicator of a hard failure or incorrect setting and will persist until corrected. 6.2 TVCT Error conditions for the Terrestrial Virtual Channel Table are classified in Table 6.2. Table 6.2 TVCT Error Conditions Error Condition Error Qualifier TOA POA CM QOS TNC TVCT repetition TVCT repetition interval error (400ms < cycle time ≤ X error 800ms) TVCT repetition TVCT repetition interval error (800ms < cycle time ≤ X X error 2000ms) TVCT absence TVCT not found (cycle time > 2000ms) X3 X X X X error TVCT syntax error CRC is incorrect for table_id 0xC8 1 X TVCT syntax error scrambling_control_field is not ‘00’ for packets containing X3 X X X X TVCT 2 Legend: TOA: TS Off Air, POA: Program Off Air, CM: Component Missing, QOS: Quality of Service, TNC: Technically Non- Conformant Notes: 1) Each instance of an incorrect CRC should be interpreted as the table not being present. This occurrence should be considered as part of a repetition or absence error determination. 2) An error in the setting of the scrambling control field is most likely an indicator of a hard failure or incorrect setting and will persist until corrected. 3) PSIP system is off-air. Tuning algorithms based on PSIP will fail. The FCC rules require PSIP (A/65B). 11 Advanced Television Systems Committee Document A/78A 6.3 CVCT Error conditions for the Cable Virtual Channel Table are classified in Table 6.3. Table 6.3 CVCT Error Conditions Error Condition Error Qualifier TOA POA CM QOS TNC 1 CVCT CVCT repetition CVCT repetition interval error (400ms < cycle time ≤ X error 800ms) CVCT repetition CVCT repetition interval error (800ms < cycle time ≤ X X error 2000ms) CVCT absence CVCT not found (cycle time > 2000ms) X4 X X X error CVCT syntax error CRC is incorrect for table_id 0xC8 2 X CVCT syntax error scrambling_control_field is not ‘00’ for packets containing X4 X X X CVCT 3 Legend: TOA: TS Off Air, POA: Program Off Air, CM: Component Missing, QOS: Quality of Service, TNC: Technically Non- Conformant Notes: 1) The CVCT is not required to be present in a terrestrial broadcast. 2) Each instance of an incorrect CRC should be interpreted as the table not being present. This occurrence should be considered as part of a repetition or absence error determination. 3) An error in the setting of the scrambling control field is most likely an indicator of a hard failure or incorrect setting and will persist until corrected. 4) CVCT generator is off-air. Tuning algorithms based on CVCT will fail. 6.4 RRT Error conditions for the Rating Region Table are classified in Table 6.4. Table 6.4 RRT Error Conditions Error Condition Error Qualifier TOA POA CM QOS TNC RRT 1 RRT repetition RRT repetition interval error (60,000ms < cycle time X error ≤120,000ms) RRT repetition RRT repetition interval error (120,000ms < cycle time ≤ X X error 300,000ms) RRT absence error RRT not found (cycle time > 300,000ms) X4 X X RRT syntax error CRC is incorrect for table_id 0xCA 2 X RRT syntax error scrambling_control_field is not ‘00’ for packets containing X4 X X RRT 3 Legend: TOA: TS Off Air, POA: Program Off Air, CM: Component Missing, QOS: Quality of Service, TNC: Technically Non- Conformant Notes: 1) RRT is not required to be present for Rating Region 1. If the RRT is signaled in the MGT, then these error condi- tions apply. 2) Each instance of an incorrect CRC should be interpreted as the table not being present. This occurrence should be considered as part of a repetition or absence error determination. 3) An error in the setting of the scrambling control field is most likely an indicator of a hard failure or incorrect setting and will persist until corrected. 4) PSIP component missing. 12 Transport Stream Verification Recommended Practice 9 May 2007 6.5 EIT and ETT Error conditions for the Event Information Table and Extended Text Table are classified in Table 6.5. Table 6.5 EIT and ETT Error Conditions Error Condition Error Qualifier TOA POA CM QOS TNC EIT-0 repetition error EIT-0 repetition interval error (500ms < cycle time ≤ X3 1000ms) EIT-0 repetition error EIT-0 repetition interval error (1000ms < cycle time ≤ X4 X3 2500ms) EIT-0 absence error EIT-0 not found (cycle time > 2500ms) X4 X4 X4 X3 EIT syntax error CRC is incorrect for table_id 0xCB 1 X EIT syntax error scrambling_control_field is not ‘00’ for packets containing X5 X X EIT 2 EIT-1 repetition error EIT-1 repetition interval error (3 seconds < cycle time ≤ 6 X6 seconds) EIT-1 repetition error EIT-1 repetition interval error (6 seconds < cycle time ≤ 15 X6 X6 seconds) EIT-1 absence error EIT-1 not found (cycle time > 15 seconds) X6 X6 X6 EIT-2, EIT-3 repetition EIT-2, EIT-3 repetition interval error (1 minute < cycle time X6 error ≤ 2 minutes) EIT-2, EIT-3 repetition EIT-2, EIT-3 repetition interval error (2 minutes < cycle X6 X6 error time ≤ 5 minutes) EIT-2, EIT-3 absence EIT-2, EIT-3 not found (cycle time > 5 minutes) X6 X6 X6 error ETT syntax error CRC is incorrect for table_id 0xCC 1 X ETT syntax error scrambling_control_field is not ‘00’ for packets containing X 5 X X ETT 2 Legend: TOA: TS Off Air, POA: Program Off Air, CM: Component Missing, QOS: Quality of Service, TNC: Technically Non- Conformant Notes: 1) Each instance of an incorrect CRC should be interpreted as the table not being present. This occurrence should be considered as part of a repetition or absence error determination. 2) An error in the setting of the scrambling control field is most likely an indicator of a hard failure or incorrect setting and will persist until corrected. 3) This condition is technically compliant, but violates the A/65 recommended interval (in the U.S., the FCC recom- mended interval). 4) EIT-0 is required. Certain receivers may have difficulty tuning and EPG display may be affected. 5) PSIP component missing – may or may not affect behavior of receiver. 6) EIT1–3 are required. EPG display may be affected if missing. Recommended timings for EIT1–3 are given in A/69 (PSIP Recommended Practice). 6.6 STT Error conditions for the System Time Table are classified in Table 6.6. 13 Advanced Television Systems Committee Document A/78A Table 6.6 STT Error Conditions Error Error Qualifier TOA POA CM QOS TNC Condition STT repetition STT repetition interval error (1000ms < cycle time ≤ 2000ms) X error STT repetition STT repetition interval error (2000ms < cycle time ≤ 5000ms) X X error STT absence STT not found (cycle time > 5000ms) X2 X X error STT syntax CRC is incorrect for table_id 0xCD 1 X errors STT time STT time value is more than 30 seconds away from current X 2, 3 X X value error correct GPS second_count (including GPS_UTC_offset impact) Legend: TOA: TS Off Air, POA: Program Off Air, CM: Component Missing, QOS: Quality of Service, TNC: Technically Non- Conformant Notes: 1) Each instance of an incorrect CRC should be interpreted as the table not being present. This occurrence should be considered as part of a repetition or absence error determination. 2) PSIP component missing. 3) This error is considered large enough to have a noticeable impact on users who tune/record by time of day. 7. TIMING AND BUFFER ERRORS Timing is the key to the MPEG-2 encoding and decoding processes. MPEG-2 Systems (ISO/IEC 13818-1) [7] defines a model for the system timing, adherence to which allows independent design of encoders and decoders that can interoperate. An MPEG-2 decoder’s 27 MHz reference clock needs to be synchronized with the equipment that is creating the encoded stream. In order to achieve this synchronization, PCR (Program Clock Reference) 27 MHz clock timestamps are sent within the stream at a rate frequent enough to re-synchronize the decoder with the encoder clock. Multiple conditions can cause the decoder clock to get out of sync with the encoder: • Incorrect PCR timestamps inserted in the TS. This condition can send the decoder reference clock out of sync with the encoder clock. • PCR not inserted often enough. This condition could cause the decoder reference clock to drift away from the encoder clock as it does not receive resynchronized timestamps often enough. • Jitter introduced during the delivery process (packets arriving early or late, which creates drift of the decoder clocks and causes buffer underflow or overflow). A referenced clock is used during the decoding process to indicate when to move data between buffers and to indicate when a frame should be decoded and presented to the stream. Null packets are used to maintain a constant bit rate transport stream for the 8-VSB modulator. Some facilities see these Null packets as opportunistic packets and replace them with private data packets. This process should not impact a normal ATSC receiver. Although, physically dropping a Null packet prior to modulation, or replacing one Null packet with two private data packets, will negatively impact the PCR timing, and possibly the 8-VSB modulator also. This is why re- multiplexers must take great care to accurately restamp all PCR, PTS, and DTS values whenever transport packets are shifted in time. 14 Transport Stream Verification Recommended Practice 9 May 2007 The PTS (Presentation Time-Stamp) and DTS (Decoding Timestamps) are based on the PCR and are used by the decoder to sequence the decoding process, and to manage the decoding buffer. If the local decoder clock is not synchronized with the encoder, it may affect the decoding process by providing incorrect timing. The consequences of de-synchronization can be buffer overflow or underflow, or presentation timing mismatch. The result of this error can lead to freeze or missing frames on the display. Another result of this scenario could be poor audio/video lip- synchronization. Buffer, PTS, and DTS errors can also be introduced by encoders and multiplexers outside of PCR errors (see Section 5 in reference [8] for timestamp accuracy and consistency). They can also create some timing errors, resulting in missing or out of sync decoded elements. Note: Actual decoders may have a larger buffer than the minimum required by MPEG-2, therefore are capable of absorbing some effects of timing errors and buffer overflow. The standard provides a minimum amount of RAM so that a theoretical model will never underflow or overflow. Some receivers may provide more memory than is required by the standard. Therefore, it is possible to have some receivers working well while others fail. A model using the minimum amount of RAM as specified by the standard may fail, while another model with 50 percent more RAM works flawlessly. This is a sign that the T-STD buffers in the encoder are either underflowing or overflowing. Meeting these buffer requirements does not guarantee a high quality program, but it does mean that the program can be decoded by any receiver that complies with the ATSC standard. A maximum data rate for PSI and PSIP has been defined to keep the receiver microprocessor from overflowing. A maximum data rate of 250 kbps is defined for each of the following: 1) PSIP Base PID (0x1FFB) 2) EIT PIDs 3) ETT PIDs The upper limit for the PSI is 80 kbps, but this is only applicable to the PAT PID and the PMT PID of the program being decoded. 7.1 PCR Error conditions for the Program Clock Reference are classified in Table 7.1. 15 Advanced Television Systems Committee Document A/78A Table 7.1 PCR Error Conditions Error Condition Error Qualifier TOA POA CM QOS TNC 1 PCR PCR error Unsignaled PCR discontinuity X X PCR repetition error PCR repetition interval error (100ms < cycle time ≤ X 200ms) PCR repetition error PCR repetition interval error (200ms < cycle time ≤ X X 500ms) PCR absence error PCR not found (cycle time > 500ms) X X X X PCR error 500 ns < ⏐ PCR inaccuracy⏐ ≤ 2500 ns 2 X PCR error ⏐ PCR inaccuracy⏐ > 2500 ns X X PCR related 810 Hz < PCR frequency offset ≤ 4050 Hz X parameters PCR related PCR frequency offset > 4050 Hz) X X parameters PCR related 75 milliHerz/second (mHz/s) < PCR frequency drift ≤ X parameters 375 mHz/s PCR related PCR frequency drift > 375 mHz/s X X parameters PCR related 25 μS < PCR overall jitter ≤ 125 μs X parameters PCR related PCR overall jitter > 125 μS X X parameters Legend: TOA: TS Off Air, POA: Program Off Air, CM: Component Missing, QOS: Quality of Service, TNC: Technically Non- Conformant Notes: 1) ETSI TR 101 290 v1.2.1 Section 5.3 and Annex I [6] provide measurement techniques for these PCR-related parameters. The thresholds therein are explicitly not part of this recommendation and should be ignored in favor of those in the table. 2) “⏐ PCR inaccuracy⏐ ” refers to the absolute value of the PCR inaccuracy. 7.2 PTS Error conditions for the Presentation Time-Stamp are classified in Table 7.2. 16 Transport Stream Verification Recommended Practice 9 May 2007 Table 7.2 PTS Error Conditions Error Condition Error Qualifier TOA POA CM QOS TNC PTS interval error 2 700 ms < Interval between coded PTS values ≤ X 1400 ms 3 PTS interval error 1400 ms < Interval between coded PTS values ≤ X X 3500 ms 3 PTS absence error Interval between coded PTS values > 3500 ms 3 X X X PTS increment error PTS time not incrementing at the reciprocal of the X frame rate 1 Legend: TOA: TS Off Air, POA: Program Off Air, CM: Component Missing, QOS: Quality of Service, TNC: Technically Non- Conformant Notes: 1) Any discrepancy in the correlation of the timing between the PTS time and the frame rate could result in decoding buffer overflow or underflow. This could be manifested by the display of missing or frozen frames at an interval that depends on the size of the offset between the reciprocal of the PTS time and the frame rate. This could also contribute to changes in the apparent audio/video synchronization (“lip synch”). 2) A/53 places tighter constraints on video PES construction; see the section on PES constraints in A/53. 3) PTS values are required with intervals not exceeding 700 ms in each elementary audio and video stream. These time intervals are measured in presentation time, that is, in the same context as the values of the fields, not in terms of the times that the fields are transmitted and received. 7.3 Buffer Errors Buffer errors are classified in Table 7.3 Table 7.3 Buffer Error Conditions Error Error Qualifier TOA POA CM QOS TNC Condition Buffer errors Overflow of transport buffer X Buffer errors Overflow of system information buffer X Buffer errors Overflow of MPEG-2 Video buffer X Buffer errors Underflow of MPEG-2 Video buffer X X Buffer errors Overflow of AC-3 Audio buffer X Buffer errors Underflow of AC-3 Audio buffer X X PSIP A/65 maximum PSIP bandwidth (250kb/s for base, 250kb/s for X X bandwidth EIT and 250kb/s for ETT) 1 Legend: TOA: TS Off Air, POA: Program Off Air, CM: Component Missing, QOS: Quality of Service, TNC: Technically Non- Conformant Note: 1) Exceeding this limit may result in degradation of other services. 8. CONSISTENCY ERRORS Before a receiver can decode a transport stream, it must identify the relationship between components in the stream. Some components contain audio and video (Elementary Streams), and other components contain information describing the relationship between them (Metadata). The receiver uses metadata to identify each component, determine its function and select an appropriate set of components when the user selects a virtual channel for decoding. Conflicts and problems within the structure of metadata are called ‘consistency errors.’ Consistency errors can 17 Advanced Television Systems Committee Document A/78A result in broken decoding, missing system components (such as closed captioning), and/or missing program guide information. This section covers the types of errors that can cause these problems. Broadcasters need to concern themselves with two kinds of metadata. Metadata defined by the ATSC is called “Program and System Information Protocol” (PSIP), and Metadata defined by MPEG is called “Program Specific Information” (PSI). Some of the functions of PSI are duplicated by PSIP. The duplication creates alternative means of locating system components; problems arise if they conflict. There are three categories of metadata consistency errors: external, internal, and collision errors. External consistency errors occur when the PSIP and PSI information do not agree with one another. These errors typically occur when more than one device in the system is configured to generate metadata. If the PSIP is generated by one system device, and the PSI is generated by another system device, then it is possible for the two devices to get out of ‘sync’ with each other. For example, the PMT is a PSI table, and the TVCT is a PSIP table. Both tables contain the program_number field. Suppose the PMT (generated on a multiplexer) lists one value for the program_number, and the TVCT (generated in a PSIP generator) contains a different one. The receiver is forced to decide between the PSIP and the PSI, and cannot determine which one is correct. The response of a receiver to a specific external consistency error depends upon the receiver implementation. Two receivers from different vendors will frequently react differently to the same external table inconsistency. Internal consistency errors occur when metadata contains references that are wrong but do not cross the boundary between PSI and PSIP. For example, the Program Map Table (PMT) is a PSI table. One of the data fields in the PMT is the PCR_PID. If the value of the PCR_PID field is incorrect, then receivers may have difficulty displaying video and audio. This is an internal constancy error because the PCR_PID value is found in a PMT (a PSI table) and not carried in any PSIP table. Internal consistency errors are not limited to PSI tables; they can occur in PSIP tables too. Consider the Master Guide Table (MGT) and the Channel Extended Text Table (Channel ETT), both of which are PSIP tables. Correctly formatted PSIP requires an MGT, but the Channel ETT is an optional PSIP table. If the MGT (required table) contains a reference to the Channel ETT (optional table), but the Channel ETT does not exist in the stream, then the resulting stream has a PSIP internal consistency error. Internal consistency errors can cause receivers to search for absent stream elements, or fail to find existing elements. Program guide information, closed captioning, and secondary audio channels are examples of program elements that may be affected by internal consistency errors. Collision errors are the third kind of consistency error. They occur when similar metadata is generated on different devices in a system, and then multiplexed together into the same output. For example, multiplexers and PSIP generators are frequently capable of generating PSI. Consider the case where PSI is generated on a multiplexer, and also generated by a PSIP generator. Unless steps are taken to consolidate the PSI from these two sources, the two different versions of PSI will collide at the output of the multiplexer. The output will contain alternating versions of PSI from each source (in this case the multiplexer and PSIP generator version). Many receiver implementations will simply fail to decode in the presence of a collision error. 18 Transport Stream Verification Recommended Practice 9 May 2007 Consistency errors are classified in Table 8.1. Table 8.1 Consistency Errors Error Condition Error Qualifier TOA POA CM QOS TNC TSID values in PAT and VCT X X X X X (transport_stream_id) do not match 1 PAT/VCT mismatch 2 Different number of programs found in VCT X X X than signaled in PAT 3 VCT/PMT mismatch SLD/PMT mismatch (number of services) X X X X VCT/PMT mismatch SLD/PMT element mismatch (different X X X “parameters” for matching program elements) PMT/EIT-0 descriptor Mismatch in duplicated descriptors for X X X mismatch 4 current event between PMT and EIT-0 ETT syntax errors ETT has invalid ETM_ID or ETM_ID does X5 X X not match existing event_id in EIT (excludes channel ETT) ETT syntax errors ETT has ETM_ID of channel ETT, but MGT X X does not flag channel ETT on this PID Multiple sources of PSI Version numbers for particular PSI tables X X X X X should never decrease (except at wraparound) 6 Daylight Savings time settings STT contains invalid values for Daylight X Savings time switchover Service Location Descriptor No Service Location Descriptor in VCT X X X X missing from VCT Dangling source_id source_id mismatch (either source_id in X X X X VCT does not have a corresponding source_id in EIT or source_id in EIT does not have a corresponding source_id in VCT) MGT mismatch Version number and/or size of tables X X signaled in MGT does not match with actual table 7 MGT mismatch PSIP table found in stream, but not signaled X in MGT Legend: TOA: TS Off Air, POA: Program Off Air, CM: Component Missing, QOS: Quality of Service, TNC: Technically Non- Conformant Notes: 1) Receivers may not tune if these fields do not agree. 2) While some receivers may operate correctly, others may not display the program in this case. 3) Conditional on the state of the hidden flag in the VCT, whether the virtual channel is digital or analog and whether the virtual channel is in another transport 4) This error condition may cause captioning to not work. 5) The text description of the event is the missing component. 6) This condition is indicative of multiple sources of PSI tables (from both multiplexer and PSIP generator). 7) Tables signaled differently than the actual parameters can lead to variable behavior, dependant on the receiver and the actual table. 9. GENERAL ERRORS The errors listed within this Section cover a variety of types of problems (typically transport- related). For those listed in only the QOS and TNC columns, a single occurrence should be treated 19 Advanced Television Systems Committee Document A/78A merely as a minor problem, which, unless periodically re-occurring, is not of concern. A repeated occurrence should warrant investigation, as it might be indicative of a device approaching total failure. If the station is using Distributed Transmission systems (as defined by A/110A [5]), then test equipment not equipped for A/110A can be expected to indicate a TS Synch Loss error every 624 packets, as the synch byte is altered at that time per A/110A. Some types of errors do occur which are not listed in the following table. Specifically, they are: • One or more transport packets were detected with a PID value that was not part of a properly signaled service. This situation is sometimes referred to as an “orphan PID”. • Some TS may also contain DVB or ARIB system tables in addition to PSIP. All of these tables have fixed PID assignments and do not appear in the PMT. User notification of such occurrences in a given TS may need to be conditioned by user expectations rather than treated as errors. Note: Multiple MPEG-2 Registration Descriptors within a loop may be a commonly encountered error as the rules for usage were originally unclear and only clarified after significant amounts of equipment were deployed. General errors are classified in Table 9.1. 20 Transport Stream Verification Recommended Practice 9 May 2007 Table 9.1 General Errors Error Condition Error Qualifier TOA POA CM QOS TNC TS Synch Loss 1 Two or more sync bytes are corrupt (not 0x47) X X X X X Sync Byte Error 1 Single Sync byte is not 0x47 X X Cadence Sync byte One or more Cadence Sync bytes corrupt X X error 2 Cadence Sync byte Cadence Sync byte has not been found in 624 packets X X error 2 Cadence Sync byte Cadence Sync byte has not been found in 3120 packets X X X X X error 2 Continuity Count Packets have been lost X X Errors Transport error 3 transport_error_indicator in TS packet header is set X Multiple registration Multiple registration descriptors in any given iteration of X descriptors a descriptor loop 4 PID values below PID values below 0x30 are reserved for specific X 0x30 are used in applications as defined in the relevant standards. Any the TS other use of these PID values is an error. Missing Descriptors One or more required descriptors were not found in the X X X stream 5 Legend: TOA: TS Off Air, POA: Program Off Air, CM: Component Missing, QOS: Quality of Service, TNC: Technically Non- Conformant Notes: 1) See also special case of Distributed Transmission Adapter (Cadence Sync Byte) [5]. 2) Only of significance if Distributed Transmission (A/110A [5]) is in use and looking at the output of the Distributed Transmission Adapter. 3) Transport sync errors are indicative of missing packets. Missing packets are treated elsewhere in this document. 4) The error may be more severe, depending upon the situation. 5) The following descriptors are required to be present in the stream: AC-3 Audio Descriptor, Program Smoothing Descriptor, ISO-639 Language Descriptor (until March 2008) and Service Location Descriptor. 21 Advanced Television Systems Committee, Inc. 1750 K Street, N.W., Suite 1200 Washington, D.C. 20006