Buffer Analysis MPEG-2 TS Analysis & Monitoring
Contents
1. Products R amp S DVM50 R amp S DVM100 R amp S DVM100L R amp S DVM120 R amp S DVM400 R amp S ETX Buffer Analysis MPEG 2 TS Analysis amp Monitoring Application Note The new buffer analysis feature was introduced with firmware version 2 80 for the R amp S DVM family This application note describes this feature and explains how it represents a valuable asset to your daily business operations This application note first provides background information and then explains how monitoring and measurement have been implemented in the R amp S DVM family amp ROHDE amp SCHWARZ Subject to change H Gsoedl 11 2005 7BM53_0E Contents 7BM53_0E Buffer Analysis 1 e TEEN 3 2 WE OC UC OI EEN 3 3 en CT 4 4 Transport Stream System Target Decoder in MPEG22 5 5 Buffer Architecture Overview ccccecccseccececeeeeeeseseeeseceeeeeeeceeeeeeeaeees 5 6 Different Buffer Models for Video Audio and System Information 6 MPEG DEE 6 Advanced video coding AVC video 7 MPE 1 2 0 0 en een 7 System Titelen Le ME 7 7 R amp S DVM Graphical User Interface OvervieW eee 8 8 R amp S DVM Buffer Monitoring in Accordance with ETSI TR 101 290 12 MPES2YIIEO a een 12 MPEG 1 2 ie e a nee ne 13 9 R amp S DVM Buffer Analysis for AVC Vuen 14 VORP de EE 15 MA TMCV ACA aae a E A AE EAEE E AEE E 15 12 Additional Information ccccccccee2. 4 Rohde amp Schwarz Buffer Analysis 4 Transport Stream System Target Decoder in MPEG 2 The systems layer of the MPEG 2 standard specifies transport stream system target decoders T STD The purpose of a T STD is to ensure that transport streams that have passed this theoretical decoder can be decoded with any true decoder You can say that the T STD buffer model embodies the timely and controlled delivery of data VIDEO Decoder Video packets 1 l gt gt V AUDIO pm e E i Presentation Audio packets Bay ps YY TB transport stream buffer Fig 2 Part of T STD model B main buffer EB elementary stream buffer MB multiplex buffer Each and every service has to be decoded with its own target decoder As Fig 2 shows a single decoder path is introduced for each ES such as audio and video services Additionally the system information such as PAT and PMT also has a specific decoding path 5 Buffer Architecture Overview 7BM53_0E As Fig 2 shows every decoder path includes the transport buffer TB The task of this 4096 bit storage is to uncouple the decoder from the TS data rate in order to decode transport streams with variable transmission rates Video elementary streams have to pass the multiplex buffer MB and elementary stream buffer EB In contrast audio and system data is directed to one main buffer B The transfer rates between the buffers shown in
3. Video3 Program 2 Program 3 MPEG 2 Multiplexer Encoder Audio3 COMPRESSION MULTIPLEXING TRANSMISSION Fig 1 Generation transmission end To make the transmission of multiple services in one channel possible the content of each program is divided into small packets after compression encoder These packets are multiplexed into one stream at a constant bit rate The MPEG 2 transport stream TS has been created in accordance with ISO IEC 18131 This content is fed to the transmitter 3 Rohde amp Schwarz 3 Requirements 7BM53_0E Buffer Analysis The TS that is created may contain a large number of independent services Each service often consists of more than one elementary stream ES e g video audio data The bit rate in an ES commonly varies over time e g I Band P frames within a video ES Consider the following example If you look at the ES of video you have specific elements commonly referred to as frames see 4 which are large in size compared to the rest of the stream Now assume that the TS used has a bit rate just high enough to hold one peak at any given time If a second peak from another ES in the multiplex coincidentally occurs at the same time the TS would not be able to hold and send out the data in a manner that allows a receiver to decode both sets of content properly This leads to the challenge of fitting these services into one TS at a fixed limited bit rate To ensure proper
4. as the size of the MB depend on the content With regard to the technologies for transferring data between the MB and the EB there are a few differences as compared to MPEG 2 video e Leak method As described under the buffer model for MPEG 2 video the data transfer rate between the MB and the EB is constant However a different equation is used to determine the rate as a function of the profile and level applied e Hypothetical reference decoder HRD method A hypothetical reference decoder consisting of a coded picture buffer CPB fed by a hypothetical stream scheduler HSS and a decoder free of any time lag is used to examine the compliance of a TS with existing standards A total of 32 different CPB and HSS pairs can exist for just one TS Thus the transmission of a data stream with different data rates is possible In the T STD model the EB is equivalent to the CPB and the MB is equivalent to the HSS MPEG 1 2 audio All TS packets of the audio elementary streams are fed to the audio TB The data is read out at 2 Mbit s for MPEG 1 and MPEG 2 Only payload information from the TS packets is written to B Data transmitted twice is transferred only once B has a size of 28672 bits At the time of decoding all data of an access unit in the main buffer is instantly removed 4096 bits 28672 bits Decoder Fig 5 Audio path of the T STD TB transport stream buffer B main buffer System information All TS packets with a P
5. 02 DVM B1 Analyzer Board 2085 3283 02 DVM K1 Additional TS Input 2085 5211 02 DVM K10 In Depth Analysis 2085 5228 02 DVM400 Base Unit 2085 1800 02 DVM400 B 1 Analyzer 2085 5505 02 DVM K1 Additional TS Input 2085 5211 02 DVM K2 TS Capture 2085 5234 02 DVM K11 Data Broadcast Analysis 2085 5311 02 DVM400 B2 TS Generator 2085 5511 02 DVM400 B3 Upgrade TS Recorder up to 90 2085 5528 02 Mbit s DVM400 B4 Upgrade TS Recorder up to 2085 5534 02 214 Mbit s DV TCM Test Card M Streams 2085 7708 02 DV HDTV HDTV Sequences 2085 7650 02 DV ASC Advanced Stream Combiner 2085 8804 02 DVM DCV Documentation of Calibration 2082 0490 29 Values Service Manual 2085 1839 02 For additional information about MPEG 2 measurement equipment see the Rohde amp Schwarz website www rohde schwarz com ROHDE amp SCHWARZ ROHDE amp SCHWARZ GmbH amp Co KG M hldorfstra e 15 D 81671 Munchen Postfach 80 14 69 D 81614 Munchen Tel 089 4129 0 Fax 089 4129 13777 Internet http Awww rohde schwarz com This application note and the supplied programs may only be used subject to the conditions of use set forth in the download area of the Rohde amp Schwarz website 7BM53_0E 16 Rohde amp Schwarz
6. Fig 2 the buffer size and the decoding times can be either computed descriptor information or derived DTS from the content An access unit is defined as a coded representation of a video or audio frame An access unit is removed from the output buffer at the time of decoding The size of each access unit for video for example differs depending on the scene complexity and the type of a specific frame of video The data rates and buffer sizes specified in this document are based on 1 and 2 5 Rohde amp Schwarz Buffer Analysis 6 Different Buffer Models for Video Audio and System Information 7BM53_0E MPEG 2 video As mentioned above the main buffer of the video path of the T STD is divided into the MB and the EB Only payload information of a TS packet is written to the MB The size of the MB as well as the data rate from the TB to the MB vary depending on the video profile and level ER mn TB transport stream buffer Decoder EB elementary stream buffer MB multiplex buffer Fig 3 MPEG 2 video path ofthe T STD Two different methods for checking the compliance of an MPEG 2 video stream are available Depending on the TS content one of these two mechanisms is used The principle behind the compliance checks is to determine how effectively data is transferred between the MB and the EB e Leak method This method checks the multiplexing of the ES into the TS by transferring data with a constant da
7. ID of 0 1 or 3 and the ones listed in the PMT of the service to be decoded are collected in the TB of the system path Packets transmitted twice and packets without payload are also fed to this buffer Packets of the NIT are not written to this object because the use of the NIT is usually part of the DVB standard All packets in the TB are read at 1 Mbit s Packets that occur twice are fed to the main buffer only once This main buffer has a size of 12288 bits If B contains data it is transmitted to the system s controller at a variable data rate D Rohde amp Schwarz Buffer Analysis System var data rate 1 MBit s control 4096 bits 12288 bits TB transport stream buffer Fig 6 System path of the T STD B main buffer 7 R amp S DVM Graphical User Interface Overview 7BM53_0E To access the buffer analysis functionality select Advanced from the Measurement Group Selection Advanced Analysis PTS SE ege Gi e E K I Topology Monitoring Interpreter aan Data Broadcast Streaming Fig 7 Measurement Group Selection The results display shows various tabs To determine buffer compliance you need to use the Buffer Analysis and Buffer Model Info tabs marked in red in Fig 8 Advanced Buffer PCR Analysis PTS Analysis DVB Config DVB Buffer Analysis Buffer Model Info Service 514 ZDF PCR PID 0545 ES PID 0545 Video MPEG2 Start 1 kb Transport B
8. ceeeeceeeeeeeseeceaeeceeeeaeeseeteneeseeeeeeeseees 16 13 Ordering Information ccccccceecccsseeceeecceeeecueeeceeeseeeeeeeeseeseeesaeeeseas 16 2 Rohde amp Schwarz 1 Overview 2 Introduction 7BM53_0E Buffer Analysis The new buffer analysis feature was introduced with firmware version 2 80 for the R amp S DVM family This application note describes this feature and explains how it represents a valuable asset to your daily business operations This application note first provides background information and then explains how monitoring and measurement have been implemented in the R amp S DVM family Today exceedingly large amounts of information are transmitted throughout the world via cable satellite and terrestrial systems Bandwidth limitations have made it necessary to optimize existing resources This has led to the development of compression mechanisms such as MPEG 2 ISO IEC 13818 As an example this standard provides makes it possible to reduce the data rate for video and audio transmissions by removing unnecessary and redundant data In this context Unnecessary means detailed information that cannot be detected by human senses The term redundant means that the same information is present more than once To explain the basic steps of digital television transmission the following figure shows the generation and transmission principle Video1 Program 1 Encoder Audio Video2 Encoder Audio2
9. erate 25 00 fps Bitrate 15 00 MBit s Overflow Aspect Ratio 4 3 Chromaformat 4 2 0 Undertow Fig 12 Buffer Model Info tab Both the Buffer Analysis and Buffer Model Info tabs contain display fields for the current measurement Fos Pk 1711720 bit Limit 1535005 bit Fig 13 Display fields The Pos Pk field indicates the maximum value reached during the measurement The Limit field shows the value specified by the standard or by the user By selecting Config from the measurement control buttons Fig 14 you can define your own parameters and limits You can choose the buffer size and the method used for transferring data from the main buffer to the EB for video streams Thus a user specific configuration that differs from ETSI TR 101 290 can be applied 10 Rohde amp Schwarz Buffer Analysis Us43 Ib4uD 0338 16405 0343 16405 Log View Event Navigate te Disabled Running lapsed Time 1572 45 42 La Streaming Fig 14 Configuration 7BM53_0E 11 Rohde amp Schwarz Buffer Analysis 8 R amp S DVM Buffer Monitoring in Accordance with ETSI TR 101 290 You can monitor various conditions as defined by the different T STD paths and standards in order to determine the conformance of the TS that is used The appropriate buffer model is automatically selected for the specified stream If a condition for a buffer model is violated an error alarm will be output The fill level of the buffers as well as the ti
10. he EB is not allowed to underflow III 7BM53_0E 12 Rohde amp Schwarz Buffer Analysis e If the still_picture_flag of the video descriptor is set to 0 the data delay has exceeded 1 s IV e If the still_ picture_flag of the video descriptor is set to 1 the data delay has exceeded 60 s IV MPEG 1 2 audio As shown in the figure below the MB is not available grey for measurements on MPEG 1 2 audio signals SN Service 514 ZDF PCR PID 0545 ES PID 0546 Audio MPEG Transport System Target Decoder Model Transport Buffer Pos Pk 1277 bit Limit 4096 bit Decoder RX Rbx u e e Video Buffer Model Ts Overflow I Empty Buffer Error Decoder A Multiplex Buffer Audio Buffer Model Pos Pk Limit Bit Rates and Data Delay Rts Rix Rbg Data Delay Overflow 13 271 Mbit s Max 2 000 Mbit s Max 71 ms Current Undertow 13 270 Mbit s Min fu Oo 71 ms Max Empty Buffer Error III 1000 ms Limit Elementary Stream Info Elementary Buffer MPEG1 Audio Stream Pos Pk 18912 bit il Bitrate 256 kBit s Limit 28672 bit Sampl ng frequency 48 00 kHz Mode Stereo amp Over verflow II P Underflow Fig 16 MPEG 1 2 audio analysis Error alarm conditions e Overflow of the TB has occurred I e Underflow or overflow of the main buffer has occurred II e Data delay has exceeded 1 s III 7BM53_0E 13 Rohde amp Schwarz Buffer Analysis 9 R amp S DVM Buffe
11. me required for a byte to enter the T STD and then exit the main buffer or EB are monitored The error alarm conditions below vary depending on the buffer model used You can monitor them with the R amp S DVM MPEG 2 video Service 514 ZDF PCR PID 0545 ES PID 10545 Video MPEG2 Transport System Target Decoder Model Transport Buffer Pos Pk 0 bit Limit 4096 bit 1 Overflow Video Buffer Model Empty Buffer Error R Le H H I TB EB Multiplex Buffer dio Bu gt 1 Pos Pk 160 bit I Limit 80000 bit Bit Rates and Data Delay Ats Rx Rbx Data Delay CH Overflow 13 271 Mbit s Max 18 000 Mbit s 15 000 Mbit s Max 375 ms Brenn Underlow 13 271 Mbit s Min 15 000 Mbit s OH 458 ms Max I OH Empty Buffer Error I U 1000 ms Limit Elementary Stream Info Elementary Buffer MPEG2 VideoStream Pos Pk 1758904 bit Profile Main Level Main Lirit 1835008 bit Size 704x576 Framerate 25 00 fps Bitrate 15 00 MBit s III Overflow Aspect Ratio 4 3 Chromaformat 4 2 0 Underflow Fig 15 MPEG 2 video analysis Error alarm conditions e Overflow of the TB has occurred I e The TB is not empty at least once per second I e Overflow of the MB has occurred II e Ifthe VBV method is used an underflow of the MB has occurred II e f the leak method is used the MB is not empty at least once per second II e Overflow of the EB has occurred III e If Low Delay is selected and the DSM _trick_mode_flag is set to 0 t
12. presentation on the terminal device the encoder and the multiplexer have to create data streams that can be demultiplexed and decoded without any problems during presentation The task is to determine how data of a specific ES can be delayed compared to others in such a manner that a synchronized decoding process with audio for example will be successful in every case Therefore ISO IEC 13818 1 provides the transport stream system target decoder T STD Although real buffers are required on the decoder end in order to present the content of the transmitted packets properly the T STD is not intended as a reference design for real decoders The T STD helps to ensure that a multiplexer or encoder functions properly Network providers in particular can verify TS compliance by means of the R amp S DVM family using the T STD A solution can be found by looking at these theoretical buffers and how they are impaired when an underflow or overflow occurs due to incorrectly generated transport streams To access the buffer analysis functionality your R amp S DVM device must have the in depth analysis functionality R amp S DVM400 DVM B1 R amp S DVM100 120 DVM K10 R amp S DVM50 DVM50 K10 and an analyzer firmware version of 2 80 or later installed Understanding buffer analysis requires a basic knowledge of MPEG 2 TS Since an introduction to MPEG 2 technology is beyond the scope of this document please see 4 for an in depth description
13. r Analysis for AVC Video 7BM53_0E The graphical example of the measurement display is identical to that shown in Fig 15 The error conditions monitored by the R amp S DVM are as follows e Overflow of the TB has occurred e The TB is not empty at least once per second e Overflow of the MB has occurred e Overflow of the EB has occurred e If the low_delay_hrd_flag is set to 0 an underflow of the EB has occurred e If the AVC _still_ present flag of the AVC video descriptor is set to 0 the data delay has exceeded 10 s e If the AVC_still_ present flag of the AVC video descriptor is set to 1 the data delay has exceeded 60 s 14 Rohde amp Schwarz 10 Abbreviations 11 Literature 7BM53_0E Buffer Analysis AVC Advanced Video Coding B Main Buffer CPB Coded Picture Buffer DTS Decoding Time Stamp DVB Digital Video Broadcasting EB Elementary Stream Buffer ES Elementary Stream ETSI European Telecommunications Standards Institute HRD Hypothetical Reference Deocder HSS Hypothetical Stream Scheduler ISO International Organization for Standardization MB Multiplex Buffer MPEG Moving Picture Experts Group NIT Network Information Table PAT Program Association Table PCR Program Clock Reference PID Packet Identifier PMT Program Map Table PTS Presentation Time Stamp TB Transport Stream Buffer TS Transport Stream T STD Transport Stream System Target Decoder VBV Video Buffer Verifier Standards 1 2 3 Inte
14. rnational Organization for Standardization Ed 1999 International Standard ISO IEC 13818 1 Information Technology Generic coding of moving pictures and associated audio information International Organization for Standardization International Organization for Standardization Ed 2005 International Standard ISO IEC 14496 10 Infrastructure of audiovisual services Coding of moving video Advanced video coding for generic audiovisual services International Organization for Standardization European Telecommunications Standards Institute Ed 2001 ETSI TR 101 290 Digital Video Broadcasting DVB Measurement guidelines for DVB systems Sophia Antipolis Cedex France European Telecommunications Standards Institute Books 4 Fischer Walter 2005 Digital Television A Practical Guide for Engineers Berlin Springer 15 Rohde amp Schwarz Buffer Analysis 12 Additional Information Our application notes are updated from time to time Please visit the Rohde amp Schwarz website to download new versions Please send any comments or suggestions about this application note to Broadcasting TY Applcations rahde schw rz com 13 Ordering Information DVM50 MPEG 2 Monitoring System 2085 1900 02 DVM K1 Additional TS Input 2085 5211 02 DVM50 K10 In Depth Analysis 2085 5434 02 DVM K11 Data Broadcast Analysis 2085 5311 02 DVM100 MPEG 2 Monitoring System 2085 1600 02 DVM120 MPEG 2 Monitoring System 2085 1700
15. ta rate between the specified buffers The data rate is specified through the profile and level of the video streams e Video buffer verifier VBV method In addition to the functionality of the leak method the VBV method is able to perform a compliance check of the ES that is used by extracting VBV specific entries such as VBV buffer size and VBV delay from the TS These values are used for specifying the buffer model parameters This method defines a hypothetical decoder consisting of a VBV buffer and an ideal decoder If the VBV buffer experiences neither an underflow nor an overflow the TS has passed the test VBV Buffer MPEG 2 Encoder Decoder Fig 4 Video buffer verifier VBV method The leak method is used if one of the following conditions is met otherwise the VBV method is used note all names used below are defined in 1 e No STD descriptor is listed in the PMT for the ES e An STD descriptor exists and the leak_valid_flag is set to 1 e An STD descriptor exists the leak_valid_ flag of the descriptor is set to 0 and the parameter vbv_ delay is set to OxFFFF e The DSM_trick_mode flag is set to 1 6 Rohde amp Schwarz 7BM53_0E Buffer Analysis All data for the access unit that is present in the EB is removed at the time of decoding Advanced video coding AVC video The architecture of the T STD path for AVC video is the same as for MPEG 2 video The data rate for the input as well
16. to 00 00 00 000 D I BER Method MB gt EB Leak Aiming Elapsed Time 00 00 35 Fig 10 Buffer Analysis tab The available monitoring types vary depending on the kind of stream selected The analysis function can be started by clicking Start Fig 11 Start and stop You can stop the measurement at any time by clicking Stop The Clear button clears the measurement screen 9 Rohde amp Schwarz 7BM53_0E Buffer Analysis The monitoring and error alarm process is located on the Buffer Model Info tab The buffer model applied MPEG 2 video in the example for the specific ES is shown and color signals indicate the compliance of the buffer fill levels and delays PCR Analysis PTS Analysis Buffer Analysis Buffer Model Info Le Service 514 ZDF PCR PID 0545 ES PID 0545 Video MPEG2 Transport System Target Decoder Model Transport Buffer Pos Pk 0 bit Limit 4096 bit Overflow Video Buffer Model Empty Buffer Error Decoder di Rts Rx Bean WE _ gt e Multiplex Buffer Audio Buffer Model Pos Pk 160 bit Limit 80000 bit Bit Rates and Data Delay Rts Rx Abs Data Delay Overflow 13 271 Mbit s Max 18 000 Mbit s 15 000 Mbit s Max 455 ms Current ER Underflow 13 271 Mbit s Min 15 000 Mbit s Min 610 ms Max Empty Buffer Error 1000 ms Limit Elementary Stream Info Elementary Buffer MPEG2 YideoStream Pos Pk 1635168 bit Profile Main Level Main Limit 1835008 bit Size 704x576 Fram
17. uffer Fig 8 Results display To access buffer analysis for a specific video or audio ES select the stream from a service listed in the TS tree Fig 9 shows that an MPEG 2 video element has been selected ARTS ID 0514 1 ig PSI SI a 514 ZDF SS Mlvideo MPEG2 PID 0 Teletext PID 0551 lt AIT PID 0552 Data MHP Object Carous Data MHP Object Carous Data MHP Object Carous Data MHP Object Carous AAudio AC3 PID 0559 Ga Service 515 Info 3sat Si Service 517 Doku KiKa u Anul Packets PID 8191 54 Fig 9 TS tree The fill level of the various buffers is graphically displayed on the Buffer Analysis tab The green graph shows the measurement values over time the red horizontal line equals the limit line 8 Rohde amp Schwarz 7BM53_0E Buffer Analysis PCR Analysis PTS Analysis Buffer Analysis Buffer Model Info Service 514 ZDF PCR PID 0545 ES PID 0545 video MPEG2 1kb Transport Buffer Pos Pk 0 bit Limit 409 bit E zl Scale gt gt x Rescale 0 kb D l D I H l V N 1 08 0 8 0 6 0 4 0 2 to 00 00 00 000 1kb E Multiplex Buffer 4 Pos Pk 160 bit Limit 80000 bit E a Scale gt z Rescale 0 kb D I D D I D I 1 1 08 0 8 0 6 0 4 0 2 to 00 00 00 000 Elementary Buffer Pos Pk 1767368 bit Limit 1835008 bit el al Scale gt 3 Rescale 1 08 0 8 0 6 0 4 0 2
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